SEA ICE AS A FACTOR IN SE ABIRD DISTRIBUTION 



15 



is released into the water. These algae are im- 

 portant because they provide at least 25 to 

 30% of the productivity in coastal waters and 

 allow the biological growing season to begin 

 before the open-water plankton bloom occurs 

 (Alexander 1974). In nearshore areas close to 

 Barrow, large populations of mysids and am- 

 phipods are associated with the decomposing 

 ice. At least in certain areas, these ice-asso- 

 ciated zooplankton populations are a major 

 food source for nearshore migrants, especially 

 red phalaropes, arctic terns, and Sabine's 

 gulls (Xema sabini). 



The effects of ice scour on the shoreline and 

 the nearshore bottom of the Chukchi and 

 Beaufort seas is demonstrated by the absence 

 of sessile benthic fauna and flora. The effect 

 this absence has on birds is seen in the feeding 

 habits of nearshore birds. Oldsquaws and 

 eiders, which frequently feed on molluscs, feed 

 instead on motile benthos species such as 

 mysids, amphipods, and isopods. The emperor 

 goose (Philacte canagica) is absent from the 

 northern Chukchi and Beaufort seas, ap- 

 parently due to the absence of eelgrass beds. 

 Ice scour is the major cause of the absence of 

 eelgrass in northern Alaska (C. P. McRoy, 

 personal communication). 



The offshore ice in the Chukchi decomposes 

 more rapidly than that in the Beaufort, 

 largely because Bering Sea water enters the 

 Chukchi through the Bering Strait (Coach- 

 man and Barnes 1961). By late July the Chuk- 

 chi is usually ice free as far north as Icy Cape. 

 In the Beaufort, however, ice decomposition 

 occurs slowly through June and July, and 

 only in August does a definite strip of open 

 water develop between the shore and the edge 

 of the pack ice. The amount of open water 

 varies greatly from year to year. In certain 

 years the Beaufort is not navigable due to the 

 lack of open water. 



Aerial censusing in June and July shows 

 that bird densities on the offshore ice are ex- 

 tremely low. In August and September, when 

 shipboard censusing can be conducted, densi- 

 ties on the pack ice in both seas are about 10 

 birds/km 2 . Unlike the Bering Sea, where densi- 

 ties south of the ice are much less than on the 

 ice, bird densities south of the ice in the Beau- 

 fort and Chukchi seas are slightly higher in 

 the open water south of the ice, averaging 

 about 20 birds/km 2 . In the Chukchi the prin- 



cipal species encountered on the ice are the 

 black-legged kittiwake and the thick-billed 

 murre. In the Beaufort, red phalaropes, old- 

 squaws, and glaucous gulls are the most com- 

 mon species. 



Numerous arctic cod are associated with 

 the underside of the summer pack ice. Ship- 

 board censusing in the ice is complicated 

 when cod are stranded on ice floes, as the ice 

 shifts under the weight of the ship. Gulls, arc- 

 tic terns, and jaegers gather behind the ship 

 to feed on these fish; mixed flocks of more 

 than 100 birds are common. In the absence of 

 a ship to provide the disturbance needed to 

 make large numbers of cod available, these 

 birds are dependent on locating the fish in the 

 surface waters next to ice floes. Because cod 

 frequently swim over underwater ice shelves 

 they are highly visible from above and should 

 be easily accessible to aerial feeders. 



Fall 



Chukchi and Beaufort Seas 



By the time ice formation begins in late Sep- 

 tember or early October, most seabirds have 

 left the Arctic on their southward migration. 

 The principal exception is the oldsquaw, 

 which does not begin its migration until Sep- 

 tember. Some oldsquaws remain in nearshore 

 waters until they are driven out by the forma- 

 tion of new ice. In contrast to the spring mor- 

 tality, there are few records of extensive bird 

 mortality in the fall due to lack of open water. 

 One instance was reported for 1975, when 

 nearshore waters froze early and flightless 

 eiders were seen sitting on the ice near Pt. Lay 

 in the Chukchi Sea. The birds were in a 

 weakened condition, apparently due to their 

 inability to obtain food (W. J. Wiseman, per- 

 sonal communication). 



In the offshore waters the species asso- 

 ciated with the pack ice in September are the 

 same as those in August. In late September, 

 however, ivory and Ross' gulls become the 

 most common species at the ice edge in the 

 Chukchi. Glaucous gulls and black guillemots 

 are also associated with the advancing ice 

 edge (Watson and Divoky 1972). Except for 

 the Ross' gull, which apparently winters in 

 the arctic basin, these species remain with the 

 ice as it advances into the Bering Sea. 



