Foraging Strategies of Great Cormorants Phalacrocorax Carbo Carbo Wintering North of the Arctic Circle

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Bird Study (2001) 48, 59–67

Foraging strategies of Great Cormorants Phalacrocorax carbo carbo wintering north of the Arctic Circle

ROGER JOHANSEN1,2,, ROBERT T. BARRETT1* and TORSTEIN PEDERSEN2 1Zoology Department, Tromsø University Museum, N-9037 Tromsø, Norway and 2Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway

This study describes how 30 Great Cormorants Phalacrocorax carbo carbo managed to catch sufficient food for their daily energetic needs under conditions of reduced daylight and cold while wintering north of the Arctic Circle. Activity observations showed that the Great Cormorants’ daily foraging pattern was generally bimodal, with morning and evening feeding peaks. They compensated for shorter daylengths in midwinter by starting to forage later and ending progressively earlier at lower light intensities. Fishing constituted only a minor part of their time–activity budget, and was one of the most efficient reported in marine birds. The Great Cormorants spent less than 60 minutes a day fishing in midwinter. Although subzero ambient temperatures and blizzards contributed to increased heat loss in midwinter, this potential energy loss did not seem to be compensated for by an increase in fish intake. Instead the Great Cormorants seemed to economize energy expenditure by halving the time spent at sea, and halving the number but doubling the mass of each fish taken.

Thermoregulation in cold-climate animals that they are poorly insulated and their feathers depends mainly on body temperature, insula- become wet when diving (Rijke 1968, Siegfried tion, activity and lower critical temperature, as et al. 1975). Although feather wetting is associ- well as a combination of wind strength and ated with reduced swimming cost through ambient air and sea temperatures (Schmidt- reduced buoyancy, it also increases heat loss Nielsen 1990). In northern Norway, thousands considerably compared with other divers of visual feeding seabirds and sea ducks (Siegfried et al. 1975, Hustler 1991, Wilson et al. wintering in the fjords within the Arctic Circle 1992). This energy loss may be compensated for experience reduced daylengths, subzero ambi- by increased food intake and feeding activity ent temperatures, sea surface temperatures (Carss 1997), by increased prey capture rates around 0°C, blizzards, and sometimes ice- (Kruuk & Carss 1996) or by decreased activity covered foraging areas. These factors not only and metabolic rate, since body temperature and increase thermoregulatory demands directly, metabolic rate are activity-dependent (Birt- but also limit the time available to feed through Friesen et al. 1989, Wilson & Grémillet 1996). reductions in daylength. However, being visual predators, feather For the 5–10 000 Great Cormorants Phalacro- wetting is a further disadvantage in that it corax carbo carbo (equivalent to 15–30% of the limits the lengths of fishing bouts in what are population breeding north of the Arctic Circle) already short periods of daylight. which winter in the Arctic (N. Røv pers. To ensure sufficient energy intake, predators comm.), the situation is exacerbated by the facts can trade-off between taking numerous and abundant but small prey items, or taking few *Correspondence author. and larger but less abundant prey. This is Email: [email protected] not necessarily a simple question of prey

60 R. Johansen, R.T. Barrett and T. Pedersen

availability, but also one of cost–benefit consid- Sørfjord erations, time available for foraging and the risk of overnight starvation (Kruuk & Carss 1996). Feeding site selection has also been shown to be important for the foraging Arctic 25 success of cormorants, which catch fish on both circle vegetation-covered and naked sea beds, mostly 50 in shallow water of 0–10 m depth (Blackwell & Krohn 1997, Debout et al. 1995). Cormorants are 125 visual foragers feeding during daylight and twilight hours (Siegfried et al. 1975, pers. obs.) such that sufficient light and daylength are Main day roost essential for successful foraging. Daylength will also influence their activity pattern and diving-depths as reported in other seabirds (Cannell & Cullen 1998, Systad et al. 2000). The objectives of this study were to deter- Main night roost mine to what extent Arctic winter conditions determined the diurnal foraging pattern and ranges, time spent foraging and fishing yield 25 of the Great Cormorants. No detailed study of Great Cormorant foraging habits has previously been carried out so far north in midwinter. This investigation was undertaken in Sørfjord, an enclosed fjord in northern 50 Norway (69°32′N, 19°40′E), where 29–31 Great Cormorants roosted and fed every day from 25 late September to early April during the winter 1996/97 (Fig. 1). Although the sun remains below the horizon for two months (late N November – late January) at this latitude, there are c. 4.5 hours of twilight in mid-December 1 km when the sun is no more than 6° below the Figure 1. Study area in Sørfjord, northern Norway. horizon (Anon. 1997). Depths are in meters. Shallow waters (0–25 m) are shaded grey. MATERIALS AND METHODS

