ICES COOPERATIVE RESEARCH REPORT
RAPPORT DES RECHERCHES COLLECTIVES
NO. 232
DIETS OF SEABIRDS AND CONSEQUENCES OF CHANGES IN FOOD SUPPLY
Edited by
Edited by
Robert W. Furness Institute of Biomedical and Life Sciences Graham Kerr Building University of Glasgow Glasgow G12 8 QQ UK
and
Mark L. Tasker Joint Nature Conservation Committee Dunnet House 7 Thistle Place Aberdeen AB10 1 UZ UK
International Council for the Exploration of the Sea Conseil International pour l’Exploration de la Mer
Palægade 2–4 DK-1261 Copenhagen K Denmark
May 1999
ICES Cooperative Research Report No. 232 ISSN 2707-7144 ISBN 978-87-7482-437-4 https://doi.org/10.17895/ices.pub.5363
TABLE OF CONTENTS
Section Page
List of Working Group participants ...... 1 1 A review of issues related to seabird consumption of fish and shellfish stocks, discards and mariculture as well as the trophic role and ecology of seabirds and waders G. L. Hunt, W. A. Montevecchi, and M. F. Leopold ...... 2
1.1 Introduction ...... 2 1.2 Seabirds as indicators...... 2 1.2.1 Seabirds as indicators of prey stocks...... 2 1.2.2 Seabirds as monitors of pollutants...... 2 1.3 Processes affecting the trophic ecology of seabirds...... 3 1.4 Seabird and wader interactions with mariculture ...... 3 1.5 Seabird impacts on recruitment of fish stocks...... 4 1.6 Mortality of seabirds ...... 4 1.7 Discards and offal ...... 4 1.8 References ...... 5
2 Consumption of pre-recruit fish by seabirds and the possible use of this as an indicator of fish stock recruitment S. P. R. Greenstreet, P. H. Becker, R. T. Barrett, P. Fossum, and M. F. Leopold...... 6
2.1 Introduction - background to fish stock assessment...... 6 2.2 Introduction - background to seabird feeding ecology...... 7 2.3 Seabirds as samplers of 0-group fish: case studies on cormorants/shags...... 7 2.3.1 Shags and saithe, Norway...... 8 2.3.2 Cormorants and flatfish, Dutch Wadden Sea...... 8 2.4 Pre-recruit herring and common tern reproduction...... 8 2.4.1 Correlations of herring population parameters ...... 8 2.4.2 Correlations between pre-recruiting clupeids and diet of common tern chicks...... 9 2.4.3 Relationships between recruiting clupeids and the reproduction of terns...... 10 2.4.3.1 Minsener Oldeog...... 10 2.4.3.2 Banter See, Wilhelmshaven ...... 11 2.4.4 Conclusions ...... 12 2.5 Norwegian spring-spawning herring and north Norwegian seabirds ...... 13 2.5.1 Røst ...... 13 2.5.2 Hornøya ...... 14 2.6 Conclusions ...... 15 2.7 References ...... 16
3 Variation in prey taken by seabirds M. L. Tasker, C. J. Camphuysen, and P.Fossum ...... 18
3.1 Introduction ...... 18 3.2 Database description...... 18 3.3 Variation in species and size of seabird prey...... 18 3.3.1 General considerations...... 18 3.3.1.1 Most frequently recorded food items ...... 19 3.3.1.2 Prey size ...... 19 3.3.2 Annual variation...... 19 3.3.3 Seasonal variation ...... 21 3.3.4 Spatial variation ...... 21 3.4 Evidence for selection related to prey body condition ...... 23 3.4.1 Differential prey selection between species ...... 24 3.4.2 Differential selection of prey within species...... 24 3.5 Differences between adult and chick diet...... 24 3.6 Discussion ...... 25
ICES Coop. Res. Rep. No. 232 i Section Page
3.7 References ...... 25
4 Evaluation of the role of discards in supporting bird populations and their effects on the species composition of seabirds in the North Sea S. Garthe, U. Walter, M. L. Tasker, P. H. Becker, G. Chapdelaine, and R. W. Furness ...... 29
4.1 Introduction ...... 29 4.1.1 The shrimp fishery off Niedersachsen, Germany...... 29 4.2 Consumption of discards by seabirds ...... 31 4.2.1 Offshore fisheries in the North Sea ...... 31 4.3 Diets of seabirds that scavenge discards in the North Sea...... 33 4.4 Numbers of seabirds supported by discards in the North Sea ...... 33 4.5 Direct effects of discard consumption on species composition of seabirds in the North Sea...... 33 4.5.1 Increase in population size of seabird species...... 33 4.5.2 Population increase and changes in composition of seabird communities...... 36 4.6 Indirect effects of discard consumption on species composition of seabirds in the North Sea ...... 38 4.7 References ...... 38
5 Exploration of the short-and medium-term consequences of a reduction in the amounts of fish discarded M. L. Tasker, P. H. Becker, and G. Chapdelaine...... 42
5.1 Short term effects ...... 42 5.1.1 Introduction...... 42 5.1.2 Loss of feeding opportunities...... 42 5.1.3 Change in bird distribution...... 42 5.1.4 Competition at trawlers...... 42 5.1.5 Changing diets...... 43 5.16 Reproduction...... 43 5.2 Medium term effects ...... 43 5.2.1 Introduction...... 43 5.2.2 Population size of consumper species...... 43 5.2.3 Population size and species composition ...... 44 5.3 References ...... 45
6 Evidence for decadal scale variations in seabird population ecology and links with the North Atlantic oscillation J. B. Reid, P. H. Becker, and R. W. Furness...... 47
6.1 Introduction ...... 47 6.2 Materials and methods...... 47 6.3 Results ...... 47 6.4 Discussion ...... 49 6.5 References ...... 49
7 A review of the causes, and consequences at the population level, of mass mortalities of seabirds ...... 51 C. J. Camphuysen, P. J. Wright, M. Leopold, O. Hüppop, and J. B. Reid ...... 51
7.1 Introduction ...... 51 7.2 Presumed causes ...... 51 7.3 Frequency and seasonal occurrence of wrecks...... 52 7.4 Vulnerability of seabird species to wrecks ...... 53 7.5 Consequences to populations ...... 55 7.6 References ...... 56
Appendix 7.1 – List of Wrecks ...... 64
ii ICES Coop. Res. Rep. No. 232 The following nationally appointed members of the Seabird Ecology Working Group participated in the meetings from which this report is derived:
R.T. Barrett Norway P.H. Becker Germany C.J. Camphuysen The Netherlands G. Chapdelaine Canada P. Fossum Norway R.W. Furness (Chair) UK S. Garthe Germany S.P.R. Greenstreet UK G.L. Hunt Jr USA O. Hüppop Germany M.F. Leopold The Netherlands W.A. Montevecchi Canada J.B. Reid UK M.L. Tasker UK P.J. Wright UK
ICES Coop. Res. Rep. No. 232 1 1 A review of issues related to seabird consumption of fish and shellfish stocks, discards and mariculture as well as the trophic role and ecology of seabirds and waders
G. L. Hunt1, W. A. Montevecchi2, and M. F. Leopold3
1Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697, USA. 2Biopsychology Programme, Memorial University, St John’s, Newfoundland, Canada A1C 5S7. 3IBN–DLO, PO Box 167, 1790 Den Burg, Texel, The Netherlands.
1.1 Introduction fishery can be harmed by competition or other trophic interactions. As these questions and issues are addressed, both experimental approaches within the North Sea and The Working Group on Seabird Ecology was requested comparisons with fisheries experience elsewhere in the by the Biological Oceanography Committee to assess the world will be required. issues most likely to be raised within the ICES community concerning the foraging ecology of seabirds and waders, and the potential interactions between these 1.2 Seabirds as indicators groups of birds and fisheries. In responding to this request, the Working Group has listed a number of issues 1.2.1 Seabirds as indicators of prey stocks likely to be of importance. The Working Group recognized that each of these issues by itself is potentially the subject for new research and/or for a 1. Changes in the distribution, abundance, species major review. The Working Group restricted itself to the composition and breeding biology of seabirds can identification of issues, and has used this list as the basis indicate changes in the distribution, abundance, and for developing possible future reports by the Working size classes of their preferred prey (reviewed by Group on Seabird Ecology, singularly, or in co-operation Montevecchi, 1993). with other ICES Working Groups or Committees. a) Seabirds may be sensitive indicators of In the present listing of issues, we have grouped issues interannual and seasonal variation in the timing into several large subcategories, but we have not ranked of life history events in prey stocks. either the issues or the subcategories by importance, since that is probably not feasible except on a local or b) Seabird diets may provide indications of species-specific basis. In the first subcategory we list changes in the biodiversity of prey populations issues related to the use of seabirds and waders as not otherwise monitored by fisheries managers indicators of conditions within the ecosystems of which (Springer et al., 1984; Montevecchi, 1993). they are a part. These issues include the distribution and abundance of prey organisms, the presence of pollutants c) There is a need to calibrate the relationship and the need to calibrate the signals received from the between the responses of seabirds and the birds with the absolute values of the parameters of variations in abundance or recruitment in prey interest. stocks of concern.
There are also important issues that focus on the basic 1.2.2 Seabirds as monitors of pollutants ecology of seabirds that are of interest (Hunt et al., 1996), particularly insofar as they illuminate processes that control the structure and energy flow within marine 2. Seabirds accumulate a wide variety of organic ecosystems. In the second subcategory, we focus on the chemicals and heavy metals and provide an use of seabirds as model systems for investigating indication of the prevalence of these pollutants in processes in marine ecosystems that are of broad interest, the marine ecosystem. and for which seabirds may be useful windows into processes that are otherwise difficult to study. a) As wide ranging top predators, seabirds provide sampling opportunities that integrate In the remaining subcategories, we list issues concerning pollutant transfers up food chains which is the effects of seabirds and waders on fisheries, and especially useful in the monitoring of lipophilic conversely, the effects of fisheries on seabirds and pollutants. They provide a better indication of waders. It should be noted that, in the cases where birds possible hazards to humans than does sampling and the fishing industry utilise the same resource, there is from low trophic levels. Because their biology the possibility that either, or both, the birds and the is generally well known, the interpretation of pollutant burdens is easier. By integrating over
2 ICES Coop. Res. Rep. No. 232 time and spatial scales they permit more cost column, since many species of seabirds are effective sampling. (see reviews in Furness and restricted to forage in the top 2 m? Greenwood, 1993). 6. What are the consequences for seabirds (and other b) The pathways of pollutant to seabirds may be marine predators) of prey switching as a traced by using stable isotope analyses and consequence of changes in the availability of fatty acid tracers to identify the trophic preferred prey? How are adult survival and pathways and carbon source areas supporting reproductive performance affected? seabird populations. 7. Evaluate evidence for decadal-scale variation in the 1.3 Processes affecting the trophic population sizes, reproductive ecology or food habits of seabirds in the North Atlantic. Can these ecology of seabirds changes be related to the North Atlantic oscillation and other long-term cycles? 1. How does the abundance of fish predators relative to the abundance of their prey influence the availability of food to seabird populations? 1.4 Seabird and wader interactions with mariculture a) There is a need to evaluate the evidence that the removal of large piscivorous fish by the 1. Shellfish fishery has enhanced prey availability to seabirds and thereby caused a related increase a) Blue mussel Mytilus edulis consumption by in seabird populations. waders, especially oystercatchers Haematopus ostralegus, gulls, and seaducks, especially b) There is a need to evaluate the evidence that eiders Somateria mollissima and scoters, on changes in the species composition of both natural and artificial mussel beds in predominant fish consumers of zooplankton competition with mussel fisheries (e.g., has affected seabird populations. Wadden Sea, coastal U.K., Baltic).
2. At the population level, is most seabird foraging b) Cockle Cerastaderma edule consumption by concentrated in a few critical areas where birds are oystercatchers and eiders that compete with present in high concentrations, or is most seabird cockle fisheries (e.g., Wadden Sea, foraging accomplished by widely dispersed Camphuysen et al., 1996). individuals (e.g., Wright et al., 1996)? c) Spisula consumption by scoters Melanitta spp. a) Are preferred foraging areas, with seabird and eiders (e.g., German and Southern Bight). aggregations, characteristic of some species or regions, but not of others? d) What are the population-level and local consequences for seabirds, seaducks and 3. Given that certain forage fish species show strong waders of the availability of commercial stocks relationships to bottom type and other species may of shellfish, and what changes in these avian respond to physical processes that concentrate populations would be predicted should the planktonic prey, what is the importance of bottom commercial stocks of shellfish no longer be sediment type versus hydrographic processes and available to these birds? This topic was briefly structures in determining foraging location, foraging reviewed in the previous meetings of the success and the role of seabirds in trophic transfer? Working Group (Leopold et al., 1996).
a) Can we determine or predict where the highest 2. Finfish mariculture concentrations of foraging seabirds are likely to be found, and the temporal stability of these a) Salmon consumption by cormorants preferred foraging locations? Phalacrocorax carbo, gulls, grey herons Ardea cinerea, ospreys Pandion haliaetus and other 4. What are the winter foods of seabirds at sea? Are birds taking fish from penned stock. there seasonal changes in the species or types of prey taken, and if so, are these changes more b) What are the local population-level marked for planktivorous than for piscivorous consequences to birds of the availability of seabirds? farm-raised fish, and the protection of mariculture by shooting birds that take or 5. What influences the vertical distribution of forage damage product? fish, in particular their abundance in the upper water
ICES Coop. Res. Rep. No. 232 3 1.5 Seabird impacts on recruitment of 1.6 Mortality of seabirds fish stocks The key question is: What are the relative impacts, at the Seabirds predominantly consume small fish, particularly population level, of various anthropogenic and natural 0-group fish. We believe that in almost all situations the sources of seabird mortality at sea? In particular in our local impact of this predation is likely to be less than that context: What is the relative impact of fisheries-related from predatory fish, and mostly trivial in terms of fish mortality compared to other sources of mortality? We stock dynamics, but in some situations it has been appreciate that these topics are to some extent included suggested that recruitment to fish stocks might be in the remit of the Working Group on Ecosystem Effects affected by seabird predation rates. of Fisheries. a) Consumption of pre-recruit gadids by cormorants 1. Wrecks. and shags Phalacrocorax aristotelis (Norway, Barrett et al., 1990). a) Wrecks or mass mortalities of seabirds may occur for a variety of reasons, one of which can b) Consumption of juvenile herring Clupea harengus be acute local food shortage. This issue is by puffins Fratercula arctica and other seabirds considered in greater depth later in this report (Norway, Anker-Nilssen, 1992). (Section 7). c) Consumption of 0-group flatfishes by cormorants 2. Mariculture and other seabirds (Damme, 1995). a) Drowning of cormorants, shags and other birds d) Consumption of salmon smolt by gulls, cormorants in fish pens. and other seabirds (West coast and Alaska, USA). Although not within the ICES geographical area, 3. Net fishing the situation in western North America provides a useful example of predator build-up at an artificial a) Entanglement of seabirds in set nets, drift nets feeding opportunity. Similar problems for released and fish traps. smolts may arise in Canada, the west of Scotland and in Norway (Greenstreet et al., 1993). b) Are there specific areas of the oceans where Examination of food samples from gannets Morus seabirds are present in high densities, and thus bassanus, which consume low levels of salmon, has particularly vulnerable to entanglement? yielded important information about the salmon migration routes in eastern Canada (Montevecchi et al., 1988). 4. Long line fishing e) Consumption of forage fish by seabirds and the a) Hooking and subsequent death of seabirds, in potential for competition with fisheries for these particular fulmars Fulmarus glacialis, great stocks, particularly when local stocks near seabird skuas Catharacta skua and gulls in long-line colonies are depleted (e.g., sandeels Ammodytes fisheries in the ICES areas and albatrosses and marinus near Shetland; Sprat Sprattus sprattus near large petrels in southern oceans. Firth of Forth, juvenile herring in German and Southern Bight). 5. Other sources of mortality (e.g., oil, chemical pollution, weather, etc.). Such effects often act in f) How does the rate and total take of the commercial synergy. Are these influences greater or less than harvest of forage fish stocks impact their effects attributable to fisheries activities, and are availability to seabirds? This question is most likely synergies evident? to be an issue in ICES IVa west and possibly in ICES IVb (Tasker and Furness, 1996; Wright and 1.7 Discards and offal Tasker, 1996; Wright et al., 1996). Much of the following is addressed in greater detail later g) Are the perceived problems for seabirds indicative in this report in Sections 4 and 5 on Discards. of similar problems for other marine predators? 1. To what extent are present populations of seabirds h) consumption of freshwater fish stocks by dependent on discards and offal? cormorants and sawbill ducks (Russell et al., 1996). 2. What impacts on seabird and other predator populations would be expected if food from discards and/or offal become reduced or unavailable? What secondary and tertiary impacts
4 ICES Coop. Res. Rep. No. 232 might be anticipated (e.g., gull predation of other Hunt, G. L., Barrett, R. T., Joiris, C., and Montevecchi, seabirds in Newfoundland; skua predation on W. A. 1996. Seabird/fish interactions: an kittiwakes Rissa tridactyla in Shetland; increased introduction. In: Seabird/fish interactions with predation on fish populations of value). particular reference to seabirds in the North Sea, pp. 2–5. Ed. by G. L. Hunt and R. W. Furness. ICES 3. Do discards and offal play different Cooperative Research Report 216. ecological/trophic roles? Leopold, M. F., Nehls, G., and Skov, H. 1996. 4. Given that different fisheries produce different Consumption of shellfish by seaducks and proportions of discards and different ranges of fish oystercatchers. In: Seabird/fish interactions with sizes in the discards, to what extent do seabirds particular reference to seabirds in the North Sea, pp. show preferences between fisheries, and how does 55–63. Ed. by G. L. Hunt and R. W. Furness ICES the rate of dumping of discards after a trawl affect Cooperative Research Report 216. their fate? Are seabirds feeding on discards that might otherwise survive? Montevecchi, W. A. 1993. Birds as indicators of change in marine prey stocks. In: Birds as monitors of environmental change, pp. 217–266. Ed. by R. W. 5. Is there a seasonal variation in the importance of Furness and J. J. D. Greenwood. Chapman and Hall, discards to seabirds? London.
6. Effects of fishing moratoria/ closed areas/ discard Montevecchi, W. A., Cairns, D. K., and Birt, V. L. 1988. bans require to be evaluated as these present Migration of post-smolt Atlantic salmon (Salmo valuable experiments permitting responses of salar) off northeastern Newfoundland, as inferred seabirds to be studied. from tag recoveries in a seabird colony. Canadian Journal of Fisheries and Aquatic Science, 45: 568– 1.8 References 571. Russell, I. C., Dare, P. J., Eaton, D. R., and Armstrong, J. Anker-Nilssen, T. 1992. Food supply as a determinant of D. 1996. Assessment of the problem of fish-eating reproduction and population development in birds in inland fisheries in England and Wales. Norwegian puffins Fratercula arctica. Doctor of Directorate of Fisheries Research, Lowestoft. 130 pp. Science thesis in terrestrial ecology, University of Trondheim. 46 pp. Springer, A. M., Roseneau, D. G., Murphy, E. C., and Springer, M. I. 1984. Environmental controls of Barrett, R. T., Rov, N., Loen, J., and Montevecchi, W. A. marine food webs: food habits of seabirds in the 1990. Diets of shags Phalacrocorax aristotelis and eastern Chukchi Sea. Canadian Journal of Fisheries cormorants P. carbo in Norway and implications for and Aquatic Sciences, 41: 1202–1215. gadid stock recruitment. Marine Ecology Progress Series, 66: 205–218. Tasker, M. L., and Furness, R. W. 1996. Estimation of food consumption by seabirds in the North Sea. In: Camphuysen, C. J., Ens, B. J., Heg, D., Hulscher, J., Van Seabird/fish interactions with particular reference to der Meer, J., and Smit, C. J. 1996. Oystercatcher seabirds in the North Sea, pp. 6–42. Ed. by G.l. Hunt winter mortality in The Netherlands: the effect of and R.W. Furness ICES Cooperative Research severe weather and food supply. Ardea, 84a: 469– Report 216. 492. Wright, P., Barrett, R. T., Greenstreet, S. P. R., Olsen, Damme, C. J. G. van 1995. Predation on juvenile flatfish B., and Tasker, M. L. 1996. Effect of fisheries for by cormorants Phalacrocorax carbo in the Dutch small fish on seabirds in the Eastern Atlantic. In: Wadden Sea. NIOZ Rapport 1995–10, Netherlands Seabird/fish interactions with particular reference to Instituut voor Onderzoek der Zee, Texel. 46 pp. seabirds in the North Sea, pp. 44–55. Ed. by G. L. Hunt and R. W. Furness. ICES Cooperative Research Furness, R. W., and Greenwood, J. J. D. 1993. Birds as Report 216. monitors of environmental change. Chapman and Hall, London. 368 pp. Wright, P., and Tasker, M. L. 1996. Analysis of fish consumption by seabirds by age class of prey fish. In Greenstreet, S. P. R., Morgan, R. I. G., Barnett, S., and Seabird/fish interactions with particular reference to Redhead, P. 1993. Variation in the numbers of shags, seabirds in the North Sea, pp. 42–44. Ed. by G. L. Phalacrocorax aristotelis, and common seals, Phoca Hunt and R. W. Furness ICES Cooperative Research vitulina, near the mouth of an Atlantic salmon, Salmo Report 216. salar, river at the time of the smolt run. Journal of Animal Ecology, 62: 565–576.
ICES Coop. Res. Rep. No. 232 5 2 Consumption of pre-recruit fish by seabirds and the possible use of this as an indicator of fish stock recruitment
S. P. R. Greenstreet1, P. H. Becker2, R. T.Barrett3, P. Fossum4 and M. F. Leopold5
1F.R.S. Marine Laboratory, PO Box 101, Victoria Road, Aberdeen AB11 9DB, U.K. 2Institut für Vogelforschung, Vogelwarte Helgoland, An der Vogelwarte 21, D–26386 Wilhelmshaven, Germany. 3Zoology Museum, University of Tromsø, Tromsø, Norway. 4Institute Marine Research, PO Box 1870 Nordnes, N–5024 Bergen, Norway. 5IBN–DLO, PO Box 167, 1790 Den Burg, Texel, The Netherlands.
