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