Study area Gadus morhua and Saithe Pollachius virens (Johansen et al. 1999). Sørfjord is an inner extension of a larger fjord, Ullsfjord with a 300-m wide and 8-m deep sill Data collection over which there is a strong tidal current. The fjord has steep bathymetric gradients and a Roosting Great Cormorants were counted at maximum depth of 133 m. The main area in their main night and day roosts, and on all sea which the Great Cormorants were active had a marks and skerries in the enclosed fjord. maximum depth of 65 m (Fig. 1). The Great Counts were made every 15 minutes from Cormorants in Sørfjord were suitable subjects dawn to dusk on three days in the middle for foraging behaviour studies due to their of each month from October 1996 to March numbers, known roost attendance and limited 1997 for a total of 165 hours. On 16 of the 18 foraging ranges. Their roost and feeding areas observation days, we also recorded which for- were within easy view from land along the aging sites they used and how far they ranged narrow fjord (Fig. 1) and their diet was from their night roost. relatively uniform and dominated by Cod Observation positions were c. 20–50 m from

Great Cormorants foraging in arctic winter 61

Table 1. Measured and predicted environmental conditions in Sørfjord in the winter 1996/97, or at nearest meteor-/hydro- logical station in northern Norway.

Condition Oct Nov Dec Jan Feb Mar

Measured Ambient temperaturea (°C) +3.9 –0.9 –4.4 -– 3.9 –4.2 –2.9 Wind forcea (m/s) 2.5 4.2 3.4 5.4 3.0 2.0 Precipitationa (mm/month) 186 79 101 234 114 100 Light intensity at zenithb (lux) 90.1 597.2 11.8 93.9 763.8 7465.0 Daylengthc (hr:min/day) 12:01 08:33 04:30 06:16 09:20 14:33 Predicted Sea temperatured 0–10 m (°C) 7.0 5.8 4.4 3.1 1.5 1.2 1% light depthe (m) 49 34 (5) (15) 37 48

Results are averages. aDet Norske Meteorologiske Institutt (unpubl. data); bJohansen (unpubl. data Anon (1999)); cin- cluding twilight periods; dfrom 1995–97 (Normann unpubl. data); efrom 1990–92 (Hegseth et al. 1995, Hegseth pers. comm.). Periods with the most critical values are in bold type.

the tide line and 10–30 m above sea level, calculate the daily individual time at sea (in min- mainly on the roads running along the west utes) = flying, loafing, diving and resting at sea and east side of Sørfjord. Mirador 15–-45 × 60 = (daily mean % Great Cormorants not on a mm telescopes on tripods, Noctron night vision roost × minutes from dawn to dusk)/100. For scope (model V) with 135-mm f1.8 lens and monthly comparisons of daily fish consumption Zenith marine 7 × 50 mm twilight binoculars rates, an index of total fish mass (g)/pellet exam- were used for observations. Concurrent light ined was used, assuming a more or less constant intensities were recorded using a Lambda I.C. but unknown pellet production rate (Johnstone quantum-meter (400–700 nm PAR) with a plane et al. 1990, Veldkamp 1995, Zijlstra & Van Eerden sensor (Table 1). Weather data were provided 1995). Statistical analyses were carried out using by the nearest meteorological station in Statistica version 5 (Anon. 1994). Tromsø, 30 km west of the study area (Det Norske Meteorologiske Institutt unpubl. data). RESULTS The tidal state was defined using local tide tables. Sea temperatures were estimated from Diurnal foraging pattern and ranges data collected at 0–10 m depth in the Sørfjord/ Ullsfjord ecosystem by the University of The Great Cormorants’ night roost was on or Tromsø and similarly, depths of the euphotic around a small lighthouse c. 800 m from land zone (as 1% of the surface light irridance) were (Fig. 1). In general, they foraged in two distinct estimated from data from adjacent fjords periods, once in the morning and once in the (Hegseth et al. 1995, Hegseth pers. comm.) early afternoon with the intervening period Diet was determined using otoliths of fish spent roosting (Fig. 2). In October, a maximum found in 135 regurgitated pellets of indigestible of 65% of all Great Cormorants foraged at any food remains. Remains of prey items were one time (Fig. 2a). As daylength and hence counted and identified and otolith length/fish available foraging time decreased from October mass relationships were taken from Breiby to December, feeding started progressively (1985), Härkönen (1986), Pedersen (1997) and later and ended earlier in the day (Fig. 2a). Johansen et al. (1999). The pellets were sampled The light intensity at the start and end of forag- at the night roost after the birds had departed ing also decreased from month to month (Table on their first daily feeding trip, on three days in 2). Even on the shortest days, no more than the middle of each month. Pellets were mostly 50% of the birds foraged at any one time (Fig. collected after the observations to avoid inter- 2a). fering with the behaviour studies. In December and January, the Great Because individual birds could not be recog- Cormorants often experienced difficult condi- nized, an ‘average individual’ was used to tions, including heavy snow falls. In the two