2.1 Introduction – background to fish and the VPA recruit estimates are fairly close; the survey data provides a reasonably accurate estimate of current stock assessment recruit numbers. However, for species such as whiting and saithe, this is not the case. For these species it would In order to address the consumption of pre-recruit fish by be particularly useful if alternative means of estimating seabirds it is necessary to define exactly what is meant the numbers of recruits were available. Even in the case by the terms “recruitment” and “pre-recruit”. These of haddock and cod it is worth exploring whether seabird terms mean different things to different people. Many diet data might provide a useful independent estimate to consider “recruits” to be those fish maturing in a compare with young fish surveys or fisheries-derived particular year to become part of the spawning stock. estimates. Consequently relatively old fish of some considerable length, two year old cod Gadus morhua of 30 cm or Assessments of the major roundfish species are carried more for example, could be considered as “pre-recruits” out over a large geographic scale. The stock “units” were because they had yet to mature. This definition however, re-evaluated as recently as 1995, following which, is not the one adopted by those carrying out assessments ACFM concluded that, for assessment purposes, the of, for example, the demersal fish stocks. They consider stocks of whiting and cod in VIId (eastern Channel) recruits to be those fish entering the population of a should be combined with those in the North Sea. particular species at the youngest exploited age, i.e. fish Conclusions for the IIIa (Skagerrak) stocks were less of an age which occur in the catch or discard data. This clear cut, but there were indications that the cod and varies between species. Thus the youngest haddock haddock stocks were linked with those in the North Sea Melanogrammus aeglefinus and whiting Merlangius and that there were therefore grounds for combining merlangus which occur in catches are 0-group fish in the these assessments. Most seabird diet data have been latter part of the year, while cod and saithe Pollachius collected during the breeding season and generally virens of this age are rarely encountered in the catch. reflects the diets of birds feeding in the immediate Consequently, assessment working groups consider cod vicinity of particular colonies. It is questionable whether and saithe recruits to be 1-group fish turning up in the data collected on such a limited spatial scale could ever catches in the year following their birth. As a result of be used to provide indices of numbers of recruits in areas these between species differences, the VPA population as large as the North Sea, Skagerrak and eastern Channel assessments provide estimates of the numbers of 0-group combined, but we address this issue with real data below. whiting and haddock for quarters 3 and 4 in any given year, but not for the equivalent aged cod and saithe. The backcast VPA estimates of the numbers of recruits in past years is highly dependent upon estimates of The numbers of recruits (0-group whiting and haddock in natural mortality (by definition, these age classes do not year x and 1-group cod and saithe in year x+1, where occur in the catch so fishing mortality is zero). Constant year x refers to the year class) can be calculated back values of natural mortality have been assumed for each down a time series using straightforward VPA. However, species in carrying out the VPA assessments. If, at the time when each working group meets, an estimate however, natural mortality has varied as a result of of the numbers of fish in the current recruiting year class between year variation in the diet of seabirds, then the is required in order to attempt to extrapolate forward to VPA estimates of the numbers of recruits in each year predict future recruitment. Clearly catch data for these could be seriously in error. Furthermore, the predation fish are unavailable. In order to estimate current, or loading inflicted by seabirds on young gadid species future recruitment, fisheries survey data are used. The (e.g., Barrett, 1991) may be independent of the numbers historic VPA recruitment estimates are regressed against of young gadids available to seabirds, and may instead recruitment indices for the various species derived from be dictated by fluctuations in the abundance of pelagic survey data and, using the relationship obtained, the most species such as mackerel Scomber scombrus, herring recent survey recruitment indices are used to estimated Clupea harengus, sprats Sprattus sprattus and sandeels the current numbers of fish in the recruit age classes. For Ammodytes spp., which tend to be the preferred prey of species such as cod and haddock, the relationship most seabird species. Such a situation has been between recruitment indices derived from survey data
6 ICES Coop. Res. Rep. No. 232 demonstrated for common seals Phoca vitulina in the also unaffected. Great skuas Catharacta skua switched Moray Firth (Tollit et al., 1997). diet away from sandeels and their breeding success was reduced, but not as much as that of kittiwakes. Breeding 2.2 Introduction - background to seabird numbers of Arctic terns fell drastically as these birds mostly chose not to breed while sandeels were scarce, feeding ecology whereas great skuas continued to attempt to breed even though food was short. Great skuas incurred reductions Because most seabirds feed their chicks small fish, often in adult survival rate through having to work harder for the juvenile stages of large fish, studies of seabird diet food, whereas Arctic terns possibly did not because they can provide information on the local abundance of the mostly refrained from breeding. Thus each seabird youngest age classes (0- and 1-group) of fish in the species may respond in a species-specific way to a immediate area around a seabird colony. As reproductive change in food abundance, and may depend on different success of seabirds depends on the availability of prey species, or combinations of species, in different adequate food resources, several parameters of their regions. We show below, that kittiwake populations in reproductive biology or diet and feeding can be used as different parts of Norway show opposite responses to indicators of the availability and distributions of prey increased local abundance of herring - in one case species on which they feed (reviewed by Montevecchi, kittiwake breeding success increases with herring 1993). However, breeding seabirds only sample fish abundance and in the other it decreases. Such local within a short distance of their colony. Foraging ranges relationships are to be expected since responses depend vary among species, and according to food abundance, both on the ecology of the seabird species but also on the but tend to be tens of kilometres at most. Thus diet, combination of preferred prey fish species on which the provisioning rate, or some surrogate measure such as birds depend. chick growth rate, of seabirds at a single colony cannot sample an entire fish stock. The extent to which local Total food availability will affect seabird condition in sampling may reflect the wider situation is uncertain, but terms of average body mass, breeding output, growth and will be considered below. survival of young. Different parameters that can be measured in seabirds may thus provide information on Fish stocks are sampled on a daily basis by seabirds total food abundance and the composition of the fish whose diet is likely to reflect relative abundance of fish, community on which the birds feed. In situations where both by size (year class) and by species. Among the young fish make up most of the diets, seabirds may different species of seabirds available for research, the provide an additional means to sample younger stages of generalists will have diets that are most likely to reflect fish at a high temporal resolution, and at low cost the overall, local fish community structure, while compared to traditional ways of monitoring fish. Using specialists’ diets will reflect yearly or within-season the additional indications provided by seabirds may add differences in stocks of a particular species or group of little to assessment costs. species. Examples of specialist feeders are the sandeel- dependent seabirds of Shetland (Martin, 1989; 2.3 Seabirds as samplers of 0-group fish: Monaghan et al., 1989), the herring-dependent puffins Fratercula arctica in western Norway (Anker-Nilssen, case studies on cormorants/shags 1992) and the terns in the south-eastern North Sea that prey mainly on sandeels and clupeids. Cormorants Cormorants and shags Phalacrocorax aristotelis Phalacrocorax carbo and gannets Morus bassanus are regurgitate indigestible prey remains in discrete pellets, good examples of fish-eating seabirds that may take a probably on a daily basis (Harris and Wanless, 1993). large variety of fish species. Diets of cormorants include These pellets are relatively easily collected and can be both demersal and schooling, pelagic fish. Gannets analysed for the presence of fish otoliths, or other sample from the pelagic fish in surface waters. As a identifiable remains, which can be related to fish size. As consequence, the local and temporal variation in gannet such, these pellets provide an easy means to sample the or cormorant diet can reflect differences in relative prey diet and to get information on the state of the fish abundance. community at high temporal and spatial resolution.
It is important to note that there are major differences Unfortunately, few long-term data sets exist for between species of seabirds and between populations of cormorant diets in relation to prey availability in any one a single species in different regions. For example, the locality. However, there is evidence that, for example, sandeel ‘crisis’ in Shetland in the 1980s caused different double-crested cormorant diet can change considerably responses in different seabird species in Shetland. Arctic over time, in response to changes in the prey fish tern Sterna paradisaea and kittiwake Rissa tridactyla community (Rail and Chapdelaine, 1998). diet remained predominantly sandeel during the period of food shortage, but these birds failed to breed Here we consider the potential of cormorants and shags successfully. In contrast, gannets switched diet to other to be used as a tool in assessing the relative abundance of fish species and their breeding success was unaffected. 0-group gadids and flatfish, using case studies made in Guillemots Uria aalge continued to feed almost European waters. The first is a study on shags feeding exclusively on sandeels yet their breeding success was
ICES Coop. Res. Rep. No. 232 7 mainly on saithe in Norway, the second is on cormorants however. Cormorants have established several breeding feeding mainly on flatfish in the Wadden Sea. colonies in the Dutch Wadden Sea in recent years, so there is now also potential for studies that relate diet to 2.3.1 Shags and saithe, Norway breeding parameters such as growth rate and survival of chicks in these parts. 0-group saithe live in shallow, inshore waters that are notoriously difficult to sample. It is relevant to note that 2.4 Pre-recruit herring and common tern the relationship between VPA estimates of 1-group reproduction saithe and the numbers of young fish detected in surveys is very weak (ICES, 1997a). In such a situation, Pre-recruit fish have special importance as food for small “systematic surveys of prey harvests of shags breeding seabirds such as terns. These birds have difficulty taking on inshore islands as supplementary inputs to [models on fish longer than 20 cm. Small fish species or juvenile fish fish abundance]” could be useful (Barrett, 1991). Pellets were sampled in the 1985 and 1986 breeding seasons on therefore form the basis of their diet, consequently terns Bleiksøy, N. Norway (69°17’N, 15°53’N). Gadid may be especially useful as indicators of pre-recruit fish otoliths, all believed to be saithe, made up 81% and 58% abundance. Their overall energy reserves are low, so of all items identified in these two years and the birds food availability immediately affects body condition and mainly took 0- and 1-group fish. When comparing 1985 reproduction in adults (Frank and Becker, 1992; to 1986, in the second year the diet contained fewer Wendeln, 1997) and growth of young (Becker and saithe with a shift toward a higher proportion of older Specht, 1991; Mlody and Becker, 1991). They transport fish. This indicates that 1986 was a poor year for 0-group single food items in the bill, making it easy to obtain recruits in the area. This corroborated results of newly information on prey identity. Common terns Sterna developed 0-group surveys which ran in 1985–92. It is hirundo and Arctic terns are distributed widely around worth noting here that the data from sampling shag diet the coasts of the North Sea, and the accessibility of many provided indications of low saithe production two or colony sites make them ideal as monitors of the temporal three years sooner than could be determined from VPA and spatial variations of 0-group fish. Thus the breeding data (ICES, 1997a). failures among Arctic terns in Shetland during the 1980s (Furness, 1987; Monaghan et al., 1989, 1992; Uttley, 2.3.2 Cormorants and flatfish, Dutch Wadden 1992) coincided with a period of exceptionally low Sea sandeel recruitment (Bailey et al., 1991). In the southern North Sea, however, sandeels are not important prey for terns. Instead, clupeids, especially herring, but also sprat, Cormorant pellets from several major roosts (1993) and are the dominant prey of terns (Frank, 1992; Frick and one colony (1992) were sampled in late summer at locations throughout the Dutch Wadden Sea. Flatfish Becker, 1995; Tasker and Furness, 1996; Becker, 1996a; were the most important prey, representing 73% of the Stienen and Brenninkmeijer, 1998). Thus in this section, total diet by numbers (Leopold et al., 1998). Total we link common tern reproduction in the Wadden Sea consumption of flatfish was estimated at 28.5 million with IBTS information collected by ICES on herring fish, of which 44.6% were plaice Pleuronectes platessa, stock size. In a long-term project, two colonies in the 30.9% dab Limanda limanda, 21.7% flounder German Wadden Sea, Minsener Oldeoog and Banter See, Platichthys flesus and 2.8% sole Solea vulgaris. Flatfish Wilhelmshaven, have been studied since 1981, to look abundance was estimated from a combination of a for relationships between breeding performance dedicated 0-group flatfish survey and the Demersal parameters and fish availability. A preliminary analysis Young Fish Surveys. Cormorant predation was estimated has already been presented by Becker (1996b). to range from 30–50% of the total mortality of the 0- group fish of these species. Both the figures for 2.4.1 Correlations of herring population consumption and for fish abundance should be taken parameters with considerable caution, as the first are as yet uncorrected for lost otoliths (by digestion) and fish abundance may have been underestimated. Despite these Herring larval abundance for the whole North Sea and uncertainties, and also considering that absolute numbers for just the south-eastern North Sea are significantly and of flatfish were low in the years of study, these figures positively correlated, indicating that variation in the still suggest that cormorant predation was significant and abundance of herring larvae over the North Sea as a that these birds relied on juvenile flatfish to a large whole parallels that in the south-eastern North Sea alone. extent. This implies that the cormorants sample 0-group Larval abundance and the IBTS herring index are also fish with great efficiency and that at least relative correlated (1-ring, Table 2.1, ICES, 1997b, c). The 1995 differences between species of fish should be represented value, however, does not fit the regression line. The in the birds’ diet. IBTS herring estimate of the 1995 year class appears to be an outlier (ICES, 1997b). Clearly, studies that only lasted 1–2 years cannot be used to describe long-term changes in fish stocks. Acquiring longer time series of diet analyses seem promising,
8 ICES Coop. Res. Rep. No. 232 Table 2.1. Spearman correlation coefficients between various parameters estimating the clupeid stock in the North Sea (IBTS). n = 17 year classes (1979–1995).
IBTS herring index IBTS sprat index IBTS clupeids Herring larvae density North Sea North Sea IBTS sprat North Sea –.07 IBTS clupeids .76 *** .52 * Herring larvae density .43 –.44 .14 Herring larvae abundance .77 *** –.04 .62 ** .59*
2.4.2 Correlations between pre-recruiting class 1993, ICES 1997c). In the other years, herring clupeids and diet of common tern chicks dominated (92–99%; Behnke, 1996; Ludwigs, 1998) in the stow net catches, and in the samples of dropped clupeids recovered from the vicinity of nests. Clupeids are the most important common tern food in the Wadden Sea. On Minsener Oldeoog and Baltrum, they In the Wadden Sea colonies, the amount of clupeids in represent 29–70% of the chick diet (mean=49%, 9 years; chick diet was positively correlated with the herring Frank, 1992, 1998; Frick and Becker, 1995; Ludwigs, larvae density in the south eastern North Sea (c.f. Figure 1998). In the colony Banter See in Wilhelmshaven, 3– 2.1, Minsener Oldeoog, r =0.70, p<0.05, n=9, Table 2.2; 15% of the chicks' food (mean=10%, n=6; unpubl. data) s with IBTS herring index r =0.45, n=9, n.s.; but not with and 11–48% of the courtship food are clupeids s IBTS sprat index r =0.03, n=9, n.s.). In the Banter See (mean=24%, n=7 years; Wendeln, 1997 and unpubl.). s colony, the amount of clupeids in chick diet was The common terns feed on 0-group herring, 1-group positively correlated with herring larvae abundance herring, and on 1-group sprat which are about 6–13 cm (r =0.95, p<0.05, n=6, Table 2.3) and IBTS herring index long during spring in the Wadden Sea. s (rs=0.89, p<0.05, n=6). A high clupeid proportion in tern diet apparently indicates a good stock of pre-recruiting It is difficult to distinguish visually between herring and clupeids, especially herring. sprat in the bill of a tern. Herring was, however, the dominant species in the local waters: in stow net catches There was no significant correlation between herring during 8 years (1985–1996) on Minsener Oldeoog, sprat larvae density and amount of clupeids in tern diet in the dominated the clupeids (92%) in 1994 only, same year. corresponding to a very high IBTS sprat index (year
Figure 2.1. Data sampling of herring recruits and common tern reproduction. Key tern diet is herring spawned in autumn two calendar years (20 months) before the tern breeding season. These herring are sampled by the IBTS surveys during spring as larvae the year before the tern breeding season, and as 1-ringers in the same season as tern breeding.
ICES Coop. Res. Rep. No. 232 9 Table 2.2. Spearman correlation coefficients between clupeid stock data of the North Sea (IBTS) and seabird data (common tern, Minsener Oldeoog, 1981–1997)
Herring larvae density South East Herring larvae abundance North Sea (n=17) (n=17) % clupeids in chick diet (n=9) .70 * .32
growth rate of chicks (n=14) .74 ** .58 *
growth rate of fledged chicks (n=11) .77 ** .73 *
% chick losses by food shortage (n=17) –.58 * –.49 *
age of fledging (n=13) –.56 * –.46
chick fledged/pair .23 .17
no. breeding pairs –.32 –.47
Table 2.3. Spearman correlation coefficients between clupeid stock data of the North Sea (IBTS) and seabird data (common tern, common tern, Wilhelmshaven, 1991–1997). n = 7 except % clupeids in chick diet.
IBTS herring IBTS sprat index IBTS clupeids Herring larvae Herring larvae index density abundance North Sea North Sea North Sea South East North Sea % clupeids in chick diet (n=6) .89* .43 .43 .66 .95**32
growth rate of chicks .21 .89** .75 .79* .61
growth rate of fledged chicks .36 .68 .49 .94* .77 *
chick fledged/pair – .75 .61 .50 .32
2.4.3 Relationships between recruiting chicks and survivors grew better. (Chick growth rate clupeids and the reproduction of terns being interpreted as a surrogate for food provisioning rate) in years with high rate of herring larvae density (Figure 2.2). There are significant correlations between 2.4.3.1 Minsener Oldeoog herring larvae production two years before and common tern chick growth rate, fledging age and chick loss Between 1981 and 1997, tern breeding success fluctuated (Table 2.2, Figure 2.3). The linear modelling of chick greatly between 0 and 1.6 chicks per pair per year, owing growth rate vs herring larvae index for the south east to variation in the annual food availability as well as to North Sea was y=2.95x + 5.517. Consequently, an the influence of predators (Becker, 1998). increase of the larvae index by 0.1 would increase the chick growth rate by 0.3 g/d, and reduce the chick losses. Comparison of herring larvae density and common tern No significant correlations were found with sprat index. chick losses through starvation, over a 17 year period (1981–1997), showed that common terns lost fewer
10 ICES Coop. Res. Rep. No. 232 Figure 2.2. Time trends of herring larval density (south-east North Sea, x1000; year class=year–2; IBTS) and losses of common tern chicks (in %x10) on Minsener Oldeoog, Wadden Sea, from 1981–1997; Becker, unpublished data.
Figure 2.3. Correlation of herring larval density (south-east North Sea, x1000; year class=year–2; IBTS) with growth rate of common tern chicks that fledged on Minsener Oldeoog, Wadden Sea, from 1981–1997; Becker, unpublished data.
2.4.3.2 Banter See, Wilhelmshaven reproductive success increased positively with herring availability (but n.s., Table 2.3). Chick growth was The number of fledglings per pair per year varied especially good in 1994, the year with high sprat between 0.2 and 2.4 chicks (1991–1997; Becker, 1998). abundance (see also Minsener Oldeoog Figure 2.3). Thus The correlations of reproductive parameters with herring sprat abundance can confuse the relationship with stock density were similar to those at Minsener Oldeoog herring abundance, but in most years sprat abundance (Table 2.3, Figure 2.4). Chick loss through predation was was too low to cause this problem. not so important as on Minsener Oldeoog, and the
ICES Coop. Res. Rep. No. 232 11 Figure 2.4. Correlation of herring larval density (south-east North Sea, x1000; year class=year–2; IBTS) with growth rate of common tern chicks that fledged at Banter See, Wilhelmshaven, from 1991–1997; Becker, unpublished data.
The regression of chick growth rate on the herring larvae However, to conclude that tern breeding ecology can be index (Figure 2.4) was y=4.0x+6.845. Thus an increase used as a measure of herring juvenile abundance, some in the herring index value of 0.1 would improve chick important points have to be clarified: growth by 0.4g/d. Using the herring larvae density or abundance for the same year the terns bred, the • Decisive for the terns was the herring year class two correlations described above were not significant at calendar years before the respective breeding either colony. season (i.e. 1-ring in the breeding season, Figure 2.1), or the sprat year class one year before the tern 2.4.4 Conclusions breeding (age 1 in the breeding season). This may indicate that the 1-group herring is more important for tern reproduction than 0-group. This should be Although not related to herring larval production in the investigated further. same year (these larvae being too small to provide much food for terns), the data show that the reproduction of • The correlations of tern data with the herring larvae terns on the southern North Sea coast is strongly linked abundance estimates are much closer than those to the annual stock of juvenile herring, their main food with IBTS herring index (1-ring). This suggests that source. Consequently, terns can be used in addition to the the sampling of larvae gives a better annual figure of the herring population 1 year later than the fisheries' data to indicate abundance of the young herring sampling of 1-ringers in the current year. Fish stock. Conversely, fisheries' data on clupeids may be catches of 1-ringers may be taken more by chance used to predict growth and reproductive success of terns. than larvae sampling. A reduced common tern breeding population in the Wadden Sea in 1996 and 1997 (Südbeck et al., 1998) • The dependence of the reproduction of terns on pre- also may be due to the reduced occurrence of juvenile recruit clupeids should be verified at another colony clupeids. Despite the short foraging range of breeding site, for example on Griend in the Dutch Wadden Sea where terns are also studied (by Stienen and common terns (max ca 7 km), the correlation with Brenninkmeijer). herring abundance over the entire south-eastern North Sea is quite strong. This suggests that herring are fairly • To investigate interactions between seabird uniformly distributed over this region, or at least that reproduction and fish stocks, breeding seabird local abundance near these tern colonies is closely numbers or overall breeding success are often related to abundance at the wider scale. considered, but parameters more directly linked to
12 ICES Coop. Res. Rep. No. 232 food availability such as chick growth, rate of chick between the amount of herring fed to chicks and starvation or fledging success should be assessed as independent assessments of amounts of young herring in they may be expected to provide a more direct and the sea (Anker-Nilssen, 1992; Anker-Nilssen and Øyan, sensitive indicator of food supply (Table 2.2). The 1995; Barrett and Krasnov, 1996). data presented also underline the importance of long term data series as the key tool to understand 2.5.1 Røst interactions between seabirds and fish. There is clear evidence that seabirds breeding at Røst, 2.5 Norwegian spring-spawning herring Lofoten Islands breed successfully only when larval and and north Norwegian seabirds 0-group stages of herring are abundant. In years with low abundance of herring, puffins and common guillemots The Norwegian spring-spawning stock of the Atlanto- have produced few chicks of poor quality, Scandian herring has shown huge fluctuations in size during the last half century. Between 1957–1971, it or no chicks at all, and kittiwakes have had reduced collapsed from ca. 18 million tonnes to an estimated breeding success (Bakken, 1989; Anker-Nilssen, 1992; 12000 tonnes, remained very low (<1 million tonnes) Anker-Nilssen and Øyan, 1995; Anker-Nilssen et al., until 1985, and has since been increasing (ICES, 1997a). 1997).
Norwegian seabirds feed their chicks mainly on small There is, however, no clear causal relationship between fish, samples of which are easy to obtain. Several studies breeding success and abundance of herring larvae. have documented that several seabird species feed Between 1979–1994, herring content in puffin chick diet significant amounts of 0- and 1-group herring to their varied between 0% and 89% wet mass, with sandeels, chicks and studies along the coast of Norway have saithe and haddock making up most of the remainder. shown that the amounts of herring in the samples vary There is no simple relationship between the amount of considerably from year to year (Barrett et al., 1987; herring fed to the chicks and 0-group abundance. This is Barrett, 1996). This has been highlighted in two long- possibly due to a preference for other prey species such term studies on two colonies in North Norway, Hernyken as sandeels or saithe which tended also to be abundant in at Røst (67o26'N, 11o52'E) and Hornøya in East years with high herring abundance (e.g., 1983, 1992, Finnmark (70o22'N, 31o10'E). Close correlations exist 1994).
Figure 2.5. The relationship between the abundance indices of age-0 herring in the Barents Sea and adjacent waters in early autumn and the fledging success of puffins at Røst, northern Norway, in 1975–1996. Eight points are located close to the origin. From Anker- Nilssen et al. (1997).
