INTERACTIONS AMONG MARINE BIRDS AND COMMERCIAL FISH 



209 



dance in areas as shallow as 5 m (Fadeev 1965; 

 Moiseev 1953). 



The commercially important king and snow 

 crabs of the eastern Bering Sea also have lar- 

 val stages that are pelagic (Table 3). Zoeae 

 and megalopa of snow crabs are found near 

 the surface where they are vulnerable to 

 plankton-feeding marine birds. The eggs of 

 king crabs are attached to the abdomen of the 

 female, but after hatching, the larvae become 

 pelagic and occur near the surface. They are 

 planktonic through five larval stages before 

 settling to the bottom to take up demersal 

 residence (Kurata 1960, 1964). These larvae 

 attain a length of 5.5-6.5 mm and spend 33 

 days or more in the plankton (Kurata 1960). 

 Even after the young king crabs have settled 

 to the bottom, they may still frequent water 

 shallow enough to make them vulnerable to 

 predation by some marine birds. Juvenile king 

 crabs 1 and 2 years of age appear to prefer 

 shallower water than do older crabs. In south- 

 eastern Alaska, during the spring, small juve- 

 nile crabs have been observed in pods at 

 depths as little as 1 m below the low tide level. 



The available life stages of king and snow 

 crabs and commercially important demersal 

 fish (Table 3) represent an enormous food 

 supply for other fishes and marine birds. Pre- 

 dation by marine birds on pelagic eggs and on 

 the larval and juvenile stages of demersal fish 

 is not well documented, probably because the 

 rapid digestion rate of birds makes species 

 identification of these stages difficult. Inves- 

 tigators must often depend on the presence of 

 the hard parts of fish (such as scales and oto- 

 liths) in the stomachs of birds to identify the 

 species eaten. Because these hard parts have 

 not yet formed in the larvae and most juve- 

 niles, predation by marine birds on older fish 

 is more apparent on examination of stomach 

 contents. Full understanding of predation by 

 marine birds on demersal fish and shellfish re- 

 quires additional data on when and where the 

 egg, larval, and juvenile stages are present. 



Pelagic Fish 



Many fish, such as herring, capelin, smelt, 

 and salmon, are pelagic for part of their lives, 

 particularly during the spring and summer 

 feeding periods. The extent of predation by 

 marine birds on these species depends pri- 



marily on the location of their spawning 

 grounds, their growth rates, and the size of 

 the adults. The spawning location determines 

 the extent of predation on eggs, whereas 

 growth rate and adult size determine during 

 how much of its lifetime a given fish species is 

 vulnerable to the wide variety of marine birds. 



Herring spawn in intertidal and subtidal 

 zones and spend most of their postlarval lives 

 in bays or estuaries near the coast. They de- 

 posit their adhesive eggs primarily on vegeta- 

 tion, and the eggs are particularly vulnerable 

 to predation by a wide variety of marine and 

 terrestrial birds. Outram (1958) estimated 

 that gulls alone accounted for 39% of the egg 

 loss on the spawning grounds at Vancouver 

 Island, British Columbia. When herring lar- 

 vae hatch, they are between 0.7 and 0.8 cm 

 long; when they metamorphose about 6-8 

 weeks later, they are between 2.6 and 3.5 cm 

 long. Thereafter, juvenile herring grow 

 rapidly and reach a length of about 7-10 cm 

 before winter. Although herring as old as 13 

 years and up to 38 cm long have been reported 

 in Alaska, they seldom exceed 30 cm and 1 1 

 years of age (Rounsefell 1929). During spring 

 and summer, herring are commonly within 

 10 m of the surface, but in winter, they are in 

 water 100-140 m deep. Although herring are 

 particularly vulnerable to predation in spring 

 and summer, they are available to marine 

 birds during most of their life. 



The life history of capelin is somewhat dif- 

 ferent than that of herring they live in the 

 open sea near the surface and throughout the 

 water column most of their lives. Sometime in 

 June or early July, they migrate in large 

 schools toward shore to spawn (Musienko 

 1970). In British Columbia, capelin bury their 

 eggs in coarse sand and gravel in the inter- 

 tidal and subtidal zones. The larvae are 0.5- 

 0.7 cm long at hatching and are carried by 

 currents to the open sea where they develop in 

 the plankton. Capelin attain an age of 5 years 

 and a maximum length of about 22 cm; their 

 small size makes them vulnerable to preda- 

 tion by marine birds most of their lives, and 

 they are an important pelagic food fish for 

 other commercial fish in the Bering Sea. 



The sand lance reaches a maximum size of 

 20-26 cm and is vulnerable to bird predation 

 during most of its life. Little information is 

 available on the maximum age attained by 