62 R. Johansen, R.T. Barrett and T. Pedersen

Darkness Twilight Obs.day 1 Obs.day 2 Obs.day 3 a b 60 October January

60 November February

10 Cormorants at sea (%) 0

60 December March

0 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 Time of day (h) Time of day (h) Figure 2. Diurnal foraging pattern of Great Cormorants wintering in Sørfjord, northern Norway. (a) 30–31 individuals, October–December 1996 and (b) 29 individuals, January–March 1997. Observations every 15 minutes from dawn to dusk, on three days in the middle of each month.

shortest days of this study (days 2 and 3 in increased further and became more evenly December), there were heavy blizzards with spread across the day and the birds left the strong winds (>10 m/s) and reduced light con- roost at a lower mean light intensity than ditions (c. 12 lux at zenith). On these days there in January (Table 2). In March, the activity was no evidence of a bimodal activity periods were even more extended, with distinct pattern (Fig. 2a). In January, foraging started foraging breaks at noon (Fig. 2b). earlier and ended later in the day and at higher There was no evidence of a tidal influence on light intensities than in December (Table 2), and fishing activity; foraging peaks occurred at the bimodal activity pattern again became both high and low tide, or during ebb or flow. evident (Fig. 2b). In February, time spent at sea Most of the Great Cormorants fished within the

Great Cormorants foraging in arctic winter 63

Table 2. Mean light intensity (95% CI) when the Great Cormorants started and ended their daily fishing activities in the winter 1996/97 in Sørfjord, northern Norway.

Light intensity Oct Nov Dec Jan Feb Mar

Start (lux) 160.7 (141.4) 17.4 (6.1) 0.9 (0.7) 15.3 (13.3) 9.7 (5.1) 332.6 (279.0) End (lux) 149.5 (19.4) 21.3 (9.1) 2.7 (2.4) 18.7 (9.0) 48.2 (52.8) 439.6 (360.0)

Start, when more than 10% of the Great Cormorants have left the night roost in the morning; end, when more than 90% of the Great Cormorants have returned to the night roost in the evening. Periods with the most critical values are in bold type.

shallow area around the roosts (Fig. 3). A total Foraging yield and effort of 72% of the foraging sites were within 4 km of the night roost and 96% within 8 km (n = 181). Fish were the most important prey, and gadoids The foraging ranges were similar across the constituted 97% by number and biomass of the study period (mean = 3.2 km ± 2.0 sd). fish prey. Cod and Saithe constituted 86% and 11% respectively of the total prey biomass (Johansen et al. 1999). Based on the analysis of 15 pellets each month, the amount of fish (num- Tidal current bers × mass) represented in each pellet was 8 km 25 relatively constant, with no significant difference between months (Kruskal–Wallis; h = 4.08, df = 5, P = 0.54) (Fig. 4). 50 However, Great Cormorants fed on signifi- cantly fewer but larger fish in the midwinter 125 6 km months than earlier and later in the season (Table 3). Although the overall fish content per Main day roost pellet tended to be higher in midwinter than in early/late winter, the difference was not 4 km significant (Table 3). Comparable results from the previous winter showed the same patterns in seasonal changes in mean fish numbers, mass Main night roost and overall content per pellet (Table 3). There

300

25 Nov 2 km 200 4 km Jan Dec Oct 25 6 km 100 Mar Feb N Mean individual fish mass (g) 0 8 km 1 km 0 1 2 3 4 5 6 7 8 9 10 11 12 Figure 3. Sites where Great Cormorants were observed Fish numbers per pellet foraging in Sørfjord, northern Norway in 1996/97 in (❍) Figure 4. Relation between fish numbers per pellet (15 October, February and March (n = 138) and () in pellets each month) and individual fish mass (mean incl. November, December and January (n = 43). Foraging 95% CI) taken by the Great Cormorants during the radii from their night roost are in km and depths are in m. winter 1996/97 in Sørfjord, northern Norway.