ICES Coop. Res. Rep. No. 232 13 Although Anker-Nilssen et al. (1997) demonstrated a relationship between kittiwake breeding success and 0- strong positive relationship between breeding success group herring abundance at Røst was also found between and independently obtained indices of 0-group herring 1980–1996 (Figure 2.6, rs=0.815, p<0.001, Anker- abundance over a 22 year period (1975–1996, Figure 2.5, Nilssen et al., 1997), but again there is a threshold above rs=0.898, p<0.001), there was a clear threshold above which breeding success does not increase further. which fledging success was at a maximum and could not Because the species composition of the diet does not increase with increases in herring abundance. This relate directly to the availability of herring, and due to suggests that any changes above e.g., 1.0 in the the shape of the relationship curves between availability presently-used logarithmic index of herring 0-group of 0-group herring and breeding success, it is impossible abundance (ICES 1997a) will not be detectable in puffin to predict levels of herring 0-group fish at scales finer breeding success alone. A similar positive than high (log. index >1.0) or low (< 0.3) from breeding success data.
Figure 2.6. The relationship between the abundance indices of age-0 herring in the Barents Sea and adjacent waters in early autumn and the breeding success of kittiwakes at Røst, northern Norway, in 1980–1996. From Anker-Nilssen et al. 1997.
Puffin diet data from several northwestern Norwegian The herring, however, are consumed by seabirds as 1- colonies have, however, indicated the presence of some group fish, and while there were no relationships 0-group herring in the Barents Sea in years when surveys between the previous years’ 0-group herring abundance failed to document any (1981, 1982, 1986, 1987, indices and the amount of herring in the chick diet on the index=0, Barrett et al., 1987; Barrett, 1996; ICES, Kola Peninsula, there were clear positive correlations for 1997a). kittiwakes, common guillemots and puffins further west on Hornøya (Figure 2.7). There are also positive 2.5.2 Hornøya correlations between herring content in the diet of common guillemot and puffin chicks and independent assessments (ICES, 1997a) of 1-group herring in the Whereas seabirds breeding at Røst depend heavily on Barents Sea (r2=44%, df=9, p=0.027 and r2=76%, df=8, herring to feed their chicks, the main diet of seabird p<0.0001 respectively; Barrett, unpubl.). The correlation chicks in the southern Barents Sea consists of varying for kittiwakes was not significant (r2=32%, df=10, proportions of herring, sandeels and capelin Mallotus p=0.07). Food data collected nearly every year since villosus (Barrett and Krasnov, 1996). Sandeels and 1980 showed that herring first appeared in food samples capelin are caught mainly as adult fish (Barrett and in 1985, was absent in the late 1980s and appeared again Furness, 1990; Barrett and Krasnov, 1996). It has proved in 1990. In 1993 and 1994 herring constituted >90% of impossible to relate amounts of capelin caught by kittiwake diet samples and 30–50% of the puffin and puffins, kittiwakes or common guillemots with common guillemot diet samples. The appearance in 1985 independent measures of capelin abundance, probably corresponds with the only large cohorts of 0-group due to the differences in spatial scale at which the herring spawned in the 1980s (1983, 1984), whereas the parameters were measured (Barrett and Krasnov, 1996). presence in all diets in the early 1990s corresponds with There was, however, a suggestion that the kittiwakes several years of successful spawning (1989–1994, ICES found smaller capelin (mean 114±40 mm) in 1989 than 1997a). in all but one of the other years (130–140 mm, 1980– 1994), due to low recruitment of capelin after the collapse of the stock in 1987.
14 ICES Coop. Res. Rep. No. 232 Figure 2.7. The relationship between the percent of herring in common guillemot, kittiwake and puffin chick diets on Hornøya, north Norway (solid squares) and Kharlov, northwest Russia (open circles), and the log index of 0-group herring abundance in the previous year. Each graph has several points at the origin, not plotted. From Barrett and Krasnov (1996).
Contrary to the situation at Røst, it seems that increased measure of this such as chick growth rate) and the amounts of herring in chick diet corresponds to a decline abundance of pre-recruit fish. Fisheries-derived and in the breeding success of kittiwakes at Hornøya survey-derived estimates of recruitment apply to entire (r2=0.821, p>0.01, n=10, Anker-Nilssen et al., 1997). stocks or to very large geographical areas, so are on a There were, however, no significant relationships much larger spatial scale than the distribution of fish between kittiwake breeding success and indices for the providing food to seabird chicks at a particular colony. 1-group or the previous years’ 0-group herring Nevertheless, correlations between common tern abundance. At present, the only parameters for seabirds breeding parameters and herring abundance in the south- on Hornøya which corroborate the fisheries’ assessments eastern North Sea provide an example of a correlation of the 0- and 1-group cohorts of herring in the Barents where it seems that the local performance of terns does Sea are the amounts of herring in the diets of chicks of reflect the changes in herring abundance over a larger kittiwake, common guillemot and puffin. scale. This may not always be the case. Thus it would be essential to be very cautious if using seabird data to infer 2.6 Conclusions the level of recruitment into a fish prey population over a wide area. In addition to the relationships discussed here, good examples can be found in the literature, as for We conclude that there are some case studies of seabirds example Montevecchi and Myers (1995), Montevecchi that show fairly strong correlations between diet (1993). composition or food provisioning (or a surrogate
ICES Coop. Res. Rep. No. 232 15 2.7 References Becker, P. H. 1996a. Flußseeschwalben (Sterna hirundo) in Wilhelmshaven. Oldenburger Jahrbuch, 96: 263– 296. Anker-Nilssen, T. 1992. Food supply as a determinant of reproduction and population development in Norwegian puffins Fratercula arctica. Doctor of Becker, P. H. 1996b. Relationships between fish Science thesis in terrestrial ecology, University of populations and reproductive biology of common Trondheim. 46 pp. terns in the Wadden Sea. In: Seabird/fish interactions, with particular reference to seabirds in the North Sea, pp. 65–67. Ed. by G.L. Hunt and R.W. Furness. ICES Anker-Nilssen, T., Barrett, R. T., and Krasnov, J. 1997. Cooperative Research Report 216. Long- and short-term responses of seabirds in the Norwegian and Barents Seas to changes in stocks of prey fish. Proceedings of a symposium on forage Becker, P. H. 1998. Langzeittrends des Bruterfolgs der fishes in marine ecosystems. Alaska Sea Grant Flußseeschwalbe und seiner Einflußgrößen im College Program AK–SG–97–01: 683–698. Wattenmeer. Vogelwelt, 119: 223–234.
Anker-Nilssen, T., and Øyan, H. S. 1995. Becker, P. H., and Specht, R. 1991. Body mass Hekkebiologiske langtidsstudier av lunder på Røst. fluctuations and mortality in common tern Sterna NINA Fagrapport, 15. 48 pp. hirundo chicks dependent on weather and tide in the Wadden Sea. Ardea, 79: 45–56. Bailey, R. S., Furness, R. W., Gauld, J. A., and Kunzlik, P. A. 1991. Recent changes in the population of the Behnke, A. 1996. Vergleich verschiedener Fang- und sandeel (Ammodytes marinus Raitt) at Shetland in Analysemethoden zur Fluktuation von Kleinfisch- relation to estimates of seabird predation. ICES Beständen im Wattenmeer. Diplomarbeit University Marine Science Symposium, 193: 209–216. of Oldenburg. 104 pp.
Bakken, V. 1989. The population development of Frank, D. 1992. The influence of feeding conditions on common guillemot Uria aalge on Vedøy, Røst. food provisioning of chicks in common terns Sterna Fauna norvegica Series C, Cinclus, 12: 41–46. hirundo nesting in the German Wadden Sea. Ardea, 80: 45–55. Barrett, R. T. 1991. Shags (Phalacrocorax aristotelis L.) Frank, D. 1998. Bruterfolgsmonitoring an der as potential samplers of juvenile saithe (Pollachius Flußseeschwalbe als Instrument ökologischer virens L.) stocks in northern Norway. Sarsia, 76: Begleituntersuchungen zu einer Pipeline-Verlegung. 153–156. Vogelwelt 119: 235–241.
Barrett, R. T. 1996. Prey harvest, chick growth, and Frank, D., and Becker, P. H. 1992. Body mass and nest production of three seabird species on Bleiksøy, reliefs in common terns Sterna hirundo exposed to North Norway, during years of variable food different feeding conditions. Ardea, 80: 57–69. availability. In: Studies of high-latitude seabirds. 4. Trophic relationships and energetics of endotherms in Frick, S., and Becker, P. H. 1995. Unterschiedliche cold ocean systems, pp. 20–26. Ed. by W. A. Ernährungsstrategien von Fluß- und Montevecchi. Canadian Wildlife Service Occasional Küstenseeschwalbe (Sterna hirundo und S. Paper 91. paradisaea) im Wattenmeer. Journal für Ornithologie, 136: 47–63. Barrett, R. T., Anker-Nilssen, T., Rikardsen, F., Valde, K., Røv, N., and Wader, V. 1987. The food, growth Furness, R. W. 1987. The impact of fisheries on seabird and fledging success of Norwegian puffin chicks populations in the North Sea. In: The status of the Fratercula arctica in 1980–1983. Ornis North Sea environment; reasons for concern, Vol. 2, Scandinavica, 18: 73–83. pp. 179–192. Ed. by G. Peet. Werkgroep Noordzee, Amsterdam. Barrett, R. T., and Furness, R. W. 1990. The prey and diving depths of seabirds on Hornøy, North Norway Harris, M. P., and Wanless, S. 1993. The diet of shags after a decrease in the Barents Sea capelin stocks. (Phalacrocorax aristotelis) during the chick-rearing Ornis Scandinavica, 21: 179–186. period assessed by three methods. Bird Study, 40: 135–139. Barrett, R. T., and Krasnov, J. V. 1996. Recent responses to changes in stocks of prey species by seabirds ICES 1997a. Report of the northern pelagic and blue breeding in the southern Barents Sea. ICES Journal whiting fisheries working group. ICES CM of Marine Science, 53: 713–722. 1997/Assess:14. 188 pp.
16 ICES Coop. Res. Rep. No. 232 ICES 1997b. Report of the herring assessment working Montevecchi, W. A., and Myers, R. A. 1995. Prey group for the area south of 62 N. ICES CM harvests of seabirds reflect pelagic fish and squid 1997/ASSESS:8. 392 pp. abundance on multiple spatial and temporal scales. Marine Ecology Progress Series, 117: 1–9. ICES 1997c. Report of the International Bottom Trawl Survey in the North Sea, Skagerak and Kattegat in Rail, J. F., and Chapdelaine G. 1998. Foods of double- 1996: Quarter 1. ICES CM 1997/H:8. 52 pp. crested cormorants Phalacrocorax auritus in the gulf and estuary of the St Lawrence River, Québec Leopold, M. F., van Damme, C .J. G., and van der Veer, Canada. Canadian Journal of Zoology, 76: 635–643. H. W. 1998 Impact of cormorant predation on juvenile flatfish in the Dutch Wadden Sea. Stienen, E. W. M., and Brenninkmeijer, A. 1998. Netherlands Journal of Sea Research, 40: 93–108. Population trends in common terns Sterna hirundo along the Dutch coast. Vogelwelt, 119: 165–168. Ludwigs, J. D. 1998. Kleptoparasitismus bei der Flußseeschwalbe (Sterna hirundo) als Anzeiger für Südbeck, P., Hälterlein, B., Knief, W., and Köppen, K. Nahrungsmangel. Vogelwelt, 119: 193–203. 1998. Bestandsentwicklung von Fluß- und Küstenseeschwalbe an den deutschen Küsten. Martin, A. R. 1989. The diet of Atlantic puffin Vogelwelt, 119: 147–163. (Fratercula arctica) and northern gannet (Sula bassana) chicks at Shetland colony during a period of Tasker, M., and Furness, R. W. 1996. Estimation of food changing prey availability. Bird Study, 36:170–180. consumption by seabirds in the North Sea. In: Seabird/fish interactions, with particular reference to Mlody, B., and Becker, P. H. 1991. Körpermasse- seabirds in the North Sea, pp. 6–42. Ed. by G.L. Hunt Entwicklung und Mortalität von Küken der and R.W. Furness. ICES Coop. Res. Report 216. Flussseeschwalbe (Sterna hirundo L.) unter ungünstigen Umweltbedingungen. Vogelwarte, 36: Tollitt, D. J., Greenstreet, S. P. R., and Thompson, P. M. 110–131. 1997. Prey selection by harbour seals Phoca vitulina in relation to variations in prey abundance. Canadian Monaghan, P., Uttley, J. D., and Burns, M. D. 1992. Journal of Zoology, 75: 1508–1518. Effect of changes in food availability on reproductive effort in Arctic terns Sterna paradisaea. Ardea, 80: Uttley, J. D., 1992. Food supply and allocation of 71–81. parental effort in Arctic terns Sterna paradisaea. Ardea, 80: 83–91. Monaghan, P., Uttley, J. D., Burns, M. D., Thaine, C. and Blackwood, J. 1989. The relationship between Wendeln, H. 1997. Body mass of female common terns food supply, reproductive effort and breeding success (Sterna hirundo) during courtship: relationships to in Arctic terns Sterna paradisaea. Journal of Animal male quality, egg mass, diet, laying date and age. Ecology, 58: 261–274. Colonial Waterbirds, 20: 235–243.
Montevecchi, W. A. 1993. Birds as indicators of change in marine prey stocks. In: Birds as Monitors of Environmental Change, pp. 217–266. Ed. by R.W. Furness and J.J.D. Greenwood. Chapman and Hall, London.
ICES Coop. Res. Rep. No. 232 17 3 Variation in prey taken by seabirds
M. L. Tasker1, C. J. Camphuysen2, and P. Fossum3
1Joint Nature Conservation Committee, 7 Thistle Place, Aberdeen AB10 1UZ, U.K. 2Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands. 3Institute Marine Research, PO Box 1870 Nordnes, N–5024 Bergen, Norway.
3.1 Introduction place) covering 38 species of birds and 518 different prey types. For the present review, a list of 1680 references dealing with seabird diets was consulted. Seabird diet in the ICES area has been described by These references are not included in the present studies that have used a variety of techniques; principally document, but are available in digitised format for future these are: analysis of regurgitated samples from living consultation. We are aware that the database is still birds or from the contents of regurgitated pellets, incomplete and it will be enlarged in the future. observations of prey being carried to chicks and analysis of stomach contents of dead birds killed either deliberately or accidentally, for instance in an oil spill or 3.3 Variation in species and size of as by-catch (Duffy and Jackson, 1986). Each technique seabird prey used has some bias attached; these biases may be large and unquantifiable. Analyses of regurgitated samples or of otolith pellets are likely to miss small prey items with 3.3.1 General considerations few or no hard parts. Observations of prey brought to colonies may not represent adult diet, and may also be Several general points should be noted in relation to this biased by the difficulty of identifying prey from a review. First, prey abundance may be very different from distance. Killing live birds may be the least biased prey availability. While ‘prey populations’ may remain method, but there may in turn be problems in ensuring constant over time or may be equally abundant in that a representative sample of birds is taken. There may neighbouring areas, spatial differences or temporal also be substantial cultural difficulties in killing birds changes in prey availability can influence whether or not (e.g., Coleridge, 1854). Analysis of the stomach contents such prey is taken. Secondly, there has been a tendency of oil spill victims may be biased by behavioural changes to study those organisms that appear in seabird diets, of birds prior to death. It may therefore often be difficult rather than the full range of potential prey items. There is to distinguish real patterns in seabird diet from patterns rarely any insight as to why a potential prey item is not caused by the study method. taken. Food aversions, other than the complete unsuitability of prey (too large, out of reach), are There have been many studies of seabird diet; rather than normally ignored. A third point is that modern review these exhaustively for evidence of dietary technology has challenged a number of common variation at various spatial and temporal scales, these assumptions on foraging performance. Seabirds tagged studies have been gathered into a database which is with satellite and/or radio transmitters or other data described below. Examples to describe the various loggers can be followed and detailed activity and prey aspects of variation have then been drawn from this consumption on their feeding trips recorded (Wanless et database to form this section of the report. We also draw al., 1985; Wilson et al., 1986; Burger and Wilson, 1988; attention to Tasker and Furness (1996) who reviewed Wanless et al., 1992; Briggs et al., 1993; Weimerskirch dietary variation of seabirds in the North Sea. and Robertson, 1994; Falk and Møller, 1995; Georges et al., 1997). These studies, despite the possible negative effects of some devices on foraging performance, have 3.2 Database description demonstrated that the feeding range of some seabirds is considerably greater than previously assumed, and that A relational database was established by members of the the diving depth of birds previously assumed to be working group to facilitate this and future reviews of the surface feeders may be comparatively large. All of these diet of seabirds. For all seabirds within the ICES and studies indicate that assumptions on the size of foraging NAFO areas, dietary information was collected from niches are usually too limited. published (including ‘grey literature’) and unpublished sources, and coded in a standard format. The first version Some studies may assume that a change in prey of this database was launched (SEABDIET 1.0) at this consumption by a predator population from one study to meeting. Each reference is coded with the ICES or the next represents a change in the availability of the NAFO area in which the samples were collected, such original prey stock. However, optimal diet models that most frequent prey items can be searched from the predict that predators will select the most ‘profitable’ database using area codes. It contains 838 study reports prey in terms of yield per unit handling time of each food (diet studies of a given predator at a given time and type encountered and rank this relative to profitabilities
18 ICES Coop. Res. Rep. No. 232 of other types. The implication is that a forager should Newfoundland colonies in response to the sometimes always accept the most profitable food type and that it abundant supply of these fish in these areas should accept successively less profitable types only (Montevecchi and Porter, 1980; Martin, 1989). when encounter rates with higher ranking types fall below critical levels (Hughes, 1993). This would mean that the representation or the absence of a given prey in Table 3.1. Size range (fish length in mm) of fish prey in some the diet could have been caused by changes in the North Atlantic seabirds (SEABDIET 1.0 database). availability of another prey species perhaps as a consequence of changes in the local abundance (Tollitt et Species Min Max al., 1997). Optimal diet theory predicts that the diet of a Red-throated diver 42 – 200 species should expand and contract according to the Great northern diver 35 – 500 quality and availability of alternative foods. Fulmar 40 – 330 Gannet 70 – 550 3.3.1.1 Most frequently recorded food items Shag 80 – 160 Great skua 100 – 360 Most seabirds, even those with highly specialised Black-headed gull 35 – 210 foraging methods, appear to feed on a great variety of Common gull 60 – 210 prey types, though primarily on small pelagic fishes, Lesser black-backed gull 40 – 470 squids and crustaceans (Montevecchi 1993). However, relatively few prey items are taken as staple foods Herring gull 20 – 530 (represented in at least 50% of the diet samples in a Great black-backed gull 20 – 450 study), while many organisms are only rarely recorded in Kittiwake 50 – 360 dietary studies. A survey of the available literature on Arctic tern 30 – 160 seabird diets contained in SEABDIET 1.0 found that 13 Common tern 30 – 160 prey species or groups have each been recorded in at Guillemot 20 – 270 least five studies as ‘staple food’ in any species of North Brünnich's guillemot 126 – 184 Atlantic seabird, either in the form of discards from Razorbill 20 – 237 fisheries or as prey taken during more ‘natural’ feeding . Black guillemot 40 – 220 Puffin 10 – 170 Sandeels (in particular Ammodytes marinus), capelin Mallotus villosus, polar cod Boreogadus saida, clupeid fish (herring Clupea harengus and sprat Sprattus sprattus), a variety of small crustaceans (mainly These results show firstly that smaller scavenging Euphausiids and amphipods), and squid (usually seabirds under similar conditions select smaller food unidentified Cephalopods, Loligo spp and Gonatus spp), items than larger seabirds, and secondly that larger in decreasing order of importance, were the most seabirds may select considerably larger roundfish than frequently encountered staple foods (at least 10 studies). are generally available. It has been shown that the Staple foods (>50% of the diet by mass) or common prey tendency for smaller seabirds to take small prey is at (26–50%), at any year of study in a given area, are in this least partly motivated by the presence of other, more powerful scavengers or more dominant conspecifics. An study considered ‘preferred prey’, whereas infrequently increase in handling time would increase the risk that a taken prey items (2–25%) or rare prey are considered fish was lost through kleptoparasitism (Hudson, 1989; ‘alternative prey’. Hudson and Furness, 1989). Size selection under natural conditions in foraging seabirds is an aspect which 3.3.1.2 Prey size deserves more attention in future studies.
The size of fish prey of North Atlantic seabirds generally 3.3.2 Annual variation varies between 100 and 300 mm, although larger as well as smaller prey are also taken (Table 3.1). Not There are few long-term studies of seabird diets. surprisingly, larger seabirds tend to feed on larger prey Examples of dietary change between years are more than smaller species, as clearly demonstrated in the numerous and the following examples illustrate the scale studies of the use of discards by scavenging seabirds in of change which has been observed. the North Sea (Table 3.2) (Camphuysen et al., 1995) but also in other multi-species diet studies (e.g., Swennen Large changes in staple foods from one prey and Duiven, 1977; Knopf and Kennedy, 1981; Götmark, species/group to another are not uncommon in seabird 1984; Sanger and Ainley, 1988; Camphuysen, 1990, populations. Changes in diet composition may be sudden 1996). There are notable exceptions. Gannets Morus or more gradual. Sudden changes are usually more easily bassanus, the largest seabird breeding in the North linked with a drastic change in foraging conditions or Atlantic, are capable of taking larger prey than most prey abundance than are gradual changes. An excellent other seabirds (e.g., roundfish of 300–450 mm). They example of a combination of rather sudden and gradual can, however, take small sandeels and capelin; these shifts is provided by diet studies of great skuas have been recorded as staple food in Scottish and Catharacta skua in Shetland (Furness, 1997). In the late
ICES Coop. Res. Rep. No. 232 19 1970s, when the breeding population of great skuas on seabirds in a single area. Surface feeding birds, such as Foula reached its highest level ever, the diet of great arctic terns Sterna paradisaea and kittiwakes Rissa skuas was dominated by sandeel (Table 3.3). After 1979, tridactyla, experienced great difficulty in obtaining sandeels were suddenly considerably less important and sufficient prey, and breeding failures or abandoned over 50% of the samples studied comprised a mixture of breeding attempts were widespread in the archipelago whitefish, most probably obtained as discards. A shift (Monaghan et al., 1989; Hamer et al., 1991, 1993). Other from sandeel towards whitefish discards had been seabirds, such as the gannet, showed marked shifts in observed also in 1974 and 1975. Since 1983, seabird their diets in response to this crash towards a wider prey predation by great skuas has increased significantly. spectrum that included much herring and gadids (Martin, Interestingly, when sandeel gradually returned in the diet 1989). Species, such as the guillemot Uria aalge and of great skuas in the 1990s, and while discards remained shag Phalacrocorax aristotelis, which dive to pursue fish in the diet, the habit of bird predation persisted. underwater, were hardly affected by the collapse and continued feeding chicks with sandeels and to reproduce The collapse of sandeel stocks around Shetland in the with reasonably high fledging rates. 1980s provided a number of examples of annual variation in seabird diet, involving several species of
Table 3.2. Median length (cm) of roundfish and median width of flatfish (0.5 cm) consumed by scavenging seabirds (arranged by body mass) in relation to the size of roundfish and flatfish offered in sessions of experimental discarding in the North Sea (from Camphuysen et al., 1995).
