Seabird distribution in the northern Barents Sea marginal ice-zone during late summer FRIDTJOF MEHLUM Mehlum, F. 1990: Seabird distribution in the northern Barents Sea marginal ice-zone during late summer. Polar Research 8 , 61-65. The abundance of five seabird species in ice-covered parts of the northern Barents Sea in late summer 1982 was analysed in relation to differences i n sea-ice coverage. The dominant species was the Little Auk ANe alle. Differences between the seabird species in the use of the sea-ice habitat are partly explained by differences in their feeding ecology, and, for alcid species, by the need for open water for getting airborne. All species, except for the Fulmar Fulmarus glacialis, took as part of their diet organisms associated with the subsurface of the sea-ice. The Little Auk fed mainly on large copepods and pelagic amphipods, and was observed mainly in areas with low ice-cover where such zooplankters are abundant. Fridtjof M e h b m , Norsk Polarinstifutt, P. 0. Box 158, N-1330 Oslo Lufthaun, Norway; September 1989 (revised February 1990). The marginal ice-zones in arctic and antarctic waters are generally thought to be important to seabirds (Brown 1989). The pelagic distribution patterns of several species, e.g. the Ivory Gull Pagophila eburnea and Ross' Gull Rhodostethia rosea in the Northern Hemipshere (Mehlum 1989), and Snow Petrel Pagodroma niuea, Emperor Penguin Aptenodytes forsteri, and Adelie Penguin Pygoscelis adeliae in the Southern Hemisphere (Cline et al. 1969; Ainley et al. 1984), are limited by the extent of the sea-ice. Other species occur in high densities along ice-edges and in the marginal ice-zone (Irving et al. 1970; Divoky 1979; Renaud et al. 1982; Brown 1985; Fraser & Ainley 1986). However, few studies have conclusively demonstrated that the occurrence of seabirds in sea-ice is a direct result of searching for prey, or of physical features (fronts, convergences, etc.) which may coincide with concentrations of prey (Fraser & Ainley 1986; Hunt this volume). In summer, in the northern Barents Sea, the ice-edge and the marginal ice-zone retreat north- wards (Vinje 1985). A high biological production is associated with the ice-zone during this retreat (Sakshaug & Skjoldal 1989), and provides food for the large number of seabirds (Mehlum 1989). Different seabird species in this area prey upon different food types according to their mor- phological limitations. Some are surface feeders, while others are pursuit divers which hunt their prey in the free water masses or at the under- surface of the sea-ice. Sea-ice will naturally limit the access t o the sea for foraging seabirds, especially for surface feeders. Because planktonic organisms are patchily distributed at scales of metres to hundred of metres (Haury et al. 1978), it is likely that the distribution of the different seabirds at sea and within the sea-ice zone will reflect the availability of suitable prey. In this paper 1 present seabird abundances in different concentrations of sea-ice east of Svalbard, and discuss the results in relation t o the feeding eco- logy of the birds. Material and methods The survey was made from R/V 'Lance' 22-25 August 1982 in the marginal ice-zone between Kong Karls Land and Frans Josef Land. I made 122 10-minute counts between 79" 17'N, 40"24'E and 79"00'N, 31" 14'E. Ice-cover was recorded and averaged over each 10-minute period on a scale from O / 8 (no ice) to 8/8 (100% ice-cover). Data from 1-2, 3-4, 5-6, and 7-818 were pooled in order to increase the sample size in each category. No 10-minute periods had 018 ice-cover. Seabird counts were made by recording all birds observed by the naked eye within 180 degrees i n front of the vessel when moving at >5 knots (Mehlum 1989). The survey occurred after the breeding season, so as not to bias the results by foraging distance 62 F. M e h l u m limitations of birds associated with breeding col- onies. Some seabirds were collected for stomach con- tent analyses. The procedure and complete results were reported in Mehlum & Gjertz (1984). Results The most commonly seen species was the Little Auk Alle alle, which accounted for 63.4% of the birds observed (Table 1); the Black Guillemot Cepphus grylle was the second most numerous species. Only 17 Briinnich’s Guillemots Uria lom- uia, one of the most numerous species in the northern Barents Sea seabird community (Mehlum 1989), were seen. Twenty-one Ross’ 50 40 30 20 10 Table I . Total number of individuals observed among the seven most numerous seabird species between Kong Karls Land and Frans Josef Land, August 1982. Median ice-cover for the obser- vations of each species i s also presented. ~~ - Species Number % Median ice-cover - Little Auk Black Guillemot Kittiwake Fulmar lvory Gull Ross’ Gull Briinnich‘s Guillcmot Alle ulle 4,360 63.4 318 Rissa triductyla 736 10.7 418 Pagophila ehurnea 196 2.8 418 Rhodostethiu rosea 2 I 0 . 