64 R. Johansen, R.T. Barrett and T. Pedersen

Table 3. Seasonal changes in fish numbers, mass and content in Great Cormorant pellets in Sørfjord, northern Norway.

Mean fish numbers/ Mean mass/fish (g) Mean fish content/ Time (months) pellet (95% CI) (95% CI) pellet (g) (95% CI)

(A) Nov, Dec & Jan 1996/97 3.1 (2.4, 3.9) 190 (158, 221) 595 (506, 683) (B) Oct, Feb & Mar 1996/97 6.0 (4.5, 7.5) 95 (76, 114) 586 (483, 688) Mann–Whitney U-test (A vs. B) U = 592, P < 0.001 U = 13014, P < 0.001 U = 995, P = 0.88 (C) Jan 1996 2.5 (1.6, 3.4) 247 (169, 323) 625 (458, 791) (D) Feb & Mar 1996 5.8 (3.9, 7.7) 103 (77, 130) 595 (446, 744) Mann–Whitney U-test (C vs. D) U = 111, P = 0.008 U = 1671, P < 0.001 U = 214, P = 0.93

Mean fish content = mean fish numbers × mean fish mass.

was also a significant difference in estimated foraging in cold water are limited to short time spent at sea (Mann–Whitney; U = 0, P < feeding bouts (Wilson & Grémillet 1996, Bullies 0.001) between midwinter (58.2 min/day ± 5.9 et al. 1986). There is also need for regular resting sd) and early/late winter (117.4 min/day ± 38.2 limit time for each bout (Wilson et al. 1992). sd) (Fig. 5). Birds are therefore forced to fill their stomachs and oesophagi in concentrated foraging bouts DISCUSSION with enough prey to meet the metabolic energy demands during inactive periods on land at The bimodal daily foraging pattern as observed noon and night. When the days are short, there in the Sørfjord Great Cormorants (Fig. 2) has might not be time for more than two or three several possible explanations. As regards their such foraging bouts each day. Although prey, Cod undertake a light-dependent vertical reduced light conditions in midwinter (Table 1) migration which may make them more avail- did not seem to affect their fishing effort in the able to the cormorants at dawn and dusk by morning and afternoon twilight (Fig. 2), the bringing them up into the normal diving range Great Cormorants in Sørfjord compensated for of the cormorants (Dickson 1989). the reduced daylength by starting and ending Furthermore, the daily pattern of morning fishing at progressively lower light intensities and afternoon feeding is largely due to both (Table 2). This strategy was also observed in environmental variability and to digestive Common Somateria mollissima and King Eiders constraints. Due to poor insulation and the S. spectabilis wintering in a fjord 35 km west of need for regular feather-drying, cormorants our study area (Systad et al. 2000). Under experimental conditions, another 8 visual predator, the Little Penguin Eudyptula 7 minor caught no fish at light intensities below 6 0.6 lux (Cannell & Cullen 1998). The 50% Oct, Feb, Mar threshold of catching fish was around 2.9 lux, 5 and the time the penguins spent searching for 4 the fish also declined with decreasing light. This, and Wilson et al’s (1993) implications 3 that five African penguin species stopped dives Nov, Dec, Jan 2 at light thresholds of c. 1 lux corroborate our observations of mean light intensities (0.9 Fish numbers per pellet 1 and 2.7 lux) recorded at the start and end of 0 foraging in December (Table 2). 0 30 60 90 120 150 When the chances of finding food are Time spent at sea (min per day) uncertain, diurnal birds are expected to start foraging early in the day, giving them time Figure 5. Relation between daily time spent away from roost per Cormorant and numbers of fish in each pellet to hunt further after unsuccessful feeding (mean incl. 95% CI), during the winter 1996/97 in periods. In February, the Sørfjord Great Sørfjord, northern Norway. Cormorants showed a weaker bimodality in