Species body mass (g) roundfish flatfish consumed offered consumed offered Kittiwake 300–500 15 18 3.5 6.5 Common gull 300–500 14 15 Fulmar 700–900 16 18 3.5 6.5 Lesser black-backed gull 700–1000 18 18 4.5 6.5 Herring gull 800–1200 18 19 5.0 6.5 Great skua 1300–1800 25 19 Great black-backed gull 1100–2000 24 18 6.5 6.5 Gannet 2800–3200 25 19 6.5 6.5
Table 3.3. Representation of sandeel A. marinus, whitefish discards, birds and other prey (% frequency of occurrence in all samples studied per year) in the diet of great skuas on Foula (Shetland Islands), simplified after Furness (1997).
Year sandeel discards birds other Year sandeel discards birds other 1973 71 27 2 0 1985 1 72 20 7 1974 24 71 5 0 1986 0 81 14 5 1975 21 69 6 4 1987 9 77 10 4 1976 72 26 2 0 1988 1 72 24 3 1977 59 35 4 2 1989 2 67 29 2 1978 64 35 1 0 1990 1 38 38 23 1979415432 19921566257 1980177463 199331871110 1981187741 199419811334 1982138034 19957363324 1983 9 70 17 4 1996 55 73 42 23 1984 0 74 23 3
20 ICES Coop. Res. Rep. No. 232 In northern Norway, changes in the stocks of capelin in importance of sandeels, clupeids and gadids in guillemot the 1970s and 1980s were tracked by changes in diets in the post- (August–October) and pre-breeding breeding performance of seabirds in the area (Wright et seasons (March–April). Camphuysen (1996) summarised al., 1996). In 1986, when the capelin stock was at its published information on guillemot diets outside the lowest, several species of seabird produced very few breeding season in the North Sea and demonstrated young. Massive declines in both the number of breeding consistent features, such as a greater importance of guillemots at northern Norwegian colonies and of sandeels in late spring and early autumn, substantial use guillemots on their wintering grounds were recorded. of prey that were available for only a short time (small Since 1989, capelin stocks have increased and bird scad in the southern North Sea), and a greater importance numbers have started to recover. In this instance, there of clupeids and gadids in winter. Future versions of the was no alternative in the late 1989s to capelin in the diet, diet database, SEABDIET, will facilitate a more detailed so that switching was not possible. Since then, herring summary of seasonal changes in diets, for a considerably stocks have increased in the area, and this species has larger number of species. To achieve that, a substantial reappeared in bird diets. This may be a reversion to the amount of so far unpublished data will need to be situation in the 1930s and 1940s when Belopol'skii computer coded. (1957) recorded herring as important constituent of the summer diet of many seabirds breeding in the region. 3.3.4 Spatial variation
3.3.3 Seasonal variation Spatial variation in seabird diets is particularly interesting on the small scale, as it indicates that local Much of the dietary work carried out on seabirds has populations use different, but perhaps overlapping, food been from or around colonies in the breeding season. resources. A demonstration of regional variation in Even within that window of courtship, incubation, seabird diets was provided by Lilliendahl and raising chicks and fledging of young, rather radical shifts Solmundsson (1997), who described summer food in the diet (both of adults and in the prey delivered to the consumption of six seabirds in Iceland (Table 3.5). For chicks) have been demonstrated. For example, the razorbill Alca torda, guillemot and Brünnich’s guillemot, gradually altering energetic demands of the growing sandeels predominated in the diet (>50% in percent wet chick(s) has to be met with by the provisioning adults prey mass) to the south, west and east of Iceland, while (Harris and Wanless, 1986; Anker-Nilsson and Nygård, capelin was their main prey (generally >90% of wet prey 1989; Annett and Pierotti, 1989; Beers and Habraken, mass) in the north-west and north-east. Euphausiids were 1993; Hill and Hamer, 1994; Anker-Nilssen and Øyan, of significance mainly for Brünnich’s guillemots, 1995). So, even in the absence of obvious changes in particularly to the east and north-west. Capelin formed food resources, there may be differences in the nearly 100% of the prey of kittiwakes to the north, while exploitation of their prey by seabirds, which have to sandeels predominated in the south and mixtures meet constantly changing energy and nutrient (capelin/sandeel and capelin/sandeel/Euphausiids and requirements during breeding. In the post-breeding other prey) in respectively the west and east sectors. season, most seabirds become more mobile, because the Fulmars Fulmarus glacialis have a more mixed diet in all constraints imposed by central place foraging are no sectors, although the overall trend of capelin longer in effect. In winter, the energetic requirements consumption in the northern sectors and sandeels may be elevated due to harsh environmental conditions, representing a significant part of the diet in the south and such as severe storms or very cold weather. Even from a west can be seen also in this species. Much of this purely energetic point of view, seasonal changes in diet variation in seabird diet can be linked to oceanographic and food preferences are likely to occur. Since many fish differences between regions. are known to show rather different distribution and activity patterns in the course of a year (e.g., spawning, Because the birds studied by Lilliendahl and buried phases in sand, migration), dietary changes in Solmundsson (1997) were shot at sea (presumably at or seabirds will probably be even more obvious between the near feeding locations) rather than at colonies, these seasons. results show the use of a common resource by predators with different foraging capabilities and prey preferences. Elliot et al. (1990) demonstrated substantial changes in So, while capelin is virtually absent from the diet of the the diet of Brünnich’s guillemots Uria lomvia, such as three auk species east of Iceland, both fulmars and shifts from predominantly fish in birds in the autumn to kittiwakes still consumed considerable amounts of these crustaceans in birds wintering off Newfoundland and fish. As the latter are surface foragers and the former are Labrador (Table 3.4). Blake et al. (1985) produced deep diving seabirds (but capable of feeding near the similar information from various locations off the surface as well as over 100m deep), we might conclude Scottish east coast, showing shifts in the relative that these auks prefer sandeel over capelin as prey in these waters.
ICES Coop. Res. Rep. No. 232 21 Table 3.4. Seasonal changes in diets of Brünnich’s guillemots in Labrador and Newfoundland from birds shot at sea (% frequency in total number of stomachs examined per season per region), after Elliot et al. (1990).
Study area Prey (genus/group) Nov Dec Jan Feb Mar
E Newfoundland Decapoda 0 3 1 Euphausiacea 1 100 100 85 Gammarus 5 1 1 Hyperiidae 3 6 3 22 31 Thysanoessa 32 92 96 Boreogadus 5737349 Gadus 9 10 29 3 10 Mallotus 18 87 3 Mollusca 1 2 Nereis 1 Cephalopoda 40 25 Labrador Decapoda 10 3 Euphausiacea 5 14 Gammarus 11 Hyperiidae 13 7 Thysanoessa 6 Boreogadus 85 66 Gadus 26 3 Mallotus 65 29 Mollusca 3 Cephalopoda 13 7 NE Newfoundland Decapoda 2 Euphausiacea 4 Hyperiidae 27 Boreogadus 51 88 Gadus 18 Mallotus 54 Cephalopoda 74 4 S Newfoundland Decapoda 3 5 Euphausiacea 69 80 Gammarus 1 Hyperiidae 9 55 Parathemisto 82 Thysanoessa 1 Boreogadus 12 5 Gadus 9 5 Mallotus 43 10 Nereis 1 SE Newfoundland Decapoda 7 Euphausiacea 80 92 Gammarus 2 Hyperiidae 7 26 Boreogadus 16 33 Gadus 5 Mallotus 8 Mollusca 7 Cephalopoda 5
22 ICES Coop. Res. Rep. No. 232 Table 3.5. Summer prey (% wet mass, rounded figures to nearest 5%) of seabirds feeding in five sectors off Iceland, as an example of spatial variation in diets. Shown are prey fractions representing at least 5% of wet mass (after Lilliendahl and Solmundsson (1997).
Species Prey South West North-west North-east East Fulmar Capelin 5 25 15 45 Sandeel 60 40 5 10 Euphausiids 5 10 Other3555707545
Kittiwake Capelin 15 55 95 100 40 Sandeel 80 45 35 Euphausiids 5 10 Other 5 15
Guillemot Capelin 30 90 90 5 Sandeel 90 65 90 Euphausiids 5 5 5 Other 10 5
Brunnich’s guillemot Capelin 10 70 100 Sandeel 75 50 Euphausiids 5 25 50 Other 10 5
Razorbill Capelin 10 95 95 Sandeel 95 80 5 100 Euphausiids 5 5
Puffin Capelin 30 25 90 Sandeel 100 65 20 55 Euphausiids 5 10 5 40 Other 45 5 5
Camphuysen et al. (1995) experimentally discarded fish was true for the even more abundant fulmar, which from survey vessels in seven subregions in the North Sea obtained the greater part of discarded fish only in and Skagerrak in four seasons. There was considerable summer and spring (see also Camphuysen and Garthe, variation in the selection of discarded items by different 1997). In brief, these studies demonstrated a mixture of species of scavengers in different areas through the year spatial and seasonal trends in discard consumption by in relation to the type and size of discards. Spatial different seabirds, which was at least partly related to variation in consumption rates (% of the discarded changes in dietary preferences or changing feeding fraction of the fish caught actually taken by seabirds) opportunities in these birds. showed that competition for fishery waste is considerably more intense in some areas and less in 3.4 Evidence for selection related to prey others. This variation could not always be explained by the relative abundance of scavenging seabirds in relation body condition to the number of fishing vessels in those areas. Specific dietary preferences of species of birds meant that some The quality of prey varies both between species and species did not occur at fishing vessels in some seasons within species. because other food resources were exploited instead. For example, kittiwakes were abundant and widespread all year round, but were most common scavenging around fishing vessels only in winter and autumn. The reverse
ICES Coop. Res. Rep. No. 232 23 3.4.1 Differential prey selection between difference in size of fish fed to chicks of different ages 2 2 species (Table 3.6) (Chi for herring/sprat = 78.1, p<0.001; Chi for smelt = 93.3, p<0.001). In an analysis of dietary selection, Harris and Hislop (1978) described the biomass and quality of various prey It may also be that seabirds prey selectively on ripe, pre- species fed to young puffins Fratercula arctica at ten spawning fish rather than spent or non-spawning fish of colonies around Scotland during six years in the early the same size. Furness and Barrett (1985) found that 1970s. This dietary information was compared with the guillemots at a colony in northern Norway took "availability" of these prey as described in experimental predominantly gravid female capelin, containing 6.6% mid-water trawl catches made in areas off north and east lipid and 15.2% protein, which compared with spent fish Scotland. There are obvious methodological problems containing only 2.5% lipid and 14% protein. These involved that are acknowledged by the authors. In terms authors could not demonstrate whether guillemots prey of biomass, sandeels and sprats predominated in the diets selectively on the young ripe late-spawning capelin in of chicks at most colonies in most years. Rocklings the area or whether the behaviour of these capelin makes Ciliata mustela and whiting Merlangius merlangus them more available to the birds. Montevecchi and formed a more important part of the diet at western Myers (1996) also indicate some prey selection by rather than eastern Scottish colonies. In calorific terms, guillemots on Funk Island, Newfoundland. Almost all large sprat (>100mm long) had a considerably higher capelin delivered to chicks between 1977 and 1994 were energy density (10.9 kJ/g wet weight) than any other gravid female, providing higher energy densities than prey species and were twice the value of saithe and found with male or spent female capelin (Montevecchi whiting (5.1 kJ/g and 4.05 kJ/g respectively). Between and Piatt, 1984). In contrast, Montevecchi and Myers these limits, in decreasing order, came rockling, small (1996) found that gannets landed about equal proportions sprats (43–93mm long), sandeels and small larval forms. of male and female capelin. Guillemots hunt by pursuit The percentage fat increases significantly in both sprat diving underwater, so may have a greater opportunity to and sandeel with the length of fish (Love, 1970). The assess the state of individual fish than would the plunge- diet of these puffin chicks, when looked at in calorific diving gannet. In addition, greater selectivity is to be terms, accentuates the importance of sprat and devalues expected among single prey loading species (e.g., the importance of whiting. guillemots or terns) than among multiple prey loading species (e.g., gannets, puffins). There was a broad similarity between the composition of trawl catches and puffin chick diets, with some 3.5 Differences between adult and chick exceptions. Sandeels were one hundred times as diet numerous as sprats in the trawl catches, but only three times as common in puffin chick loads, suggesting that A simultaneous study of diet as assessed by three puffins differentially select sprat. Rockling were methods was carried out by Harris and Wanless (1993) uncommon in the trawl, but this may have been due to on shags on the Isle of May in the Firth of Forth during the young of this species living near the surface, above the chick-rearing period. Regurgitated samples were the level at which the trawl was fishing. Conversely, collected from chicks, stomach contents of adults were Norway pout Trisopterus esmarkii and long rough dab sampled by flushing with water and mucous pellets Hippoglossoides platessoides were common in the trawl, (which contain hard parts of prey) were retrieved from a but only recorded once each in puffin diets. roost site. The roost site samples (mostly non- and failed breeders) were from a wider spectrum of prey than the A later study, around the Isle of May off eastern chicks. Stomach contents of adults returning to feed Scotland, found that as the North Sea sprat stock chicks were very similar to those regurgitated by the declined, these were replaced in the diet by herring, chicks (i.e. almost entirely sandeels), however Wanless whose stock was increasing (Hislop and Harris, 1985). et al. (1993) concluded that adults ate a wide spectrum of fish from other fish families and probably digested these 3.4.2 Differential selection of prey within before returning to the colony. Fish in these families had species low calorific densities compared to sandeels indicating that adults transported highest calorific items for their chicks. Several authors have suggested that seabirds may differentially select individuals of the same species with higher calorific values. The most obvious selection Courtship feeding of females by their mates is a feature would be for different sized individuals of the same of the biology of several seabirds. We contrasted the sizes of fish fed to common tern females and to chicks species (e.g., Harris and Hislop, 1978). Wright and (Table 3.7). In general females were fed on a Bailey (1993) showed that diving birds tended to bring in significantly wider size range of fish than were chicks a higher proportion of older age-classes sandeels than (Chi2 for herring/sprat = 80.1, p<0.001; Chi2 for smelt would be expected if they were selecting fish randomly. Becker (unpubl.) examined the changes in size classes of Osmerus eperlanus = 70.2, p<0.001). fish brought to chicks by common terns at Wilhelmshaven. There was a clear and significant
24 ICES Coop. Res. Rep. No. 232 Table 3.6. Proportions (%) of size classes (multiples of bill length) of fish fed to different ages of common tern chicks at Wilhelmshaven in 1995 (P.H. Becker, unpubl.).
Smelt Sprat/herring Age 0–7 days 8–14 days >14 days 0–7 days 8–14 days >14 days Fish size class 1 19.7 2.7 2.9 37.7 15.3 3.6 2 62.5 48.1 36.5 56.9 60.2 60.8 >2 17.8 49.2 60.6 5.4 24.4 35.5 Sample size 152 187 170 130 98 166
Table 3.7. Proportions (%) of size classes (multiples of bill length) of fish fed to female and chick common terns at Wilhelmshaven in 1995 (P.H. Becker, unpubl.).
Fish size class Smelt Sprat/herring Females Chicks Females Chicks 1 19.6 6.9 20.7 17.6 2 32.6 43.1 31.1 56.9 3 34.6 41.3 32.4 23.1 >3 13.2 8.7 15.8 2.5 Sample size 613 813 241 615
3.6 Discussion have tried to address the aspect of prey selection from a known resource of potential prey. There are very obvious methodological problems involved with the assessment From the examples of annual, seasonal and spatial of food resources (a function of prey stock size, variation in seabird diets provided in this chapter it may suitability and availability) for piscivorous seabirds, but seem that we have a reasonable overview of its in the absence of any insight it remains speculative why variability in most common species of seabirds in the certain seabirds rely on sandeels in one year and perhaps ICES area. In fact, this is not the case. Of 767 studies in clupeids in the next. Size selection (e.g., Swennen and which the study season was specified, 64% were Duiven, 1977, 1991; Camphuysen et al., 1995), conducted during the breeding season or in summer. differential selection of prey of a certain ‘quality’ or Only 8% of all studies were conducted in the pre- calorific value (e.g., Harris and Hislop, 1978, Wright and breeding season, 12% during post-breeding (early Bailey, 1993) and prey choice or dietary shifts in relation autumn) and 15% in winter. Logistic problems have to the prey stock (e.g., Doornbos, 1979; Vader et al., prevented large scale studies of the diets of most pelagic 1990) are very important aspects which all deserve a lot seabirds outside the breeding season, simply because more attention in future studies. most birds are ‘out of reach’ (away from land). From the examples given earlier and from many published papers on variability in seabird diets, it should be emphasised 3.7 References that the results obtained in one area, in one season, in any one year are not necessarily valid with that same predator Anker-Nilssen, T., and Nygård, T. 1989. Notes on the species in other circumstances. However, on the larger food choice of adult and young guillemots Uria aalge scale it will soon be possible to come up with some during post-breeding migration in central Norway. generalisations. There is no need to become side-tracked Fauna Norvegica Series C, Cinclus, 10: 53–56. as a result of the immense variation in prey, since most items form only a very small part of the diet. Rather few Anker-Nilssen, T., and Øyan, H. S. 1995. species/types are ‘preferred’ prey for seabirds while very Hekkebiologiske langtidsstudier av lunder på Røst. many should be labelled ‘occasional prey’. It is very NINA Fagrapport, 15. 48 pp. important, however, that additional information is collected on seabird prey preferences, particularly Annett, C., and Pierotti, R. 1989. Chick hatching as a outside the breeding season and away from the colonies. trigger for dietary switching in the western gull. Colonial Waterbirds, 12: 4–11. A second point which should be highlighted, particularly from the fisheries point of view, is that very few studies
ICES Coop. Res. Rep. No. 232 25 Beers, P. W. M. van, and Habraken, J. M. P. M. 1993. Elliot, R. D., Ryan, P. C., and Lidtser, W. W. 1990. The De invloed van getij, wind en leeftijd van de kuikens winter diet of thick-billed murres in coastal op de voedselaanvoer van de grote stern Sterna Newfoundland waters. In: Auks at sea, pp. 125–138. sandvicensis. Doctoraalverslag KU Nijmegen / Ed. by S.G. Sealy, Studies in Avian Biology, 14. Instituut voor Bos- en Natuuronderzoek, Arnhem, 23 pp. Falk, K., and Møller, S. 1995. Satellite tracking of high- arctic northern fulmars. Polar Biology, 15: 495–502. Belopol'skii, L. O. 1957. Ecology of sea colony birds of the Barents Sea. Israel Program of Scientific Furness, R. W. 1997. The impact of predation by great Translation 1961. 346 pp. skuas on other seabird species, with particular reference to Special Protection Areas in Shetland. Blake, B. F., Dixon, T. J., Jones, P. H., and Tasker, M. L. Northern Isles Area Report for contract 5092AA, 1985. Seasonal changes in the feeding ecology of Scottish Natural Heritage, Lerwick. 99 pp. guillemots (Uria aalge) off north and east Scotland. Estuarine, Coastal and Shelf Science, 20: 559–568. Furness, R. W., and Barrett, R. T. 1985. The food requirements and ecological relationships of a seabird Briggs, D. R., Prince, P. A., and Croxall, J. P. 1993. community in North Norway. Ornis Scandinavica, Movements and interactions of wandering 16: 305–313. albatrosses: the roles of satellite tracking and direct observations. Sea Swallow, 42: 41–44. Georges, J.-Y., Guinet, C., Jouventin, P., and Weimerskirch, H. 1997. Satellite tracking of seabirds: Burger, A. E., and Wilson, R. P. 1988. Capillary-tube interpretation of activity pattern from the frequency depth gauges for diving animals: an assessment of of satellite locations. Ibis, 139: 403–405. their accuracy and applicability. Journal of Field Ornithology, 59: 345–354. Götmark, F. 1984. Food and foraging in five species of Larus gulls in the breeding season: a comparative Camphuysen, C. J. 1990. Fish stocks, fisheries and review. Ornis Fennica, 61: 9–18. seabirds in the North Sea. Technisch Rapport Vogelbescherming, 5. Vogelbescherming Nederland, Hamer, K. C., Furness, R. W., and Caldow, R. W. G. Zeist. 122 pp. 1991. The effects of changes in food availability on the breeding ecology of great skuas Catharacta skua Camphuysen, C. J. 1996. De verspreiding van zeevogels in Shetland. Journal of Zoology, London, 223: 175– in de Noordzee: naar een beter begrip van patronen 188. en verbanden. Sula, 10(special issue 2): 41–88. Hamer, K. C., Monaghan, P. Uttley, J. D., Walton, P., Camphuysen, C. J., Calvo, B., Durinck, J., Ensor, K., and Burns, M. D. 1993. The influence of food supply Follestad, A., Furness, R. W., Garthe, S., Leaper, G., on the breeding ecology of kittiwakes Rissa Skov, H., Tasker, M. L., and Winter, C. J. N. 1995. tridactyla in Shetland. Ibis, 135: 255–263. Consumption of discards by seabirds in the North Sea. Final report to the European Commission, study Harris, M. P., and Hislop, J. R. G. 1978. The food of contract BIOECO/93/10, NIOZ–Report 1995–5, young puffins Fratercula arctica. Journal of Netherlands Institute for Sea Research, Texel. 202 Zoology, London, 185: 213–236. pp. Harris, M. P., and Wanless, S. 1986. The food of young Camphuysen, C. J., and Garthe, S. 1997. An evaluation razorbills on the Isle of May and a comparison with of the distribution and scavenging habits of northern that of young guillemots and puffins. Ornis fulmars (Fulmarus glacialis) in the North Sea. ICES Scandinavica, 17: 41–46. Journal of Marine Science, 54: 654–683. Harris, M. P., and Wanless, S. 1993. The diet of shags Coleridge, S. T. 1854. The rime of the ancient mariner. Phalacrocorax aristotelis during the chick-rearing period assessed by three methods. Bird Study, 40: Doornbos G. 1979. Winter food habits of smew (Mergus 135–139. albellus) on lake IJssel, The Netherlands: species and size selection in relation to fish stocks. Ardea, 67: Hill, J. K., and Hamer, K. C. 1994. Do great skuas 42–48. Catharacta skua respond to changes in the nutritional needs of their chicks? Seabird, 16: 3–7. Duffy, D. C., and Jackson, S. 1986. Diet studies of seabirds: a review of methods. Colonial Waterbirds, Hislop, J. G., and Harris, M. P. 1985. Recent changes in 9: 1–17. the food of young puffins Fratercula arctica on the Isle of May in relation to fish stocks. Ibis, 127: 234– 239.
