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

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Foraging Strategies of Great Cormorants Phalacrocorax Carbo Carbo Wintering North of the Arctic Circle 481059_v2 23/3/01 7:08 am Page 59 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 © 2001 British Trust for Ornithology 481059_v2 23/3/01 7:08 am Page 60 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 mid- winter. 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 © 2001 British Trust for Ornithology, Bird Study, 48, 59–67 481059_v2 23/3/01 7:08 am Page 61 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.
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