Great Cormorants foraging in arctic winter 65

feeding activity (Fig. 2b), and they tended to row their search image towards larger and leave their night roost at a light intensity lower more conspicuous prey, as reported in, for than that expected when compared to the example, pelicans (McMahon & Evans 1992). darker January period (Table 2). Furthermore, Secondly, low water temperatures might result they also caught more but smaller fish in in reduced swimming speeds among Cod February than during the rest of the winter (Schurmann & Steffensen 1997), thereby (Fig. 4). February is a period of normally low increasing the likelihood of being caught by ambient and sea temperatures (Table 1), and pursuing Great Cormorants. Thirdly, the over- the low catches of Cod larger than 25 cm in the all dark body (of adult Great Cormorants) may trap-nets set in Sørfjord (Johansen et al. 1999) benefit predators hunting in dense seaweed, suggest that the Great Cormorants’ preferred turbulent waters or in twilight (Siegfried et al. large cod had left the foraging area. Thus, 1975, Bretagnolle 1993), enabling them to take to catch sufficient numbers of smaller fish larger and more profitable prey items in the to satisfy their needs, the Great Cormorants dark. This may be compared with diving birds needed to start foraging earlier than expected. with a white anterior and ventral plumage (of In Sørfjord, Cod and Saithe were abundant juvenile Great Cormorants) which is presumed within the seaweed-covered shallow areas near to be advantageous under normal daylight the night roost and a Salmon Salmo salar farm conditions (Nelson 1980, Götmark 1987). near their main day roost, where most of the Finally, the change in size selection by the Great Cormorants foraged (Fig. 3) (Johansen et Great Cormorants between October and al. 1999). Great Cormorants generally feed on the November, and both winters between January sea bed in shallow water, but in Sørfjord some and February, may be due to a change in fish feeding sites were deeper than 25 m (Fig. 3). This behaviour. For example, a large and hungry suggests that the Great Cormorants either dived prey fish may take the risk of overlooking a deeper than normal or carried out more pelagic suddenly approaching predator under poor pursuits than previously reported for cor- midwinter feeding conditions (Milinski 1993). morants (Grémillet et al. 1998, Trayler et al. 1989). The light-dependent vertical migration of Cod As the winter deepened, the Great Cor- would also make them more available if they morants in Sørfjord caught fewer but larger spread more within the normal diving range of fish. This was probably advantageous to the the cormorants when it is darker, such as in birds since it enabled them to spend less time at midwinter (Dickson 1989). sea (Fig. 5), thereby reducing energy expendi- In Sørfjord, the Great Cormorants’ diurnal ture. The fact that the amount of fish per pellet rhythm and time-budget resembled that of the did not seem to increase suggests that, contrary same species wintering in France (Bullies et al. to expectations, low midwinter temperatures 1986). In Mississippi, wintering Double-crested and typical winter wind conditions (Table 1) Cormorants foraged for c. 60 min/day, but flew did not affect the cormorants’ energy intake c. 16 km to their fishing grounds and spent only during the winter in question. c. 190 min/day away from the roost (King et al. Another possible strategy documented in 1995). In our study, the Great Cormorants spent cold-water Bank Cormorants P. neglectus might even less time away from any available help explain this pattern. Wilson & Grémillet roost catching their daily food ration (Fig. 5),

(1996) found that the body temperature (Tb) probably due to the proximity to the food was environment- and activity-dependent. By resources in Sørfjord, resulting in the c. 3 km ° allowing Tb to drop by more than 5 C, mean flying distance (Fig. 3). The foraging effi- cormorants may be able to forage more efficiency of the Great Cormorants in Sørfjord ciently at water temperatures that would seemed to be higher in November–January otherwise entail a considerable energy loss. when they were able to catch sufficient food in Several hypotheses might explain why the c. 60 min than in October, February and March Great Cormorants maximized their energy when they used c. 120 min. intake by taking fewer, larger fish at low Daily fish intake in non-breeding Great ambient temperatures and reduced daylength Cormorants of subspecies carbo in low temper- in midwinter. It is possible that a lower visual ature waters has recently been predicted to be sensitivity during reduced daylight might nar- up to c. 800 g per day (Carss 1997, Grémillet