26 ICES Coop. Res. Rep. No. 232 Hudson, A. V. 1989. Interspecific and age-related Sanger, G. A., and Ainley, D. G. 1988. Review of the differences in the handling time of discarded fish by distribution and feeding ecology of seabirds in the scavenging seabirds. Seabird, 12: 40–44. oceanic subarctic North Pacific Ocean. Bulletin of Ocean Research Institute, University of Tokyo, 26: Hudson, A. V., and Furness, R. W. 1989. The behaviour 161–186. of seabirds foraging at fishing boats around Shetland. Ibis, 131: 225–237. Swennen, C., and Duiven, P. 1977. Size of food objects of three fish-eating seabird species: Uria aalge, Alca Hughes, R. N. 1993. Diet selection: an interdisciplinary torda, and Fratercula arctica (Aves, Alcidae). approach to foraging behaviour. Blackwell, London. Netherlands Journal of Sea Research, 11: 92–98. 232 pp. Swennen C., and Duiven P. 1991. Diving speed and Knopf, F. L., and Kennedy, J. L. 1981. Differential food-size selection in common guillemots Uria predation by two species of piscivorous birds. Wilson aalge. Netherlands Journal of Sea Research, 27: 191– Bulletin, 93: 554–556. 196.
Lilliendahl, K., and Solmundsson, J. 1997. An estimate Tasker, M. L., and Furness, R. W. 1996. Estimation of of summer food consumption of six seabird species in food consumption by seabirds in the North Sea. In: Iceland. ICES Journal of Marine Science, 54: 624– Seabird/fish interactions, with particular reference to 630. seabirds in the North Sea, pp 6–42. Ed. by G. L. Hunt, and R. W. Furness. ICES Cooperative Love, R. M. 1970. The chemical biology of fishes. Research report No. 216. Academic Press, London. 547 pp. Tollitt, D. J., Greenstreet, S. P. R., and Thompson, P. M. Martin, A. R. 1989. The diet of the Atlantic puffin 1997. Prey selection by harbour seals Phoca vitulina Fratercula arctica and northern gannet Sula bassana in relation to variations in prey abundance. Canadian chicks at a Shetland colony during a period of Journal of Zoology, 75: 1508–1518. changing prey availability. Bird Study, 36:170–180. Vader W., Barrett R. T., Erikstad K. E. and Strann K. B. Monaghan, P., Uttley, J. D., Burns, M. D., Thaine, C., 1990. Differential responses of common and thick- and Blackwood, J. 1989. The relationship between billed murres Uria spp. to a crash in the capelin stock food supply, reproductive effort and breeding success in the southern Barents Sea. In: Auks at sea, pp. 175– in arctic terns Sterna paradisaea. Journal of Animal 180. Ed. by S. G. Sealy. Studies in Avian Biology 14. Ecology, 58: 261–274. Wanless, S., Corfield, T., and Buckland, S. T. 1992. Montevecchi, W. A. 1993. Birds as indicators of change Diving behaviour of shags Phalacrocorax aristotelis in marine prey stocks. In: Birds as monitors of in relation to habitat and prey. In: European seabirds, environmental change, pp. 217–266. Ed. by R. W. Proceedings of the Seabird Group conference, Furness and J. J. D. Greenwood. Chapman and Hall, Glasgow 27–29 March 1992, p. 26. Ed. by M. L. London. Tasker, Seabird Group, Sandy.
Montevecchi, W. A., and Myers, R. A. 1996. Dietary Wanless, S., Harris, M. P., and Morris, J. A. 1985. changes of seabirds indicate shifts in pelagic food Radio-monitoring as a method for estimating time webs. Sarsia, 80: 313–322. budgets of guillemots Uria aalge. Bird Study, 32: 170–175. Montevecchi, W. A. and Piatt, J. 1984. Composition and energy contents of mature inshore spawning capelin Wanless, S., Harris, M. P., and Russell, A. F. 1993. (Mallotus villosus): implications for seabird Factors influencing food load sizes brought in by predators. Comparative Biochemistry and Physiology shags Phalocrocorax aristotelis during chick rearing. A, 78: 15–20. Ibis, 135: 19–24.
Montevecchi, W. A., and Porter, J. M. 1980. Parental Weimerskirch, H., and Robertson, G. 1994. Satellite investments by seabirds at the breeding area with tracking of light-mantled sooty albatross. Polar emphasis on northern gannets, Morus bassanus. In: Biology, 14: 123–126. behavior of marine animals vol. 4, pp 323–365. Ed. by J. Burger, B. L. Olla, and H. E. Winn. Plenum, Wilson, R. P., Grant, W. S., and Duffy, D. C. 1986. London. Recording devices on free-ranging marine animals: does measurement affect foraging performance? Ecology, 67: 1091–1093.
ICES Coop. Res. Rep. No. 232 27 Wright, P. J., and Bailey, M. C. 1993. Biology of Wright, P., Barrett, R. T., Greenstreet, S. P. R., Olsen, sandeels in the vicinity of seabird colonies at B., and Tasker, M. L. 1996. Effect of fisheries for Shetland. Fisheries Research Services Report 15/93. small fish on seabirds in the eastern Atlantic. In: Marine Laboratory, Aberdeen. 64 pp. Seabird/fish interactions, with particular reference to seabirds in the North Sea, pp 44–55. Ed. by G. L. Hunt, and R. W. Furness ICES Cooperative Research report No. 216.
28 ICES Coop. Res. Rep. No. 232 4 Evaluation of the role of discards in supporting bird populations and their effects on the species composition of seabirds in the North Sea
S. Garthe1, U. Walter2, M. L. Tasker3, P. H. Becker4, G. Chapdelaine5 and R. W. Furness6
1Institut für Meereskunde, Düsternbrooker Weg 20, 24105 Kiel, Germany. 2Forschungzentrum Terramre, Schleusenstrasse 1, 26382 Wilhelmshaven, Germany. 3Joint Nature Conservation Committee, 7 Thistle Place, Aberdeen AB10 1UZ, U.K. 4Institut für Vogelforschung, Vogelwarte Helgoland, An der Vogelwarte 21, D–26386 Wilhelmshaven, Germany. 5Canadian Wildlife Service, 1141 route de l’Eglise, PO Box 10100, 9th floor, Ste-Foy, Quebec G1V 4H5, Canada. 6Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, U.K.
4.1 Introduction Shrimping is carried out with beam trawls close to the coast and inside the Wadden Sea. Large numbers of undersized shrimps, other benthic invertebrates and fish In this report, we use the term discards to describe the species are incidentally caught owing to the poor animal waste generated by fishing operations which is selectivity of the fine meshed shrimp nets (minimum jettisoned at sea. This therefore includes undersized fish mesh opening 20 mm). and shellfish, fish which cannot be taken to market because quotas are exceeded or the catch is of low relative value to other hauls etc., offal and waste from In order to quantify total amounts discarded in three cleaning fish at sea and other biota such benthos. categories (undersized shrimps, other invertebrates and fish) the discard to commercial shrimp mass ratios in 103 The amounts of discards (including offal) from offshore unsorted catch samples (November 1992–November 1993) were analysed (Walter, 1997). These ratios, fisheries in the North Sea have been evaluated by several combined with the landings statistics of brown shrimps workers. Recent evaluations were summarised by ICES for the same month, was used as a basis to estimate the (1996). Garthe et al. (1996) compiled information from a total amount of discards from the shrimp fleet of variety of sources on the amounts discarded in six Niedersachsen in the main part of the fishing season of sections of the North Sea (Table 4.1). 1993.
4.1.1 The shrimp fishery off Niedersachsen, Shrimps of marketable size comprised 11 % of mass of Germany the catch, the remainder was mostly undersized shrimps (64 %), other invertebrates (8 %) and fish (11 %). The Shrimping is the most important fishing activity off most abundant fish were flatfish such as plaice Niedersachsen. The fleet consists of 118 cutters (Prawitt, Pleuronectes platessa, flounder Platichthys flesus and 1995), which fish between March and dab Limanda limanda, and roundfish such as clupeids November/December (Gubernator, 1994) for the brown and gadids. Among the invertebrates, shore crab shrimp Crangon crangon (5–8 cm body length). Carcinus maenus and swimming crab Liocarcinus holasadus were most frequent (Walter, 1997).
Table 4.1. Estimated quantities of discards and offal in six sub regions (see Figure 4.1) in North Sea offshore trawl fisheries in 1990 (in tonnes) (Garthe et al., 1996, see also ICES, 1996) and the SE North Sea shrimp fisheries.
Roundfish Flatfish Elasmobranchs Benthic Offal Total invertebrates NW 54,890 13,130 3,380 7,760 11,750 90,910 NE 53,310 14,290 3,270 8,270 11,450 90,590 CW 26,760 14,960 1,610 7,860 5,970 57,160 C 48,010 61,450 2,710 30,580 11,690 154,440 CE 48,520 68,230 2,710 33,820 11,990 165,270 S 30,710 127,240 1,320 61,410 9,950 230,630 SE shrimp fishery 10,800 8,000 0 137,800 0 156,600 Total 273,000 307,300 15,000 287,500 62,800 945,600
ICES Coop. Res. Rep. No. 232 29 Figure 4.1. Map of the six sub regions of the North Sea used by Garthe et al. (1996).
The monthly median of the ratio of undersized to There is no direct information on discards from shrimp marketable shrimps varied considerably (between 1:2– fisheries elsewhere in the North Sea. Shrimping is 1:10), with the lowest value in spring and the highest carried out off the coasts of France, Belgium, England, ratio in August (Figure 4.2). The majority of undersized and the three Wadden Sea countries (Netherlands, shrimp are discarded alive. The proportion of Germany and Denmark). The results of the study off invertebrates (other than brown shrimps) to marketable Niedersachen might be extrapolated to the remainder of shrimps varied between 0.05:1 and 1.1:1. The equivalent the brown shrimp fishery off the Wadden Sea coast of ratios for fish showed less variation (0.5:1–1.4:1) than the North Sea, in order to provide an approximate invertebrates (excluding undersized shrimps) (Walter, estimate of total bycatch. The total landings of brown 1997). shrimps in the coastal area of the Wadden Sea average to 20,000 tonnes per year (1983–1992) (Lozàn, 1994). If Total discards of approximately 4,000 tonnes of fish the mean discards/marketable shrimp ratios in the catch (1,750 t of flatfish and 2,290 t of roundfish), 27,000 samples off Niedersachsen (0.4 for flatfish, 0.54 for tonnes of undersized shrimps and a further 2,000 tonnes roundfish, 6.4 for undersized shrimps and 0.49 for other of other invertebrate species were calculated for the invertebrates) is applied to the rest of the fishery, then a shrimper fleet in the main fishing season (April– total of more than 150,000 tonnes of discards would be November) in 1993. produced by all shrimpers of the three countries (Table 4.1).
30 ICES Coop. Res. Rep. No. 232 It should be noted that Table 4.1 does not include In the coastal area off Niedersachsen, scavenging amounts discarded from a number of inshore fisheries seabirds follow shrimp trawlers in large numbers (e.g., shrimp fisheries off countries other than those of throughout the whole fishing season (Walter and Becker, the Wadden Sea), from static gear fisheries or from 1997). Up to 3,000 birds may be found astern of an industrial fisheries (likely to be relatively small individual shrimper (Berghahn and Rösner, 1992; Walter amounts). The total amount of fishery waste discarded in and Becker, 1994, 1997). the North Sea probably exceeds 1,000,000 tonnes. The main scavenging species are herring gull Larus 4.2 Consumption of discards by seabirds argentatus and black-headed gull L. ridibundus, which together represent 93% of all recorded birds (Walter and 4.2.1 Offshore fisheries in the North Sea Becker, 1997). Both species showed the same seasonal pattern, with low numbers until June and larger numbers The proportion of discards consumed by seabirds in the in late summer and autumn. Common gulls L. canus, North Sea was studied experimentally by Camphuysen et lesser L. fuscus and great black-backed gulls L. marinus al. (1995) (summarised in Table 4.2). These proportions and common terns Sterna hirundo and arctic terns S. are broken down by species to quantify tonnages of five paradisaea were less numerous than herring and black- categories of discard (offal, roundfish, flatfish, headed gulls. Common gulls occurred throughout the elasmobranchs and benthic invertebrates) consumed by whole fishing season, but only in substantial numbers the most important scavenging seabird species in the behind shrimpers in March and in autumn. Lesser black- North Sea (Table 4.3 based on Camphuysen et al., 1995; backed gulls and common/arctic terns were summer Garthe et al., 1996), based on the numbers of discard visitors and occurred in relatively low numbers between items consumed by birds. Calculations using discard April and September. Great black-backed gulls were mass as the basis would certainly lead to somewhat scarce before July, increasing slightly in numbers in late different results since, for instance, kittiwakes Rissa summer and autumn. tridactyla take the smallest roundfish and gannets Morus bassanus the largest roundfish (Camphuysen et al., 1995). Inshore shrimp fisheries off Niedersachsen (Lower Saxony)
Table 4.2. Proportion of experimental discards and offal consumed by birds (in %) in six offshore regions (all seasons) and four seasons (all sub regions), respectively, in the North Sea offshore trawl fisheries (from Garthe et al., 1996), and in the shrimp fishery of Niedersachsen (Walter, 1997).
Roundfish Flatfish Elasmobranchs Benthic invertebrates Offal Sample size NW 90 28 12 9 99 9,132 NE 89 41 12 3 98 3,281 CW 84 32 12 1 92 5,316 C 75 14 12 1 90 8,519 CE 63 10 12 3 54 3,396 S 71 8 12 4 100 1,200 Winter 92 35 12 17 100 6,028 Spring 76 22 12 8 94 10,354 Summer 70 10 12 3 94 8,526 Autumn 82 20 12 3 97 5,936
sample size 21,848 2,345 34 902 5,715 30,844
Shrimp fishery 79 41 23 (excl. shrimp) 4291
ICES Coop. Res. Rep. No. 232 31 30 A: undersized shrimps
20
10
0
12 B: other invertebrates 8
4
0 5 C: fish 4
3
2 Mass ratio of discard category: commercial shrimps commercial category: discard of ratio Mass 1
0 Month AprMay Jun Jul Aug Sep Oct Nov Nsamples 9 13 15 15 15 15 12 9
Figure 4.2. Average seasonal discards/commercial shrimp ratio of three main discard components, undersized shrimps, other invertebrates and fish (April–November 1993, total number of catch samples = 103) (from Walter, 1997).
Table 4.3. Tonnes of discards consumed by seabird species from the North Sea offshore fisheries as a whole (based on Garthe et al., 1996; Camphuysen et al., 1995; Walter and Becker, 1997).
Offal Roundfish Flatfish Elasmobranchs Benthic Total invertebrates Fulmar 39,800 53,400 4,500 200 6,300 104,200 Gannet 300 35,900 15,300 200 0 51,700 Great skua 100 2,000 0 0 0 2,100 Black-headed gull 0 100 0 0 0 100 Common gull 100 800 100 0 0 900 Lesser black-backed gull 1,300 14,500 6,200 1,100 500 23,300 Herring gull 2,600 21,100 5,100 0 500 29,300 Great black-backed gull 300 12,600 4,800 200 600 18,500 Kittiwake 10,500 66,000 2,200 400 1,100 80,200
Total 55,000 206,000 38,000 2,100 9,000 310,000
32 ICES Coop. Res. Rep. No. 232 Feeding rates by number of items consumed were seabirds – an assumption supported only by some discard determined following the method of Hudson and Furness experiments (Garthe et al., 1996)). Discarding is not (1988). Differences between the length distribution of uniform, thus different numbers of varying species might commercial and experimental discards were be supported in separate parts of the North Sea. Garthe et compensated for (Walter and Becker, 1997). In total, al. (1996) divided the offshore areas into six sub regions 5,500 tonnes of discards from the shrimper fleet of (Figure 4.1). Niedersachsen were consumed by the birds in 1993. This comprised 41% of the discarded flatfish mass (=710 The largest number of seabirds that could potentially be tonnes), 79% of roundfish (=1,820 tonnes), 23% of four supported by fishery waste is in sub region S invertebrate species (Carcinus maenas, Liocarcinus (1,500,000), followed by CE (1,200,000) and C holasadus, Asterias rubens, Allotheutis subulata; 420 t) (1,100,000). Lower numbers might be supported by and 10% of the undersized shrimps (2,500 t). fisheries in sub regions NW and NE (800,000 individuals in each of the two regions) and CW (500,000) (Garthe et 4.3 Diets of seabirds that scavenge al., 1996). discards in the North Sea Additionally, the shrimp fishery in inshore waters off Niedersachsen supports a large number of seabirds. The Discards form only a proportion of the diet of seabirds in consumed part of the shrimper discards represents an the North Sea. Full quantification of seabird diet has not 13 been carried out, but it is known that this proportion energy value of 2.5 x 10 J per year (Walter and Becker, varies by species, by location and by season. Based on a 1997). The mean daily energy demand of a 'model' compilation of many studies, Tasker and Furness (1996) seabird (species energy demand may be weighed against make some assumptions on diets for an input to a model their relative frequency astern the shrimpers) amounts to of North Sea fish consumption by seabirds. Their results 1,145 kJ or to 418,000 kJ/year. A total of 60,000 birds for the main scavenging species are summarised in Table may potentially have been supported by the discards of 4.4. the fleet off Niedersachsen in 1993.
In the south-eastern North Sea shrimp fishery, 4.4 Numbers of seabirds supported by consumption rates by mass were applied to the estimated discards in the North Sea discard quantities. A total consumption of 27,000 tonnes of all discard categories were calculated. The most In order to assess how many seabirds can be sustained important scavenger species were herring gulls which took 55% of all consumed discards, and black-headed from discards and offal, Garthe et al. (1996) derived an gulls (39%). Using standardised energy content of the "average scavenger community" from seabird counts different discard categories (Walter and Becker, 1997) (Camphuysen et al., 1995). This is based on the typical the total amount consumed by seabirds represents an composition of those eight common seabird species 14 known to consume fishery waste regularly and is energy value of 1.22 x 10 J. This amount of energy is sufficient to support a potential number of about 340,000 calculated in proportion to the numerical and seasonal birds (Table 4.5). abundance of the species in the North Sea There are considerable variations in the distribution of the scavengers in the North Sea, with respect to both area 4.5 Direct effects of discard and season (Camphuysen et al., 1995; Stone et al., consumption on species composition 1995). Fulmars Fulmarus glacialis are most numerous in the north (particularly around Shetland), with much of seabirds in the North Sea lower numbers in the south and east. Highest numbers are present in late summer/early autumn. Gannets leave 4.5.1 Increase in population size of seabird the North Sea in autumn and winter as do lesser black- species backed gulls. Herring gulls and great black-backed gulls, in contrast, move into the North Sea during the winter. About 30% of total food consumed by seabirds in the Kittiwakes are also highly numerous, but stay in the North Sea is estimated to be discards (including offal) North Sea in considerable number the entire year. (Tasker and Furness, 1996). These foods are therefore of Common gulls are present only in winter in the south and direct importance in sustaining populations of some the eastern parts, black-headed gulls are scarce in seabirds. Furness and Hislop (1981) showed that discards offshore areas at all times, in contrast to inshore areas of formed up to 70 % of the diet of adult great skuas the south-eastern North Sea where they are common breeding in Shetland and 28 % of chick diet even when (Stone et al., 1995; Berghahn and Rösner, 1992; Walter their preferred prey, lesser sandeels Ammodytes marinus, and Becker, 1997). were abundant. When sandeel abundance declined in the late 1980s, discards formed up to 82 % of adult diet and About 5.9 million individuals in the North Sea 77 % of chick diet (Hamer et al., 1991) (Table 4.6). Breeding success was much reduced in the absence of scavenging seabird community could possibly be sandeels (Furness, 1987) and chick growth rate is sustained by offshore fisheries (this figure assumes that considerably reduced when the proportion of discards in all offal and discarded organisms are consumed by the diet is high (Table 4.7).
ICES Coop. Res. Rep. No. 232 33 Table 4.4. Foods consumed by seabirds which scavenge discards in the North Sea (after Tasker and Furness, 1996 and Walter and Becker, 1997).
Species Discards and offal Other food Fulmar (summer) 30% offal, 30% discards 10% zooplankton, 30% sandeels (winter) 50% offal, 25% discards 25% zooplankton Gannet 10% discards 30% sandeels, 30% herring, 30% mackerel
Great skua1 62% discards 26% sandeel, 10% birds, 2% other
Black-headed gull2 10% discards 50% other, 40% terrestrial food
Common gull2 10% discards 50% other, 40% terrestrial food
Lesser black-backed gull2 60% discards 40% other Herring gull 10% offal, 30% discards 30% invertebrates, 30% terrestrial foods Great black-backed gull 60% discards 20% sandeels, 20% other prey Kittiwake IVa W (summer) 100% sandeels (winter) 25% offal, 25% discards 25% zooplankton, 25% sprat, IVa E, IVb, IVc (summer) 20% zooplankton, 60% sandeels, 20% sprat (winter) 25% offal, 25% discards 25% zooplankton, 25% sprat Notes: 1. A 16 year average from non-breeding birds, based on studies on breeding grounds (Hamer et al., 1991). 2. Estimates from Arbouw and Swennen (1985), Dernedde (1993), Freyer (1995), Gorke (1990), Hartwig et al. (1990), Noordhuis and Spaans (1992), Spaans et al. (1994).
Table 4.5. Total numbers of seabird that could theoretically be supported by discards and offal in the North Sea (offshore fisheries: from Garthe et al., 1996; shrimp fisheries: Walther and Becker, 1997).
offshore fisheries shrimp fisheries Fulmar 3,200,000 0 Gannet 210,000 0 Great skua 21,000 0 Black-headed gull 0 204,000 Common gull 84,000 7,000 Lesser black-backed gull 130,000 4,000 Herring gull 670,000 115,000 Great black-backed gull 250,000 0 Kittiwake 1,300,000 0 Common/arctic tern 0 9,000 Total 5,900,000 339,000
However, with the exception of these cases, there is Sea (e.g., Hudson, 1986; Camphuysen, 1993; Walter and limited evidence that fishery waste forms the essential Becker, 1997). Since fisheries are carried out throughout part of the diet of any other population of seabirds. the study area and throughout the year, interrupted only Nevertheless, the availability of discards is believed to locally during gales and storms, one is rarely able to affect feeding strategies of the scavengers. For instance, demonstrate any effects of fishing activities on feeding Blaber et al. (1995) suspect that the greater availability ecology and reproductive output of discard consumers. of discards of similar taxa may have led to greater This might be the reason for the weak link between overlap in the diets of the seabird species of the Northern studies showing the utilisation of discards at sea and Great Barrier Reef, Australia. Blaber et al. (1995) also studies focusing on possible effects of fishing found that the diet of several species changed due to the activities.The distribution of scavenging birds, both on supply by discards, which has occurred also in the North land and at sea, is affected by the availability of discards.
34 ICES Coop. Res. Rep. No. 232 Fishing activity strongly enhanced the number of 1995), and herring and great black-backed gulls on Audouin's gulls Larus audouinii resting on the Helgoland, south-eastern North Sea (Geiss, 1994; Columbrete Islands off east Spain (Castilla and Pérez, Hüppop, 1995).
Table 4.6. Food items in pellets produced by non-breeding great skua on Foula between 1 and 15 July, for the years from 1973 to 1989 except 1985 (from Hamer et al., 1991).