3 418 Cepphus grylle 1,154 16.8 218 Fulmarus glacialis 395 5.7 318 Uriu lomoiu 17 0.2 318 0 1-2 3-4 5-6 7-8 (25.4%) (41.9%) (23.0%) (9.8%) ice-cover Fig. 1 . Rclative abundances of five seabird species in relation to ice-cover. The Y-axis represents the percentages of the 122 10- minute observation periods within which the species were present. The percentages of the total transect length in each ice-cover category are given in parentheses on the X-axis. Seabird distribution in the northern Barents Sea 63 t t occurred more frequently in areas with dense ice- cover (7/8-8/8) (X2 = 96.0, df = 1, p < 0.001). Ross' Gulls were seen in ice-cover from 3/8 to 7/ 8, and 57% of the observations were made in 3/ H / 8 . The sample size is too small to determine preferences of ice coverage. In Fig. 2. the abundance of Little Auks along the transect is compared to the salinity profile (Gammelsr0d unpublished) and the density of calanoid copepods sampled below the sea-ice sub- surface (Gulliksen 1984). In the eastern part of the section, there was a strong salinity and tem- perature stratification above 20-25 m, with low salinities and temperatures near the surface. All the under-ice zooplankton stations within the No Copepodslm' 1 161 68 132 45 7 2 t ' $ t NO. Llllle AukSI10 mln\ 10.6 1 45.7 I 57.0 1 53.0 b 1 I ' , I T ' " ' " ' ' I ' I - D ID0 200 5m SECTION: S A L I N I T Y [ P S S 181 T l M E : 8 2 . 8.2(: 8.45 - 82. 8.25:23.29 POS: 78.82ON 2 1 . 5 O 0 E - 1 9 . 5 1 ' Y 42.58OE O l S T IKMl: Fig. 2. Salinity profile for the section investigated. Mean num- bers of Little Auks A l l e alle observed per 10-min observation period and dcnsity per mz of copepods below the sea-ice are also given. (Salinity profile, after T. G a n i m e l s r ~ d unpublished; copepods, after Gulliksen 1984.) Gulls were seen. This species was recorded in 9 of the 122 10-minute periods. Both the Little Auk and the Black Guillemot were less frequent in dense ice cover (7/8-8/8) than in areas with more open water (X2 = 348.2, df = 1, p70% frequency of occurrence in stomachs investigated. * * L y c o d e s * * * L y c o d e s , Cottidac * * * * Mammalian blubber Cephalopods 64 F. M e h l u m pelagic prey, i.e. polychaetes (Nereis sp.), cepha- lopods (Gonatus sp.), and the pelagic amphipod Parathemisto libellula. The main diet of Little Auks was copepods Calanus sp. and Parathem- isto. In the northern Barents Sea the fauna of sympagic animals is well developed, reaching a mean biomass of 9.6 g per m2 sub-surface of sea- ice, with higher values in multi-year ice than in first year ice (Gulliksen 1984; Gulliksen & L0nne 1989). Discussion In late August 1982, there were large numbers of Little Auks in the marginal ice-zone in the area between the eastern parts of Svalbard (Kong Karls Land) and Frans Josef Land. According to the distribution maps presented by Mehlum (1989), this is probably a regular phenomenon. The area is also important for other seabird species, but they are less common than the Little Auk. Species such as the Ivory Gull, Ross’ Gull and Black Guillemot are often associated with areas with sea-ice in the northern Barents Sea during late summer, while others such as the Fulmar and the Kittiwake are distributed both in ice-covered and open waters farther south at this time of the year (Mehlum 1989). A high pro- portion of the Little Auks and Black Guillemots are immature and moulting individuals (Mehlum 1989). The less frequent occurrence of Little Auks and Black Guillemots in very dense sea-ice (7/8-8/8) may partly be due to their need for sufficient open water between the ice-floes to take off into the air. Gulls, on the other hand, which can take off vertically, are associated with dense ice where they search for prey which may be exposed when ice-floes grind together in the moving pack-ice or when they turn upside down. The polar cod was a major prey of Kittiwakes, Ivory Gulls and Black Guillemots. The youngest year-groups of polar cod are known to associate with sea-ice, the typical habitats being between sandwiched ice-floes or in melting holes and crev- ices in the ice ( L ~ n n e & Gulliksen 1989). In contrast, the Little Auk does not seem to be dependent on the sympagic fauna, but feeds mainly on copepods and young specimens of the amphipod Parathemisto libellula living in the water column. Bradstreet (1982) also reported that little Auks selected the oldest age-stages of the largest potential prey in his study area in the Canadian Arctic. Similar information is reported from Novaya Zemlya (Zelickman & Golovkin 1972). The strong stratification in temperature and salinity observed above 20-25 m depth along the ice-edge was caused by melting of sea-ice, forming a relatively stable and nutrient rich body of water in which primary production is high (Sakshaug & Skjoldal 1989). The algal populations are grazed upon by the growing stages of zooplankton, i n these waters dominated by Calanus glacialis. According to Sakshaug & Skjoldal’s (1989) model of the abundance of Calanus glacialis along the ice-edge in the northern Barents Sea, the largest (oldest) age-stages should be found in the outer part of the ice-zone, grazing on the phytoplankton bloom. It was in these areas with low ice-cover that we also found the highest density of Little Auks. In other geographical areas, large concen- trations of the Little Auk have been reported in association with sea-ice, continental slopes and at different oceanic ‘fronts’ (Bradstreet & Brown 1985), where they feed upon the prey that tend to accumulate there in high densities (Pingree et al. 1974; Bradstreet 1980, 1982; Brown 1980; Bradstreet et al. 1981). 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