66 R. Johansen, R.T. Barrett and T. Pedersen

1997). This is about 20% more than the 661 sity of Tromsø and the Zoology Department, g/day estimated by Barrett et al. (1990) for Tromsø University Museum. Further support breeding birds, and c. 50% more than the 551 was given by the Norwegian Sea Ranching g/day estimated by Johansen et al. (1999) for Programme (PUSH). In addition to assistance Great Cormorants wintering in Sørfjord. Both from the local staff at NFH, we thank Åse the latter estimates were however based on a Alvestad for her help during the activity obser- higher energy density of prey (5.5 kJ/g as vations, Else Nøst Hegseth (NFH) for her opposed to Grémillet’s (1997) 4.0 kJ/g) and a comments on light intensity measurements, Ulf higher assimilation efficiency (85% as opposed Normann (NFH) for the hydro- graphical data to 77%). A substitution of Grémillet’s lower and Vervarslinga for Nord Norge (Det Norske values results in an increase of daily needs to Meteorologiske Institutt) for the meteorological 800–1000 g. If the Sørfjord cormorants spent data. Dr David Grémillet and Dr Barwolt only 60 min/day fishing (Fig. 5), an intake Ebbinge are gratefully acknowledged for their of 800 g/day results in a catch rate of c. 13 g constructive comments on the manuscript. fish/min spent fishing in midwinter. Even when not limiting this figure to the time spent REFERENCES under water, this corroborates Grémillet’s doc- umentation of a high foraging efficiency among Anon. 1994. Users Manual. Statistica for Windows, Great Cormorants (15.2 g/min spent under- version 5, part I. StatSoft Inc., USA. water) compared with other marine birds and Anon. 1997. Almanakk for Norge. Inst. Teor. Astrophys., mammals. To attain this catch rate, it is how- University of Oslo. ever possible that the Great Cormorants Barrett, R.T., Røv, N., Loen, J. & Montevecchi, W.A. 1990. benefited from the high prey densities recorded Diets of shags Phalacrocorax aristotelis and in Sørfjord at that time of year (Pedersen 1997, cormorants P. carbo in Norway and possible implica- Grémillet & Wilson 1999). tions for gadoid stock recruitment. Mar. Ecol. Prog. Ser. 66: 205–218. Thus, based on the activity observations and Birt-Friesen, V.L., Montevecchi, W.A., Cairns, D.K. & pellet analysis, we were not able to detect any Macko, S.A. 1989. Activity-specific metabolic rates of midwinter increase in either foraging effort or free-living northern gannets and other seabirds. fish intake. On the contrary, fishing activities Ecology 70: 357–367. actually decreased considerably. When days Blackwell, B.F. & Krohn, W.B. 1997. Spring distribution are short and temperatures are low, an activity and habitat selection by double-crested cormorants budget with long resting periods and concen- on the Penobscot River, Maine. Colonial Waterbirds trated feeding bouts possibly conserves energy 20: 66–76. and reduces the Great Cormorants’ need to Breiby, A. 1985. Otolitter fra saltvannsfisker i Nord-Norge. increase food intake. In this study, February Tromura, Naturvitenskap, No. 45. Tromsø Museum, seemed to be their ‘bottleneck’, when both the University of Tromsø. ambient and sea temperatures were low, and Bretagnolle, V. 1993. Adaptive significance of seabird col- when there were few large Cod in the foraging oration: the case of procellariiforms. Am. Nat. 142: area. The Great Cormorants responded to this 141–173. by fishing earlier in the morning at lower light Bullies, A., Jullien, J.M., Yésou, P. & Girard, O. 1986. conditions than expected, foraging more even- Activity pattern and use of space by the cormorant ly through the day, and catching a more Phalacrocorax carbo on wintering site: example of variable number of fish with lower mass the Olonne region, Vendée. Gibier Faune Sauv. 3: compared with the other winter months. From 43–86. November to January, their foraging was Cannell, B.L. & Cullen, J.M. 1998. The foraging behav- extremely efficient in that they were able to iour of Little Penguins Eudyptula minor at different light levels. Ibis 140: 467–471. catch their daily food requirement of large fish Carss, D.N 1997. Techniques for assessing cormorant in less than 60 min. diet & food intake: towards a consensus view. In Baccetti, N. & Cherubini, G. (eds.) Proc . IV Europ. ACKNOWLEDGEMENTS Corm. Conf. (Bologna, 1995). Suppl. alle Richerche di Biol. della Selvaggina: 197–230. INFS, Italy. This study was financed by the Norwegian Debout, G., Røv, N. & Sellers, R.M. 1995. Status and College of Fishery Science (NFH), the Univer- population development of cormorants Phalacroco- © 2001 British Trust for Ornithology, Bird Study, 48, 59–67 481059_v2 23/3/01 7:08 am Page 67

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(MS received 19 November 1999; revised MS accepted 8 June 2000)