Year n sandeel (%) Whitefish (%) bird (%) other (%) (mostly discard) 1973 100 71 27 2 0 1974 100 24 71 5 0 1975 100 21 69 6 4 1976 100 72 26 2 0 1977 100 59 35 4 2 1978 100 64 35 1 0 1979 100 41 54 3 2 1980 100 17 74 6 3 1981 100 18 77 4 1 1982 100 13 80 3 4 1983 305 9 70 17 4 1984 100 0 74 23 3 1986 200 0 82 14 5 1987 98 9 77 10 4 1988 200 0 73 24 4 1989 247 4 62 30 4
Table 4.7. The relationship between an index of growth for skua chicks and the proportion of discards in their diet (data from Hamer et al., 1991).
Year % Discard Chick growth Year % Discard Chick growth index index 1975 28 30 1983 2 3 1976 14 –18 1984 33 4 1977 14 0 1985 33 7 1978 24 28 1986 30 5 1979 24 26 1987 42 –44 1980 28 8 1988 77 –129 1981 6 –40 1989 76 –62 1982 5 15
The food provided by discards may be of importance Discards may lower the costs of reproduction for adults, particularly during periods of low natural food such that survival and the future reproductive potential availability. There may therefore be positive effects on might increase. body condition, survival (including overwinter survival) of adult and sub-adult birds as well as on reproductive Examples from the Mediterranean have documented parameters such as the onset of laying, egg size, clutch various effects of the availability of discards and offal on size, chick growth, chick survival and breeding success. breeding phenology, reproductive output, foraging range,
ICES Coop. Res. Rep. No. 232 35 diet, activity and behavioural interactions of Audouin's, Hälterlein and Südbeck, 1996). Herring gulls in the yellow-legged Larus cachinnans and lesser black-backed Netherlands increased from around 20,000 pairs in 1940 gulls breeding on the Ebro Delta, north-east Spain (e.g., to 90,000 pairs in 1992 (Noordhuis and Spaans, 1992; Arcos and Oro, 1996; Oro, 1995, 1996; Oro and Dijk and Meininger, 1995). Lesser black-backed gulls Martinez-Vilalta, 1994; Oro et al., 1995, 1996; Ruiz et increased in the Netherlands from first breeding in the al., 1996). Wadden Sea in 1926 to 34,200 pairs in 1992 (Dijk and Meininger, 1995) with an additional 12,000 pairs in During the late 1980s, many seabirds in Shetland failed Germany in 1995 (Hälterlein and Südbeck, 1996). Black- to breed successfully due to low availability of sandeels. headed gulls started to use the German Wadden Sea as Only one kittiwake colony (Eshaness) fledged chicks breeding area during the 1940s. Today this gull is the successfully. This colony was mainly feeding on discards most numerous seabird in the Wadden Sea (64,000 pairs (Hamer et al., 1993). Removal of fishing offal as a food in Germany in 1995; Hälterlein and Südbeck, 1996; source has been shown to be associated with lagged 170,000 pairs in the Netherlands in 1992, including population declines in herring and great black-backed inland colonies; Dijk and Meininger, 1995). gulls in the Gulf of St. Lawrence, Canada (Howes and Fisher (1953) and Tuck (1961) considered that the Montevecchi, 1992). discards of factory trawlers on the Grand Bank of Newfoundland were responsible for the increase in 4.5.2 Population increase and changes in fulmars and kittiwakes in the British Isles prior to the composition of seabird communities 1950s.
There have been considerable changes in the breeding Herring gulls and black-headed gulls are the main avian populations of seabird species in the North Sea during consumers of the discards of the shrimp fishery. In the past century. There have further been changes in Denmark, herring gull numbers increased five years after species composition. While many species which the development of the Danish fisheries (Møller, 1981). consume discards have increased their populations, it is From 1973 to 1982 both the landings of the German difficult to discriminate between the effects of discards shrimp fishery and the discards produced by the and other factors such as enhanced bird protection and shrimpers increased in parallel with the gull populations increased stocks of small fish. The populations of some (Figure 4.3). Thereafter the gull populations continued to species groups, such as the terns, which had been the grow despite lower shrimp landings; possibly the amount most numerous species on the southern North Sea coasts of fishing continued to increase, but the catch of in the beginning of the century, have decreased in size marketable shrimps per unit effort decreased with a (e.g., Becker and Erdelen, 1987), which may be an consequential increase in amounts of discards. indirect effect of the increase in gull numbers. The increase in populations of discard-feeding seabirds The numbers of most scavenging seabird species around the North Sea has changed the balance of seabird breeding in eastern Britain have increased markedly communities towards these species. In the German since at least 1900 (Table 4.8). In the southern North Wadden Sea in 1951, the gulls (herring, lesser black- Sea, breeding numbers of offshore feeding seabirds such backed, common and black-headed) comprised 40% of as kittiwakes and fulmars have shown strong population the seabird community (44,300 pairs) and terns increases (e.g., kittiwakes: from a few pairs in the early (common, arctic, Sandwich Sterna sandvicensis and little 1950s to 7,460 pairs in 1995; Hüppop, 1995). Herring S. albifrons) the remaining 60% (Becker and Erdelen, gull numbers increased in Germany from about 7,000 1987). By 1995, gulls dominated the seabirds breeding pairs in 1910 to 45,600 pairs in 1995 (Vauk et al., 1989; community with 83 % of the total (155,000 pairs) (Hälterlein and Südbeck, 1996).
36 ICES Coop. Res. Rep. No. 232 Table 4.8. Numbers of pairs of scavenging seabirds breeding on North Sea coasts (Furness, 1992). a) Northeast Britain (Shetland, Orkney, Caithness to Cruden Bay)
Year Fulmar Gannet Great skua Lesser Herring Great Kittiwake All species black- gull black- backed gull backed gull 1900 600 3500 41 (3000) (2000) (1000) (26000) (37000) 1910 1760 3500 82 (2000) (3000) (1500) (34000) (46000) 1920 5200 3500 193 (1500) (4000) (2000) (48000) (64000) 1930 11600 3500 429 (1500) (5000) (3000) (68000) (93000) 1940 28200 8000 745 (1500) (10000) (4000) (90000) (142000) 1950 53000 8800 1350 (1500) (20000) (6000) (120000) (211000) 1960 66000 10000 2100 (1500) 40000 8000 160000 290000 1970 190000 14000 4000 1500 82000 9600 230000 531000 1980 280000 20000 6300 2500 43000 9900 210000 572000 1990 (350000 24000 7500 2500 40000 9900 180000 614000)
b) East coast of Britain from Cruden Bay to the Humber
Year Fulmar Gannet Lesser Herring Great Kittiwake All species black- gull black- backed gull backed gull 1900 0 2800 (2000?) (400?) (10?) (9000) (14000?) 1910 0 3000 – (800) – (12000) (18000?) 1920 20 3500 – (1500) – (19000) (26000?) 1930 200 4100 – (3000) – (28000) (37000?) 1940 600 4400 – 6000 – 38000 (50000) 1950 1200 4800 4000 12000 30 50000 72000 1960 2000 6800 4000 23000 30 65000 101000 1970 5800 8100 4240 45100 31 106000 169000 1980 10000 20000 5000 40000 20 200000 275000 1990 (14000 24000 5300 35000 20 210000 288000)
ICES Coop. Res. Rep. No. 232 37 Figure 4.3. Development of the gull populations in 28 areas along the German North Sea coast (Becker and Erdelen, 1987, P.H. Becker, unpubl. data) and landings of edible shrimps of the fleets of Niedersachsen and Schleswig-Holstein (Tiews, 1983; Tiews and Wienbeck, 1990; Anon., 1990/94) between 1968–1992 (from Walter and Becker, 1996).
4.6 Indirect effects of discard small seabird species caused by gulls feeding on eggs or consumption on species composition chicks (e.g., Kruuk, 1964; Hatch, 1970; Montevecchi, 1977; Wanless, 1988; Hario, 1994; Thiel and Sommer, of seabirds in the North Sea 1994; Russell and Montevecchi, 1996; Regehr and Montevecchi, 1996). The increase in population size of gulls supported by discards may have negative effects of other species of In Shetland and Orkney, great skuas Catharacta skua sea- and shorebirds. This may happen through various rely on discards and sandeels for most of their diet, but mechanisms. Nesting gulls may physically displace other will switch to killing other seabirds if sandeels and species by occupying their habitat. Larger predatory discards are in short supply, threatening the viability of species may depredate smaller species taking eggs, some seabird populations (Furness, 1997; Heubeck et al., young and adults (Regehr and Montevecchi, 1996). 1997).
In the Wadden Sea the nesting habitat of shorebirds such 4.7 References as plovers and oystercatchers has been invaded by large gulls. Some breeding sites which are well suited for Anonymous 1990/94. Die kleine Hochsee- und nesting by habitat or food availability may no longer be Küstenfischerei Niedersachens und Bremens im Jahr available for the terns because of the occupation by gulls 1989/1990/1991/1992/1993/1994 - Jahresbericht des earlier in the season. The Wadden Sea islands of Staatlichen Fischereiamtes Bremerhaven. Fi. Bl. div. Memmert and Mellum were important breeding sites for Jahrgänge. terns at the start of this century – nowadays more than 10,000 pairs of herring gulls and no terns breed on these Arbouw, G. J., and Swennen, C. 1985. Het voedsel van islands (Becker and Erdelen, 1987). Howes and de stormmeeuw Larus canus op Texel. Limosa, 58: Montevecchi (1992) describe a similar situation off 7–15. Canada. Arcos, J. M., and Oro, D. 1996. Changes in foraging Frequently, terns can only breed close to gulls, thus range of Audouin's gulls Larus audouinii in relation increasing the probability of predation. Common terns to a trawler moratorium in the western became re-established on Mellum at the end of the Mediterranean. Colonial Waterbirds, 19: 128–131. 1970s. This was not successful as herring gulls depredated most tern chicks which led to very low Becker, P. H. 1995. Effects of coloniality on gull reproductive output for five years. Subsequently the predation on common tern (Sterna hirundo) chicks. colony site was abandoned (Becker, 1995). There are Colonial Waterbirds, 18: 11–22. many other examples of reduced reproductive output of
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Møller, A. P. 1981. Problems with Danish herring gulls Spaans, A. L., Bukaciska, M., Bukaciski, D., and van Larus argentatus. Proceedings of the Second Nordic Swelm, N. D. 1994. The relationship between food Congress of Ornithology, 1979: 127–135. supply, reproductive parameters and population dynamics in Dutch lesser black-backed gulls Larus Montevecchi, W. A. 1977. Predation in a salt marsh fuscus: a pilot study. IBN Research Rep. 94/9, laughing gull colony. Auk, 94: 583–585. Institute for Forestry and Nature Research, Wageningen. 65 pp. Noordhuis, R., and Spaans, A. L. 1992. Interspecific competition for food between herring Larus Stone, C. J., Webb, A., Barton, C., Ratcliffe, N., Reed, T. argentatus and lesser black-backed gulls L. fuscus in C., Tasker, M. L., Camphuysen, C. J., and the Dutch Wadden Sea area. Ardea, 80: 115–132. Pienkowski, M. W. 1995. An atlas of seabird distribution in north-west European waters. Joint Oro, D. 1995. The influence of commercial fisheries in Nature Conservation Committee, Peterborough. 326 daily activity of Audouin's gull Larus audouinii in pp. the Ebro Delta, NE Spain. Ornis Fennica, 72: 154– 158. Tasker, M. L., and Furness, R. W. 1996. Estimation of food consumption by seabirds in the North Sea. In: Oro, D. 1996. Effects of trawler discard availability on Seabird/fish interactions, with particular reference to egg laying and breeding success in the lesser black- seabirds in the North Sea, pp. 6–42. Ed. By G. L. backed gull Larus fuscus in the western Hunt and R. W. Furness. ICES Cooperative Research Mediterranean. Marine Ecology Progress Series, 132: Report, 216. 43–46.
40 ICES Coop. Res. Rep. No. 232 Thiel, M., and Sommer, T. 1994. Wind-dependent Walter, U. 1997. Quantitative analysis of discards from impact of gulls (Larus spec.) on the breeding success brown shrimp trawlers in the coastal area of the East of common terns (Sterna hirundo). Ophelia Frisian islands. Archives of Fishery and Marine Supplement, 6: 239–251. Research, 45: 61–76.
Tiews, K. 1983. Über die Veränderungen im Auftreten Walter, U., and Becker, P. H. 1994. The significance of von Fischen und Krebsen im Beifang der deutschen discards from the brown shrimp fisheries for seabirds Garnelenfischerei während der Jahre 1954–1981. in the Wadden Sea – preliminary results. Ophelia Archiv Fischereiwissenschaft, 34: 1–156. Supplement, 6: 253–262.
Tiews, K., and Wienbeck, H. 1990. Grundlagenmaterial Walter, U., and Becker, P. H. 1996. Die Bedeutung der zu "35–Jahres–Trend (1954–1988) der Häufigkeit Fischerei und des Schiffolgens für die Ernährung von von 25 Fisch- und Krebstierbeständen an der Seevögeln im Wattenmeer. Forschungsbericht 108 02 Deutschen Nordseeküste". Veröff. Inst. Küsten- und 085/21, Ökosystemforschung Niedersächisches Binnenfischerei. 64 pp. Wattenmeer. Umweltbundesamt, Berlin. 322 pp.
Tuck, L. M. 1961. The murres. Queen’s Printers, Ottawa. Walter, U. and Becker, P. H. 1997. Occurrence and 260 pp. consumption of seabirds scavenging on shrimper discards in the Wadden Sea. ICES Journal of Marine Vauk, G., Prüter, J., and Hartwig, E. 1989. Long-term Science, 54: 684–694. population dynamics of breeding bird species in the German Wadden Sea area. Helgolander Wanless, S. 1988. The recolonisation of the Isle of May Meeresuntersuchungen, 43: 357–365. by common and Arctic terns. Scottish Birds, 15: 1–8.
ICES Coop. Res. Rep. No. 232 41 5 Exploration of the short- and medium-term consequences of a reduction in the amounts of fish discarded
M. L. Tasker1, P. H. Becker2 and G. Chapdelaine3
1Joint Nature Conservation Committee, 7 Thistle Place, Aberdeen AB10 1UZ, U.K. 2Institut für Vogelforschung, Vogelwarte Helgoland, An der Vogelwarte 21, D–26386 Wilhelmshaven, Germany. 3Canadian Wildlife Service, 1141 route de l’Eglise, PO Box 10100, 9th floor, Ste-Foy, Quebec G1V 4H5, Canada.
5.1 Short term effects Both measures could lead to reduced feeding opportunities for immature individuals since adults are 5.1.1 Introduction generally more successful than immatures of the same species (Wunderle, 1992), particularly if immatures switch to less favourable lengths of discards (Garthe, There are increasing pressures to further manage 1993). Immatures, especially birds in their first year of fisheries in order to make them sustainable, to reduce life, could suffer from higher mortality. waste and to minimise collateral damage to the environment. A reduction in the levels of discarding 5.1.3 Change in bird distribution seems almost inevitable through several possible policies. Two possibilities seem likely to occur in the All those species utilising fishery waste can be assumed near future, firstly, a general reduction in fishing effort to be somewhat influenced by the distribution of fishing and hence a general reduction in discards, and secondly vessels. Tasker et al. (1987) found positive spatial an increase in the mesh size used in fishing gears. These correlations between many species including gulls, great measures may have different effects on scavengers skuas and fulmars and the presence of trawlers. (Furness, 1992; ICES, 1994; Hubold, 1994). Seasonal However, there was substantial variability with respect to and longer-term fishery-closures are also likely to occur, season and area. Camphuysen et al. (1995) found that as at present off Canada and Spain. great black-backed gulls Larus marinus, herring gulls L. argentatus and lesser black-backed gulls L. fuscus (in 5.1.2 Loss of feeding opportunities summer) were the only species which were clearly positively influenced by the presence of fishing vessels. A general reduction in catch effort will probably lead to There was no evidence of large-scale spatial correlations more competition for available discards and larger and between trawlers and fulmars (Camphuysen et al., 1995; stronger species would be more likely to benefit at the Camphuysen and Garthe, 1997) expense of the smaller, weaker species. In other words, kittiwake Rissa tridactyla, other small gulls, great skuas Based on the above results, it is possible to speculate that Catharacta skua and fulmars Fulmarus glacialis the distribution of large gulls would be most affected by (Camphuysen et al., 1995) would suffer, while gannets a change in fisheries effort whereas that of other species, Morus bassanus would be relatively unaffected. such as gannet, would be less affected.
An increase in mesh size does not necessarily increase 5.1.4 Competition at trawlers the size of fish caught as fishermen may take counteracting measures (Reeves et al., 1992). However, Discharges of fishery waste from fishing vessels attract if the purpose of this potential measure was to be met, scavenging seabirds which compete for preferred items. the proportion and amount of small-sized fish present in For several species of seabirds, the preferred size and/or discards would decrease considerably. Furness (1992) type of the discarded items overlaps and because the calculated reductions in the mass of discarded whiting numbers of ship-followers are often high, competition Merlangius merlangus at 65%, while haddock for scraps is often intense. In the competition for the food Melanogrammus aeglefinus discard would decrease by resources provided by fishing boats some seabirds are 52% if the mesh size increased from 90 to 120 mm in more successful than others as shown by several studies North Sea fishing fleets. This increase would principally (e.g., Hudson and Furness, 1989; Camphuysen et al., reduce the small-sized discards (Furness, 1992). This 1995). Different species employ different strategies for would lead to a deterioration of feeding opportunities for obtaining discards and offal (e.g., Dändliker and the smaller gulls, such as black-headed gull Larus Mülhauser, 1988; Hudson and Furness, 1989; ridibundus, common gull L. canus and kittiwake which Camphuysen, 1993; Walter and Becker, 1994; utilise the smallest discarded fish preferentially Camphuysen et al., 1995). Small species, such as (Camphuysen et al., 1993, 1995; Garthe and Hüppop, kittiwake, have to catch and swallow prey items rapidly 1994). to avoid interactions with other, physically stronger
42 ICES Coop. Res. Rep. No. 232 species. If these small species do not succeed with this high tide, when foraging sites are flooded (Veen, 1977). strategy they will often lose their prey to larger, more Growth rates of tern chicks will be reduced, and the aggressive, species. breeding success lowered in kleptoparasitised species.
Gannets and great black-backed gulls are least vulnerable 5.1.6 Reproduction to kleptoparasitism. For these high-ranking species, kleptoparasitism is an effective strategy for obtaining Noordhuis and Spaans (1992) showed that as herring food. Large such as gannet, great black-backed gull and gulls changed diet and obtained fewer discards, there was great skua, are virtually absent during spring and summer a decrease in breeding success and numbers. Some in the eastern and southern North Sea. Smaller species examples of the dependence of seabirds on fisheries such as fulmar and kittiwake do better when robbing originate from the Mediterranean. Paterson et al. (1992) others in these regions and seasons. A reduction in total describe severely reduced breeding success in two quantities of discards produced and discharged in Spanish colonies of Audouin's gulls in 1991 that resulted commercial fisheries will probably lead to a higher from a fisheries moratorium (to preserve fish stocks) frequency of kleptoparasitic interactions. The implication during the gull's breeding season. Oro (1996) and Oro et of these size-based dominance hierarchies is that small al. (1995, 1996) demonstrated that the breeding success species, such as kittiwake, other small gulls and fulmar of Audouin's, yellow-legged and lesser black-backed will suffer the most. gulls differed significantly between years with different trawling activity at the Ebro Delta, north-east Spain. The 5.1.5 Changing diets three species of gulls compensated partly for the cessation in food supply (discards) after a trawl A reduction in the availability of, and increase in mean moratorium took place by switching to other types of size of, discards will lead to a switch in foraging methods food. Other parameters of breeding and behaviour were and diets in gulls. During the breeding season, herring affected by the availability of fishery waste. gulls would change their feeding areas and habits and exploit food of lower energetic quality such as eggs and A long-term large-scale fishery moratorium in eastern chicks of its own and other species (Regehr and Canada has been associated with increased predatory Montevecchi, 1996). Interactions between Audouin's gull pressure by great black-backed gulls on kittiwakes and Larus audouinii and yellow-legged gulls L. cachinnans Atlantic puffins Fratercula arctica, which has in turn at the colony site increase during periods with no fishing reduced breeding success (Russel and Montevecchi, activity (Gonzalez-Solis, 1996). High densities of 1996; Regehr and Montevecchi, 1996). breeders and low food supply increases cannibalism among gulls (Parsons, 1971, 1976; Spaans et al., 1987; In summary, if food supply is reduced, reproduction can Kilpi, 1989). be impaired in several ways. The numbers of non- breeders can increase, the onset of laying can be delayed, Investigations of a kittiwake colony on Great Island, clutch size and egg size can decrease, and hatching Newfoundland revealed complex relationships (Regehr success, growth rate, fledging success and recruitment and Montevecchi, 1996). A four-week delay in the can will be reduced (e.g., Pons, 1992). The weakened inshore arrival of spawning capelin and a lack of fishery condition of adults can lead to higher mortality and waste due to the closure of the ground fishing industry in lowered reproductive ability. Mortality of adult gulls, eastern Newfoundland apparently led to food shortages which is highest during August after the breeding season in herring gulls and great black-backed gulls. These (Coulson et al., 1983), may increase due to lowered adult species were forced to switch to other prey, including condition caused by the lack of food from fisheries. depredation of the eggs and chicks of kittiwakes. The low availability of capelin Mallotus villosus (also an 5.2 Medium term effects important food of kittiwakes) and the high predation pressure by herring gulls and great black-backed gulls 5.2.1 Introduction led to delayed breeding and led to extremely low breeding success. They showed that kittiwake All effects listed above as short-term will tend to reproductive success was a consequence of indirect and continue into the medium and long-term if quantities of interactive effects of food supplies on both parents and waste discarded remain at a relatively low level. Several predators. further medium-term effects might be expected.
Intra- and interspecific kleptoparasitic feeding may 5.2.2 Population size of consumer species increase at colonies owing to reductions in the availability of discards. In windy conditions black- Short-term reductions of reproductive success and headed gulls steal more sandeels Ammodytes spp. from survival of the scavenging species owing to a discard Sandwich terns Sterna sandvicensis than during calm reduction will over time result in population decreases if weather when their intertidal foraging is more successful alternative foods are not available. The capacity of the (Gorke, 1990). In addition, kleptoparasitism increases at environment enhanced by the anthropogenic food
ICES Coop. Res. Rep. No. 232 43 sources at sea will be lowered to more natural levels, and 5.2.3 Population size and species composition the numbers of seabirds using discards and offal will decline. But as gulls are opportunistic feeders, During the first years of discard reductions, those species individuals will respond by changing their scavenging preyed upon by the larger predatory and scavenging diet to other food sources, especially more terrestrial species are likely to experience a population decline. prey and garbage (Gonzalez-Solis, 1996) and may This could be through direct predation, or indirectly increase predation pressure on smaller seabird species. through reduced reproductive output due to predation of Despite this shift, however, competition between chicks and eggs. However, should the populations of the individuals could be stronger, so that populations may be larger predatory species also decline, there might be reduced anyway. some longer term recovery of the smaller species. The competition for nesting sites will be less. In consequence As an example, on the North Shore of the Gulf of St. the quality of breeding sites for those species might Lawrence the herring gull population decreased from improve, and areas abandoned will be resettled. Terns, 14,000 pairs in 1988 to 3,000 pairs in 1993 (Figure 5.1), for example, can reoccupy their former breeding sites corresponding with a moratorium on cod fishing and populations can recover in the longer term. Overall, (Chapdelaine and Rail, 1997). While kittiwakes are in the absence of other influences, population sizes are considered to be a scavenging species, they could likely to settle to different equilibria than previously. compensate the lack of discard provisioning because depredation by gulls will be less as gull populations decrease (Howes and Montevecchi, 1992). In turn the breeding success of smaller species should improve (Regehr and Montevecchi, 1996).
Furness (1992) estimated a reduction of scavenging seabirds in Scotland by 500,000 individuals if the demersal trawl mesh size were to be increased from 90 to 120 cm or if fishery effort was reduced by 30%.
Figure 5.1. Herring gull breeding numbers in sanctuaries on the North Shore of the Gulf of St. Lawrence in relation to total landings of cod (assumed to provide an index of the quantities of offal and discards made available to seabirds) on the North Shore Chapdelaine and Rail, 1997)
44 ICES Coop. Res. Rep. No. 232 5.3 References Howes, L. A., and Montevecchi, W. A. 1992. Population trends of gulls and terns in Gros Morne National Camphuysen, C. J. 1993. Een verkennend onderzoek: De Park, Newfoundland. Canadian Journal of Zoology, exploitatie van op zee overboord geworpen vis en 71: 1516–1520. snijafval door zeevogels. Het Vogeljaar, 41: 106– 114. Hubold, G. 1994. Maßnahmenkatalog für eine ausgewogenere und rationellere Bewirtschaftung der Camphuysen, C. J., Calvo, B., Durinck, J., Ensor, K., von der deutschen Fischerei genutzten Fischereiressourcen im EG Meer. Informationen zur Follestad, A., Furness, R. W., Garthe, S., Leaper, G., Fischwirtschaft, 41: 3–18. Skov, H., Tasker, M. L., and Winter, C. J. N. 1995. Consumption of discards by seabirds in the North Hudson, A. V., and Furness, R. W. 1989. The behaviour Sea. Final report EC DG XIV research contract of seabirds foraging at fishing boats around Shetland. BIOECO/93/10. NIOZ–Report 1995–5, Netherlands Ibis, 131: 225–237. Institute for Sea Research, Texel. 202 pp. ICES 1994. Report of the Working Group on ecosystem Camphuysen, C. J., and Garthe, S. 1997. Distribution and effects of fishing activities. ICES, C.M. scavenging habits of northern fulmars in the North 1994/Assess/Env:1. 109 pp. Sea. ICES Journal of Marine Science, 54: 654–683. Kilpi, M. 1989. The effect of varying pair numbers on Chapdelaine, G., and Rail, J.-F. 1997. History of the reproduction and use of space in a small herring gull herring gull on the north shore of the Gulf of St. Larus argentatus colony. Ornis Scandinavica, 20: Lawrence and its relationship with fisheries. ICES 204–210. Journal of Marine Science, 54: 708–713. Noordhuis, R., and Spaans, A. L. 1992. Interspecific Coulson, J. C., Monaghan, P., Butterfield, J., Duncan, N., competition for food between herring Larus Thomas, C., and Shedden, C. 1983. Seasonal changes in the herring gull in Britain: weight, moult and argentatus and lesser black-backed gulls L. fuscus in mortality. Ardea, 71: 235–244. the Dutch Wadden Sea area. Ardea, 80: 115–132.
Dändliker, G., and Mülhauser, G. 1988. L'exploitation Oro, D. 1996. Effects of trawler discard availability on des déchets de chalutage par les oiseaux de mer au egg laying and breeding success in the lesser black- large des Orcades et des Shetland (Nord-Est backed gull Larus fuscus in the western Atlantique). Nos Oiseaux, 39: 257–288. Mediterranean. Marine Ecology Progress Series, 132: 43–46. Furness, R. W. 1992. Implications of changes in net mesh size, fishing effort and minimum landing size Oro, D., Bosch, M., and Ruiz, X. 1995. Effects of a regulations in the North Sea for seabird populations. trawling moratorium on the breeding success of the JNCC report No. 133. Joint Nature Conservation yellow-legged gull Larus cachinnans. Ibis, 137: 547– Committee. Aberdeen. 62 pp. 549.
Garthe, S. 1993. Quantifizierung von Abfall und Beifang Oro, D., Jover, L. and Ruiz, X. 1996. Influence of der Fischerei in der südöstlichen Nordsee und deren trawling activity on the breeding ecology of a Nutzung durch Seevögel. Hamburger avifauniatische threatened seabird, Audouin's gull Larus audouinii. Beitrage, 25: 125–237. Marine Ecology Progress Series, 139: 19–29.
Garthe, S., and Hüppop, O. 1994. Distribution of ship- Parsons, J. 1971. Cannibalism in herring gulls. British following seabirds and their utilization of discards in Birds, 64: 528–537. the North Sea in summer. Marine Ecology Progress Series, 106: 1–9. Parsons, J. 1976. Nesting density and breeding success in the herring gull Larus argentatus. Ibis, 118: 537–546. Gonzalez-Solis, J. 1996. Interspecific relationships between two species of gulls breeding sympatrically: Paterson, A. M., Martinez Vilalta, A., and Dies, J. I. Larus audouinii and L. cacchinans. Thesis, 1992. Partial breeding failure of Audouin's gull in two Spanish colonies in 1991. British Birds, 85: 97– University of Barcelona. 100. Gorke, M. 1990. Die Lachmöwe (Larus ridibundus) in Pons, J. M. 1992. Effects of changes in the availability of Wattenmeer und Binnenland. Ein human refuse on breeding parameters in a herring verhaltensökologischer Vergleich. Seevögel, 11, gull Larus argentatus population in Brittany, France. Supplement 3: 1–48. Ardea, 80: 143–150.
ICES Coop. Res. Rep. No. 232 45 Reeves, S. A., Armstrong, D.W., Fryer, R.J. and Coull, Tasker, M. L., Webb, A., Hall, A. J., Pienkowski, M. W., K.A. 1992. The effect of mesh size, cod-end and Langslow, D.R. 1987. Seabirds in the North Sea. extension length and cod-end diameter on the Nature Conservancy Council, Peterborough. 336 pp. selectivity of Scottish trawls and seines. ICES Journal of Marine Science, 49: 279–288. Veen, J. 1977. Functional and causal aspects of nest distribution in colonies of the Sandwich tern (Sterna Regehr, H. M., and Montevecchi, W.A. 1996. Interactive s. sandvicensis Lath.). Behaviour Supplement, 20: 1– effects of food shortage and predation on breeding 193. failure of black-legged kittiwakes: effects of fisheries activities and implications for indicator species. Walter, U., and Becker, P. H. 1994. The significance of Marine Ecology Progress Series, 155: 249–260. discards from the brown shrimp fisheries for seabirds in the Wadden Sea – preliminary results. Ophelia Russell, J. O., and Montevecchi, W.A. 1996. Predation Supplement, 6: 253–262. on adult puffins Fratercula arctica by great black- backed gulls Larus marinus at a Newfoundland Wunderle, J. M., Jr. 1992. Age-specific foraging colony. Ibis, 138: 791–794. proficiency in birds. Current Ornithology, 9: 273– 324. Spaans, A. L., de Wit, A. A. N., and van Vlaardingen, M. A. 1987. Effects of increased population size in herring gulls on breeding success and other parameters. Studies in Avian Biology, 10: 57–65.
46 ICES Coop. Res. Rep. No. 232 6 Evidence for decadal scale variations in seabird population ecology and links with the North Atlantic Oscillation
J. B. Reid1, P. H. Becker2 and R. W. Furness3
1Joint Nature Conservation Committee, 7 Thistle Place, Aberdeen AB10 1UZ, U.K. 2Institut für Vogelforschung, Vogelwarte Helgoland, An der Vogelwarte 21, D–26386 Wilhelmshaven, Germany. 3Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, U.K.
6.1 Introduction 6.2 Materials and methods
The North Atlantic Oscillation (NAO) has an influence Four data-sets, or extracts therefrom, were used in not only on the physical oceanography of the North bivariate correlation analyses to detect possible Atlantic (Levitus et al., 1994; Hurrell, 1995; see also associations between the NAO and aspects of seabird Web site 1), but also on zooplankton (Fromentin and breeding ecology. Planque, 1996) and fish (Friedland et al., 1993). Thus an influence of the NAO on higher trophic levels of the The UK Seabird Monitoring Programme (Thompson et North Atlantic and North Sea may be anticipated. al., 1996) monitors seabird populations at several Seabirds characteristically have high adult survival rates colonies around the coasts of Great Britain and Ireland. and deferred maturity, coupled with low reproductive In addition to breeding numbers and overall success, rates. Thus we may expect some parameters of the various detailed aspects of breeding performance are also ecology of seabird populations to be buffered against measured, mainly at four key sites. Data on breeding such environmental fluctuations. In particular, we can numbers and breeding success of several species from a predict that breeding numbers may not respond to the selection of years (between 1986 and 1996) and colonies NAO or may show a long time-lag in response, whereas were used in the analyses. breeding success might correlate with the NAO as a consequence of its influence on preferred prey The second data-set on bird populations used includes breeding numbers of various species nesting along the populations of the seabirds. coast of the German Wadden Sea and Helgoland. This is a long term data-set, which, for some species dates from Studies of seabird populations in the California Current 1950 (Becker and Erdelen, 1987; Südbeck and (Ainley et al., 1995) demonstrated that the El Nino- Hälterlein, 1997; and unpublished data of Becker, Verein Southern Oscillation (ENSO) caused spectacular changes Jordsand and Institut für Vogelforschung). The third in food supply to seabirds, leading to correlations seabird data-set used in the analyses was numbers of between seabird breeding parameters and variation in the breeding pairs and breeding success of common terns Southern Oscillation and/or the Aleutian low pressure Sterna hirundo between 1981 and 1997 on Minsener system, both of which affect sea-surface temperature and Oldeoog, an island in the German Wadden Sea (Becker, thermocline depth. Some seabird species showed 1998). stronger links than others with these physical parameters. Cormorants and gulls showed stronger variations in breeding success than did common guillemots Uria Data on the NAO were slightly amended from Hurrell aalge, as might be anticipated given the smaller clutch (1995). The winter (December through March) NAO size and the greater volume of sea used by foraging index was used and is based on the difference of common guillemots. normalised sea level pressures (SLP) between Lisbon, Portugal and Stykkisholmur, Iceland. The SLP anomalies Montevecchi and Myers (1997) attributed a century-long at each station were normalised by division of each increase in northern gannet Morus bassanus numbers in seasonal pressure by the long term (1864–1983) standard Newfoundland with warming surface water conditions deviation. and increased availability of mackerel Scomber scombrus. A major dietary change during the 1990s to 6.3 Results colder-water conditions in the north-west Atlantic led to a change in prey stocks from warm-water pelagic fish and squid to cold-water fish. In this report we have No significant correlations were found between the NAO concentrated our efforts on searching for any evidence index and breeding population sizes or breeding success that the NAO influences numbers or breeding ecology of of various species breeding at several UK seabird seabirds in the north-east Atlantic. Our analysis is colonies (Tables 6.1 and 6.2). In addition, correlations constrained by the fact that, while there are data sets for between breeding success of kittiwakes Rissa tridactyla seabird breeding numbers over many decades, data on for seven or more years between 1986 and 1996 at 49 breeding success or diet at particular sites rarely provide colonies in Britain and Ireland and the NAO gave 31 more than a run of 10 years, and most sets start during negative and 18 positive correlations. the 1970s or 1980s.
ICES Coop. Res. Rep. No. 232 47 Table 6.1. Relationships between seabird breeding numbers at various UK seabird colonies and the NAO Index. Pearson correlation coefficients, r, are presented with associated p values. All correlations are non-significant.
Species Sites Dates r p Guillemot Skomer, Isle of May, 1986–96 .051 .883 Razorbill Skomer, Isle of May 1986–96 .165 .629 Puffin Isle of May 1983–1993 .142 .738
Table 6.2. Relationships between seabird breeding success at various UK seabird colonies and the NAO Index. Pearson correlation coefficients, r, are presented with associated p values. All correlations are non-significant.
Species Sites Dates r p Fulmar Fair Isle, Shetland 1986–96 –.058 .866 Fulmar Isle of May, SE Scotland 1986–96 .088 .796 Fulmar Farne Islands, NE 1986–96 –.258 .445 Fulmar Troswick Ness, Shetland 1986–96 .351 .290 Gannet Fair Isle, Shetland 1986–96 .415 .204 Gannet Noss, Shetland 1986–96 –.169 .620 Gannet Bempton, NE England 1986–96 (excluding 1985) –.087 .812 Shag Canna, NW Scotland 1986–96 –.239 .480 Shag Fair Isle, Shetland 1986–96 .141 .678 Shag Isle of May, SE Scotland 1986–96 –.262 .437 Kittiwake St. Kilda, NW Scotland 1986–96 (excluding 1995) –.397 .256 Kittiwake Isle of May, SE Scotland 1986–96 (excluding 1987) .076 .834 Guillemot Fair Isle, Shetland 1987–96 –.237 .510 Guillemot Isle of May, SE Scotland 1986–96 .082 .810 Razorbill Isle of May, SE Scotland 1986–96 .369 .265
Table 6.3. Relationships between seabird breeding numbers on the German Wadden Sea coast and the NAO Index. Pearson correlation coefficients, r, are presented with associated p values. An asterisk (*) indicates statistical significance.
Species Dates r p Fulmar most years, 1953–96 .371 .022 * Cormorant 1971–96 .183 .370 Herring gull most years, 1950–93 .456 .004 * Lesser black-backed gull most years, 1950–93 .480 .002 * Common gull most years, 1950–93 .566 .000 * Kittiwake most years, 1953–96 .423 .008 * Black-headed gull most years, 1950–93 .565 .000 * Sandwich tern most years, 1909–96 .264 .015 * Arctic tern various, 1982–96 .316 .317 Common tern various, 1982–96 .485 .156 Arctic/common tern most years, 1950–93 .145 .385 Guillemot most years, 1953–96 .536 .001 * Razorbill most years, 1953–96 .414 .010 *
48 ICES Coop. Res. Rep. No. 232 Table 6.4. Relationships between common tern breeding numbers and success on Minsener Oldeoog, German Wadden Sea, and the NAO Index between 1981 and 1997. Pearson correlation coefficients, r, are presented with associated p values. Both correlations are non-significant.
Measure r p No. Breeding pairs .233 .368 Chicks fledged per pair .252 .329
Four were significant at the 5% level, two of these were NAO index is unlikely in the case of the black-headed positive and two were negative correlations. We gull, as unlike the other species considered, the black- conclude from this result that there is no reason to headed gull is not closely linked with fish foods gathered believe that the factors that the NAO index represents at sea, but feeds mainly on Nereis in the Wadden Sea, affects kittiwake breeding success. and additionally inland (Gorke, 1990).
No significant relationship was found between the NAO If the common cause of recent seabird population index and breeding numbers of cormorant, Arctic tern or increases (the Dutch and German Wadden Sea data common tern in the German Wadden Sea as a whole. indicate that these date from the 1970s) is not the NAO However, there were significant associations between the then a more local explanation need be sought. It is likely NAO and numbers of other breeding seabirds here that the general, sustained increase is due to recovery of (Table 6.3). In some cases, the relationship was very these populations following a major pollution incident in strong. the Dutch and German parts of the Wadden Sea in the late 1960s when large amounts of organochlorine No correlation was detected between either breeding pesticides contaminated these waters from the Rhine numbers or breeding success of common terns on river (Becker, 1991). The immediate effect of this was Minsener Oldeoog in the German Wadden Sea and the widespread mortality of seabirds in the Wadden Sea NAO (Table 6.4). resulting in depressed population sizes, from which there has been a gradual increase in breeding seabirds. This population recovery has coincided with an increase in 6.4 Discussion strength of the NAO since the 1970s but there remains no evidence of a causal link between the two. No correlations were found between the breeding success of seabirds and the NAO index during the last decade. As 6.5 References expected, no significant relationships were found between numbers of seabirds breeding around the UK. Significant correlations were detected, however, between Ainley, D. G., Sydeman, W. J., and Norton, J. 1995. breeding numbers of fulmar Fulmarus glacialis, herring Upper trophic level predators indicate interannual gull Larus argentatus, common gull L. canus, lesser negative and positive anomalies in the California black-backed gull L. fuscus, black-headed gull L. Current food web. Marine Ecology Progress Series, ridibundus, kittiwake, Sandwich tern Sterna 118: 69–79. sandvicensis, razorbill Alca torda and guillemot on the German coast, and the winter NAO index. Becker, P. H. 1991. Population and contamination studies in coastal birds: The common tern Sterna That so many significant correlations should be found hirundo. In: Bird population studies: relevance to between breeding numbers and the NAO when no conservation and management, pp. 433–460. Ed. by relationship was suspected, is puzzling. Such a finding C. M. Perrins, J. D. Lebreton and G. J. M. Hirons. would be expected if high correlations also existed Oxford University Press, Oxford. among population sizes of these species. Indeed, such high, positive correlations do prevail among population Becker, P. H. 1998. Langzeittrends des Bruterfolgs der levels of all these species (all pairwise Pearson Fluβseeschwalbe und seiner Einfluβgröβen im correlation coefficients, r>0.75, p<0.001). This indicates Wattenmeer. Vogelwelt, 119: 223–234. that a common factor may account for the observed results. Such a common factor, of course, could be the Becker, P. H., and Erdelen, M. 1987. Die influence of the NAO on food resources. However, the Bestandsentwicklung von Brutvögeln der deutschen feeding requirements and general feeding ecology of Nordseeküste 1950–1979. Journal für Ornithologie, those species involved is so diverse as to render this 128: 1–32. unlikely. Furthermore, if the NAO were to contribute to processes underlying seabird population patterns, then Friedland, K. D., Redding, D. G., and Kocik, J. F. 1993. seabird life history parameters would lead to the Marine survival of North American and European expectation that there would be a time lag in the Atlantic salmon: effects of growth and environment. manifestation of NAO effects. An identical effect of the ICES Journal of Marine Science, 50, 481–492.
ICES Coop. Res. Rep. No. 232 49 Fromentin, J.-M., and Planque, B. 1996. Calanus and Montevecchi, W. A., and Myers, R. A. 1997. Centurial environment in the eastern North Atlantic. II. and decadal oceanographic influences on changes in Influence of the North Atlantic Oscillation on C. northern gannet populations and diets in the north- finnmarchicus and C. helgolandicus. Marine Ecology west Atlantic: implications for climate change. ICES Progress Series, 134: 111–118. Journal of Marine Science, 54: 608–614.
Gorke, M. 1990. Die Lachmöwe (Larus ridibundus) in Südbeck, P. and B. Hälterlein. 1997. Brutvogelbestände Wattenmeer und Binnenland. Seevögel, 11, an der deutschen Nordseeküste im Jahre 1995 – Sonderheft: 1–48. Neunte Erfassung durch die Arbeitsgemeinschaft "Seevogelschutz". Seevögel, 18: 11–19. Hurrell, J. W. 1995. Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Thompson, K. R., Brindley, E., and Heubeck, M. 1997. Science, 269: 676–679. Seabird numbers and breeding success in Britain and Ireland, 1996. U.K. Nature Conservation No. 21. Levitus, S., Antonov, J.I., and Boyer, T.P. 1994. Peterborough, Joint Nature Conservation Committee. Interannual variability of temperature at a depth of 64 pp. 125 meters in the North Atlantic ocean. Science, 266: 96–99. Web site 1: http://www.clivar.ucar.edu/vol2/pdl.html – The North Atlantic Oscillation (NAO)
50 ICES Coop. Res. Rep. No. 232 7 A review of the causes, and consequences at the population level, of mass mortalities of seabirds
C. J. Camphuysen1, P. J. Wright2, M. Leopold, O. Hüppop2 and J. B. Reid3
1Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands. 2Fisheries Research Services, Marine Laboratory, P.O. Box 101 Victoria Road, Aberdeen AB11 9DB, United Kingdom. 3IBN-DLO, P.O. Box 167, 1790 AD Den Burg, Texel Netherlands. 4Vogelwarte Helgoland, PO Box 1220, D–27494 Helgoland, Germany. 5Joint Nature Conservation Committee, 7 Thistle Place, Aberdeen AB10 1UZ, U.K.
7.1 Introduction Storm-related wrecks are those in which mortality has been linked with birds being blown away from favoured feeding areas or being prevented from feeding by wind. Of the millions of seabirds that die of natural causes each Calm weather in summer has been associated with year only a small proportion come ashore. However, wrecks of fulmars Fulmarus glacialis (Anonymous, public attention and concern is often drawn to the 1982; Camphuysen, 1989a), possibly as a result of frequent large strandings of dead or moribund birds increased energetic costs associated with flapping flight washed up on beaches. These “wrecks” may reflect mass in these birds which are adapted for gliding in the wind mortalities of seabirds at sea, but are in fact defined as (Furness and Bryant, 1996). Oil-related wrecks may be any much larger than usual concentration of seabird divided into those related to major oil pollution incidents corpses washed ashore over a short period. The or to chronic oil pollution. Effects on birds of both types definition must be applied relative to the population sizes tend to be physical, disabling the birds through plumage of the species concerned (perhaps considering only that saturation and subsequent hypothermia and also part of the population within a local area from which the physiological through the toxic effects of oil. Major oil wrecked birds probably originate). ‘Large’ numbers of incidents are widely reported because the events an uncommon species may qualify as a wreck whereas engender public awareness (major ship-wreck, blow- ‘large’ numbers of a very common species might not. out), from which stranded birds are easily detected. There is rarely any indication from the size of wrecks of Chronic oil pollution is a constant process, a more severe the total mortality of birds at sea. Mass mortalities that threat to seabirds because of a mosaic of larger and do not result in wrecks are, by their very nature, difficult smaller oil slicks which reduces the quality of the areas to study. The numbers and even species of bird involved where seabirds live, most notably around busy shipping may only be ascertained by the subsequent effect on the lanes and near larger harbours. Oil-related wrecks are size of breeding populations. However, wrecks of relatively well studied, because the effects of oil seabirds are recorded sufficiently frequently to enable pollution on seabirds has attracted attention of the some, usually qualitative, consideration of their likely ornithological community since the end of the 19th causes, seasonality and possible effects on population century (Bourne, 1969; Camphuysen, 1989a). Beached levels. This section reviews the major causes of seabird bird surveys are an appropriate method of identifying wrecks, their seasonal occurrence and the relative trends in oil contamination of stranded birds, but not vulnerability of various species to different causal (necessarily) in identifying trends in mortality patterns of agents. seabirds (Heldt, 1969; Joensen, 1972; Kuyken, 1978; Vauk, 1978; Becker and Schuster, 1980; Commecy, 7.2 Presumed causes 1982; Stowe, 1982; Heubeck, 1987; Vauk et al., 1990, 1991; Camphuysen and van Franeker, 1992; Wrecks can be explained by several factors. These Camphuysen, 1995a; Heubeck, 1995). include those related to weather (for example storms, calm conditions, severe cold), food, pollution (for Food-related wrecks are those where mortality results example oil spills, chronic oil pollution, chemical from starvation due to the birds' not being able to forage polllution), fishing activities (for example bycatch) and successfully, either through low food availability or parasites. The most frequent causes are those due to abundance. Seabird mass strandings (and also large scale storms, oil, severe cold weather, and food. Other, less fluctuations in wintering distribution of seabirds) may be commonly cited, causes include chemical pollution, indicative of changes in prey stock abundance, toxins, calm weather, diseases and parasites. In many distribution or availability. In the early 1980s, a major cases wrecks cannot be identified as being due to one south and eastward shift in the wintering distribution of single cause. For example, adverse weather conditions common guillemots Uria aalge, kittiwakes Rissa may affect foraging behaviour and success and may be tridactyla and razorbills Alca torda occurred in the North indirectly responsible for a wreck of emaciated birds. Sea. This was consistent with a decline in sprat Sprattus However, in the following treatment we distinguish sprattus availability in the northern North Sea (Corten, between the above categories and food related causes per 1990), sprats being a major prey species of these birds in se. winter. The change in the pattern of sprat distribution together with poor weather was implicated in a wreck of
ICES Coop. Res. Rep. No. 232 51 auks along the east coast of Britain in February 1983 • pollution related wrecks (e.g., oil, chemicals, (Blake, 1984), and multiple wrecks in the southern North netting) Sea (Camphuysen, 1981, 1989b, 1990a,b,c,d, 1992, 1995a,b; Camphuysen and Keijl, 1994). • weather related wrecks (e.g., storm, calm or cold weather) The rapid decline in Barent Sea capelin Mallotus villosus during the 1980s provides an even more dramatic • food related wrecks example of a starvation induced wreck. Thousands of emaciated common guillemots were washed ashore • other types. along the coasts of Finnmark during the winter 1986/87, and breeding populations in the Barents Sea collapsed, It needs emphasizing that the effect of severe winters on coincident with the decline of this important prey (Vader marine and estuarine birds is not fully addressed here, et al., 1987). although it is an important factor behind mass mortality. Several case studies indicated that very large numbers of Fishing has often been alleged to be a contributory factor seabirds can suffer from cold stress and starvation in in wrecks of emaciated birds, due to the common association with severe winter weather (Crisp, 1964; utilisation of many small fish species by seabirds and Schoennagel, 1980; van Gompel, 1987; Meininger et al., fisheries. For example, concern was expressed that the 1991; Suter and van Eerden, 1992; Beukema, 1994; sandeel Ammodytes marinus fishery off the east coast of Camphuysen et al., 1996). The literature search of this Scotland may have been involved in the large wreck of subject was not completed for this review. The very long auks and shags Phalacrocorax aristotelis along the east lists of smaller and larger scale oil-related wrecks which coast of British in February, 1994. The wreck occurred have been published in association with accounts dealing over a far more extensive region than the area where the with marine oil pollution are not repeated in Appendix fishery operated, but it is typical of wrecks that seabirds 7.1 (Bourne, 1969; Vermeer and Vermeer, 1974; Stowe affected disperse beyond their normal distribution. There and Underwood, 1984; Hooper et al., 1987; are inadequate data to attribute a cause, beyond Camphuysen, 1989a; Camphuysen and van Franeker, starvation, to this particular wreck. Wrecks due to 1992; Camphuysen, 1995b). Large numbers of recently bycatch in fishing nets of various sorts (set nets, gill nets) fledged birds occasionally wash up dead near their are locally important (e.g., Robins, 1991). colonies. These post-fledging incidents appear to be relatively common,, but are not often reported (Jones, 1980). Post-fledging wrecks can only be studied after Other, apparently less common, causes of seabird wrecks having set clear criteria from which 'wrecks' (as include mortality due to natural toxins (for example unusually large numbers of birds which died) may be botulism, red tides, paralytic shellfish poisoning). separated from the background noise. Botulism may hit coastal seabirds that utilize freshwater bodies for drinking or bathing in summer, but as yet there is little evidence that this is a major problems for Wrecks, as described earlier, were identified in the first seabirds except at a local scale (Sutcliffe, 1986). place through stranded birds and influxes of birds in Epizootics, involving e.g., Noctiluca in red-tides have areas where they do not normally occur in large been reported to kill a variety of seabirds (Coulson et al., numbers. Some of these birds showed clear symptoms of 1968; Armstrong et al., 1968; Wrånes, 1988). Toxins the cause of the event. However, the mass mortalities as inducing paralytic shellfish poisoning are known to a result from drowning and entanglements in fishing gear affect gulls in the USA that take contaminated shellfish are not easily attributed if the cause of death was not (Kvitek, 1991) and there are suggestions that such found (i.e. the net in which the birds had drowned). In poisoning might be responsible for recent die-offs of the literature, or more commonly before the stage on common guillemots in the Baltic (Hario, 1994). which things get written up, there is frequently considerable speculation as to why such birds had died (good condition, non-oiled, no adverse weather, but still Occasionally, other less common factors may cause dead in large numbers). Although such events were wrecks. For example, the production of an oily substance possibly caused by netting incidents, a firm conclusion as by a plankton bloom of Coscinodiscus concinnus in the to the cause cannot be reached. While we are aware of southern German Bight in spring 1996 resulted in several areas in which potential 'conflicts' between strandings of red-throated divers Gavia stellata, due to seabirds and fishermen in terms of unwanted bycatch of plumage contamination (Camphuysen, 1997). birds exist, there is very little factual evidence available, and several wrecks in such areas may have been mis- 7.3 Frequency and seasonal occurrence interpreted. Post-mortem analysis might help in of wrecks confirming drowning as the cause of death.
An initial literature search led to identification of over It is important to stress that an analysis of wrecks such as 100 wrecks, or events, in European waters, and a very this is inherently biased towards scarce species, and in incomplete list of events elsewhere in the world. Wrecks other words to the relatively rare ‘more interesting’ were roughly classified as: events. Strandings of common birds are often taken for granted and do not become subject of further study or be
52 ICES Coop. Res. Rep. No. 232 published in the ornithological literature. As a result, a occurred in one season only, being late summer and thorough literature search will lead to a relatively winter respectively. Food-related and storm-driven complete picture of little auk wrecks (Camphuysen and wrecks were basically autumn and winter phenomena. Leopold, 1996; Stenhouse and Montevecchi, 1996), but a Oil-related wrecks occurred through the year, but very incomplete idea of e.g., post-fledging mortality in incidents due to chronic oil pollution were concentrated herring gulls Larus argentatus. Phalarope strandings will in the half year spanning winter (Bourne, 1969; Stowe be reported even if only very few individuals were and Underwood, 1984; Camphuysen, 1989a). An overall found, whereas common guillemot strandings get noticed conclusion of this first inventory of wrecks is that the only when many hundreds wash ashore over short types of mass-mortality events which are considered here lengths of coast. occur seldom in summer, and most frequently in autumn and winter. A conclusion which might be drawn is that some (causes of) wrecks get more attention than others because the Little auk Plautus alle wrecks and influxes, which were underlying factors are more obvious. In the absence of studied in considerable detail, occurred rather frequently, adequate data relating to the underlying factors of most but not randomly during the last odd 150 years (reported) mass-strandings of seabirds, this analysis (Camphuysen and Leopold, 1996). In Europe, over 60 should only be considered as a first attempt to discuss influxes/wrecks were recorded since 1840, but these wrecks. It is not possible at present to reach firm events appeared to occur in clusters (Runs test, ts = – conclusions as to which species are more vulnerable than 2.30, n1 = 62, n2 = 94, P< 0.05). A detailed analysis of others and as to what type of wrecks most affect seabird the most recent influxes demonstrated that the events populations. were in fact related to major shifts in wintering distribution of little auks. Hence, wrecks may occur if the A final aspect which needs to be addressed before the North Sea is used as a wintering area and not, or not be frequency of wrecks is discussed is the possible overlap recorded, when the birds were wintering elsewhere. of cause of wrecks, or the accumulating effect of a These wrecks were often wind-related, and stormy number of factors which lead to mass mortality. Where weather usually suggested as the cause of the wreck, we refer to storm-driven or food-related wrecks, however several little auk influxes and wrecks took place starvation of the birds found dead is a key point, while under calm conditions. strong winds were more obvious in the first type. It is easy to understand that while wind may have been an 7.4 Vulnerability of seabird species to important factor in reducing the availability of food for wrecks certain species, a wind-driven event may be also food- related. It has been suggested that starving birds are also Most species of seabird are subject to wrecking (Tables more susceptible to the effects of oil pollution, while 7.2, 7.3) but some are more vulnerable than others. For netting as a factor behind mass mortality of auks in the example, auks tend to be wrecked more often than Skaggerak region had increased after a displacement of Procellariiformes while grebes are rarely wrecked. Of wintering auks due to poor feeding conditions in their course there is variation in the degree to which different more usual wintering areas (Peterz and Oldén, 1987). species and groups of species are vulnerable to different types of wreck, their preferred food (fish/plankton) being From our first analysis, oil-related and storm driven one component of this variability. wrecks occur very frequently. A preliminary search of the literature revealed 30 events of the former type and Species have different vulnerabilities towards the 41 of the latter in a list which only took account of major different causes of wrecks (Table 7.3). Birds spending events (Appendix 7.1). On the scale of the north-east long time swimming such as divers, grebes, duck and Atlantic, both types of wrecks probably occur annually, auks are especially vulnerable towards oil pollution but many have a local or regional character. Oil-related (Stowe, 1982; Averbeck et al., 1993; Camphuysen, wrecks include those caused by shipping accidents and 1989a, 1995b, 1997, Williams et al., 1994), whereas blow-outs, but very many more small wrecks occur as a small species that spend a higher proportion of their time result of chronic oil pollution due to deliberate, flying may be wrecked as a consequence of severe operational discharges of oil. Storm-driven events storms, e.g., storm petrels, fulmar, kittiwake, little auk overlap with 17 wrecks that were temporarily labelled as (Pashby and Cudworth, 1969; Threlfall et al., 1974; 'food-related', because both types comprise stranded Doumeret, 1979, 1980; Nakamura, 1983; Teixeira, 1987; birds that were seriously emaciated and apparently died Camphuysen and Leopold, 1996). For species that rely as a result of starvation (see above). Fewer wrecks on gliding rather than flapping,, the energy expenditure appeared to have been related to bycatch of seabirds in at sea increases with decreasing wind speed (Furness and fishing nets (7), parasites (3), chemical pollution (2), Bryant, 1996), hence they may run into energetic exceptionally calm weather (2), plankton bloom (1). bottlenecks during periods of calm weather which may in time end in a wreck. Diving birds are especially Wrecks did not occur evenly over the year, and different vulnerable to entanglement and drowning in fishing gear types of wrecks appeared associated with different (Brewka et al., 1978, 1985, 1989; Barrett and Vader, seasons (Tables 7.1, 7.2). Obviously, post-fledging 1984; van Eerden and Bij de Vaate, 1984; Peterz and wrecks of young birds and wrecks due to cold-stress Oldén, 1987; Kies and Tomek, 1990; Hüppop, 1996).
ICES Coop. Res. Rep. No. 232 53 Table 7.1. Frequency distribution of different types of North Atlantic wrecks in different seasons (see Appendix 7.1 for a list of wrecks).
Category Type spring summer Autumn winter Totals Pollution oil 9 1 5 15 30 chemicals 2 2 bycatch 7 7 plankton 1 1 Weather storm 24 17 41 cold 44 calm 1 1 2 winddrift 1 1 Food food’adults’ 1 2 14 17 post-fledging 1 1 Other parasites 2 1 3 unknown 2 1 1 4 Totals 13 7 32 61 113
Table 7.2. Frequency distribution of wrecks in the North Atlantic for different groups and species of birds (see Appendix 7.1 for a list of the wrecks; review papers and non-European wrecks were excluded for this analysis).
Species/group spring summer Autumn winter Totals divers1 124 grebes 1 2 3 storm petrels 7 1 8 shearwaters 1 1 2 Fulmar422311 Gannet 1 1 2 4 cormorants 1 1 5 7 seaduck 5 3 1 16 25 phalaropes 1 1 smaller skuas 7 7 Great skua 0 Larus gulls21159 Sabine’s gull 1 1 Kittiwake 242715 terns 1 1 2 Common guillemot 4 1 5 24 34 Brunnich’s guillemot 2 2 Razorbill 1 1 2 17 21 Puffin 1 1 8 10 Little auk 9 11 20 Black guillemot 1 3 4
54 ICES Coop. Res. Rep. No. 232 7.5 Consequences to populations such as the wreckages of the Torrey Canyon (Cornwall), Amoco Cadiz (Brittany), Sea Empress (Wales), Braer (Shetland), and Exxon Valdez (Alaska) Possible consequences of wrecks are hard to detect. (Bourne et al., 1967; Bourne, 1970; Jones et al., 1978; Further, the number of birds wrecked is not a good Anonymous, 1993; Heubeck et al., 1995; Paine et al., indicator of the true number of birds affected. As has 1996). been shown, wrecks often occur outside the breeding season. This means that birds from different populations and colonies may be affected. However, there are cases where effects at a population Consequently, due to the large numbers of most level have been detected following a wreck. For seabird species, severe effects affecting populations or example, severe and long lasting effects were observed even on single colonies are rare. With regards to oil during and following a period of pollution involving contamination, there is no evidence that chronic chlorinated hydrocarbon insecticides in the Wadden pollution has had a long-term effect on populations, but Sea. Sandwich terns Sterna sandvicensis and common this lack of apparent effect may be confounded by the eiders Somateria mollissima were the most affected effect of long-term increases in seabird numbers in birds although declines in populations of all coastal many areas. Major effects of wrecks caused by oil birds of the Wadden Sea were recorded (Koeman et al., contamination could be detected only after a small 1968, 1969, 1972; Swennen, 1972). The Sandwich tern proportion of major oil spills and only at the colony colony at Vlieland collapsed from 20,000 pairs to less level. Examples are only a few big accidents near the than 1,000 pairs within a few years. At the main eider breeding season and close to the breeding colonies, colony on the island of Vlieland numbers of breeding females dropped from c. 4000 to 800 pairs (Swennen 1972, Furness and Camphuysen, 1997).
Table 7.3. Frequency distribution of different types of wrecks in the North Atlantic for different groups and species of birds (see Appendix 7.1 for a list of the wrecks; review papers and non-European wrecks were excluded for this analysis).
Species/group oil storm Food other Totals divers 4 4 grebes 3 3 storm petrels 8 8 shearwaters 1 1 2 Fulmar 4 2 5 11 Gannet 2 1 1 4 cormorants 4 2 1 7 seaduck 15 2 8 25 phalaropes 1 1 smaller skuas 6 1 7 Great skua 0 Larus gulls52119 Sabine’s gull 1 1 Kittiwake 5 6 1 3 15 terns112 Common guillemot 17 1 10 7 35 Brunnich’s guillemot 2 2 Razorbill917522 Puffin 5 1 4 10 Little auk 4 12 3 19 Black guillemot 4 4
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ICES Coop. Res. Rep. No. 232 63 APPENDIX 7.1
LIST OF WRECKS
Shown are authors, year of publication, type of wreck (e.g., storm, oil incident, severe winter, food shortage), region of occurrence, season and species.
Author Year Wreck Region Season Species Europe Andersen 1996 storm Scandinavia autumn Auks Anker-Nilssen et al. 1988 oil Skagerrak winter Auks Anker-Nilssen and Røstad 1983 oil Norway autumn guillemot Anonymous 1869–75 storm? UK–Clyde winter auks Anonymous 1982 calm Netherlands spring fulmars Anonymous 1912 storm UK autumn auks Anonymous 1979 oil Atlantic Spain winter Anonymous 1985 food Netherlands winter auks Barrett 1979 oil Norway auks Barrett 1982 oil Norway–north auks Bibby and Bourne 1971 bycatch UK? winter auks Blake 1983 oil Skaggerak winter auks Bodenstein 1956 storm German Bight winter kittiwake Bourne 1979 oil Norway winter seaduck, auks Bourne 1990 food UK–northeast winter auks Boyd 1954 storm Europe autumn storm petrels Brewka et al. 1978 bycatch Poland winter seaduck, auks Byrkjeland 1989 oil Norway auks Campbell et al. 1978 oil UK–east winter grebes Camphuysen 1989b food Netherlands winter auks Camphuysen 1990b oil Netherlands spring auks Camphuysen 1996a food Netherlands autumn auks Camphuysen 1987 calm Netherlands summer fulmars Camphuysen 1989c postfiled North Sea summer kittiwake Camphuysen 1990b food Netherlands winter auks Camphuysen 1995a oil Netherlands autumn auks Camphuysen 1996b food Netherlands winter seaduck Camphuysen et al. 1988 oil Netherlands winter grebes, auks Camphuysen and Derks 1989 cold Netherlands winter grebes Camphuysen and IJzendoorn 1988 food Europe autumn skuas Camphuysen and Keijl 1994 oil Netherlands autumn auks Camphuysen and Leopold 1996 storm Europe aut/winter auks Clarke 1895 storm UK–northeast auks Cobb 1976 bycatch UK? winter divers, auks Craik 1992 food UK–west summer mixture Debout 1982 storm France–west winter mixture Doumeret 79 / 80 storm France–west autumn Mixture Dreckhahn 1969 oil German Bight autumn Seaduck Durinck et al. 1993 bycatch Denmark winter Seaduck Eber 1958 storm German Bight winter Gulls Engelen 1987 oil Wadden winter Seaduck Evans 1892 storm UK–north autumn storm petrels Furphy et al. 1971 ? Irish Sea autumn Auks Furtado and LeGrant 1979 storm Azores winter Auks Géroudet 1991 food Europe Seaduck Gill et al. 1967 oil UK–southwest Mixture
64 ICES Coop. Res. Rep. No. 232 Author Year Wreck Region Season Species Goethe 1978 oil Wadden Sea Mixture Greenwood et al. 1971 oil UK–northeast winter Auks Grenquist 1970 parasite Finland winter Seaduck Haila 1970 oil Finland Seaduck Ham 1989 storm Netherlands autumn Skuas Ham et al. 1991 storm Netherlands winter Auks Hanssen 1982 oil Baltic Seaduck Harris et al. 1991 food Shetland winter Auks Harris and Wanless 1996 food UK–east winter Cormorant, auks Haverschmidt 1930 storm Netherlands winter Auks Hesse 1912 storm German Bight autumn Skuas Heubeck 1991 food Shetland winter Auks Heubeck 1994 oil Shetland winter Mixture Heubeck and Richardson 1980 oil Shetland winter Mixture Heubeck and Suddaby 1991 food Shetland winter Auks Holdgate 1971 food Irish Sea Auks Joensen 1961 ? Denmark Fulmars Jones et al. 1970 oil Irish Sea spring Auks Jones et al. 1978 oil Channel spring Mixture Jones et al. 1984 food UK–east spring Auks Jouanin 1953 ? France autumn storm petrels Kennedy et al. 1954 storm Ireland autumn Skuas Kies and Tomek 1990 bycatch Poland winter grebe, duck, auks Larsson 1960 ? Sweden spring Fulmar Leopold et al. 1986 cold Netherlands winter Seaduck Leopold and Camphuysen 1992 oil Netherlands winter fulmar, gannet, auks Lloyd et al. 1974 food Irish Sea winter Mixture Lönnberg 1927 storm Sweden Kittiwake Louzis et al. 1984 storm Channel winter kittiwake MacPherson 1892 storm UK autumn storm petrels Mathiasson 1963 ? Sweden spring fulmar McCartan 1957 storm UK winter kittiwake Mead 1974 storm Irish Sea Meek 1985 oil Orkney seaduck Mehlum 1980 oil North Sea fulmar, gulls, auks Meininger et al. 1991 cold Netherlands winter grebes, seaduck Mudge et al. 1992 food UK–northeast all Nelson 1880 storm UK autumn skuas Nelson 1911 storm UK–east autumn skuas, Sabine’s gull Oldén et al. 1986 bycatch Sweden winter auks O’Donovan and Regan 1950 storm Ireland–west autumn auks Parrack 1967 oil UK–northeast winter auks Partridge 1993 oil Channel winter auks Pashby and Cudworth 1969 storm North Sea winter fulmar Poulsen 1957 storm Denmark aut/win auks Proger and Paterson 1913 storm UK–west winter auks Rittinghaus 1956 oil German Bight spring seaduck, terns Robinson 1909 storm Irish Sea autumn storm petrels Sage 1979 oil Shetland seaduck Sage and King 1959 storm? UK autumn storm petrels Seilkopf 1955 storm German Bight winter gulls Sergeant 1952 storm UK aut/winter auks Soikkeli and Virtanen 1972 oil Finland seaduck
ICES Coop. Res. Rep. No. 232 65 Author Year Wreck Region Season Species Swann and Butterfield 1996 food? UK–northeast winter auks Swennen and Smit 1991 parasite Netherlands summer seaduck Swennen and Spaans 1970 oil German Bight winter seaduck, auks Swennen and van den Broek 1960 parasite Netherlands summer seaduck Tasker 1994 wind UK–east winter mixture Teixeira 1985a storm Portugal winter storm petrels Teixeira 1985b bycatch Portugal winter auks Teixeira 1987 storm Portugal autumn storm petrels Underwood and Stowe 1984 food UK–east winter auks Wheeler 1990 storm North Sea autumn auks Witherby 1912 storm UK–east autumn auks Wrånes 1988 cold? Norway winter seaduck Wynne-Edwards 1953 storm UK–north autumn storm petrels Wynne-Edwards 1963 storm UK–east autumn skuas Zoun 1991 chemical Netherlands winter gannets, auks Zoun et al. 1991 chemical Netherlands winter seaduck, auks
Western North Atlantic Brewster 1906 storm autumn auks Cramer 1932 storm? phalaropes Eliot 1939 storm storm petrels Murphy and Vogt 1933 storm winter auks Snijder 1953 storm winter auks Sprunt 1938 storm autumn auks Stenhouse and Montevecchi 1996 storm aut/winter auks Stone 1965 storm winter auks Stone 1965 storm autumn phalaropes
Other regions: Bailey and Davenport 1972 food Alaska winter auks Batchelor 1981 ? South Africa spring petrels Bond 1971 storm NW Pacific phalaropes Bourne 1981 storm? S Pacific spring petrels Carter 1985 ? Australia inc diving petrels Crochett and Kearns 1975 food? New Zealand penguins Crochett and Reed 1976 storm New Zealand fulmars Gabrielson and Jewett 1970 storm? NW Pacific phalaropes Jury 1991 storm South Africa winter Nakamura et al. 1983 storm Japan autumn storm petrels Nevhaev 1993 ? Sakhalin summer fulmars, auks Pelt and Piatt 1995 food? Alaska winter auks Piatt and Lensink 1989 oil Alaska winter mixture Ryan et al. 1989 storm South Africa winter petrels Vernon 1988 ? South Africa winter storm petrels
66 ICES Coop. Res. Rep. No. 232