584 



Fishery Bulletin 103(4) 



(Ryer and Olla, 1999; Ryer et al., 2002); therefore it may 

 be that longer days at northern latitudes make a north- 

 ward feeding migration beneficial by possibly providing 

 an extended window of search time if the pollock happen 

 to be in a locally depauperate area. However, day-length 

 remains long enough in the entire Bering Sea for pollock 

 to feed to satiation, and their gastric evacuation rate is 

 slow (Dwyer et al., 1987), making the need to entirely 

 fill their stomachs every day very unlikely. 



62" N 



6CTN 



58 : N - 



56 : N - 



54°N 



62°N - 



60°N 



58 N - 



56"N 



54°N 



180°W 175°W 

 1 



1 70"W 



t — r 



165°W 180°W 



At this time it is impossible to assess which factor 

 is most important in driving pollock migrations, but in 

 summary we can conclude that pollock, as visual pelag- 

 ic predators, benefit from northward feeding migrations 

 during seasonal warming. Because three of the factors 

 (excluding current) are similar throughout the Northern 

 Hemisphere, we should see similar migration patterns 

 for other pelagic fish of the north. Other examples in- 

 clude Pacific hake migrating along the North American 

 west coast from California to British Columbia 

 (Francis and Bailey, 1983; Dorn, 1995). Her- 

 ring in the Norwegian Sea undergo seasonal 

 feeding migrations in the northwestern direc- 

 tion from the south-central coast of Norway to 

 the areas located northeast of Iceland (Ferno, 

 1998). Blue whiting, mackerel, and capelin 

 from the north Atlantic undergo northward 

 feeding migrations (Nottestad et al., 1999). 

 Pacific saury (Cololabis saira), chub mackerel 

 (Scomber japonicus). Pacific sardine (Sardinops 

 sagax melanosticta), and Japanese anchovy 

 (Engraulis japonicus) from the western North 

 Pacific are reported to migrate northwards 

 during the summer (Novikov, 1986). Capelin, 

 Atlantic cod, and haddock in the Barents Sea 

 migrate north towards a "preference" tempera- 

 ture during summer (Wielgolaski, 1990). All 

 these species have characteristics similar to 

 those of Bering Sea pollock — that is, a pelagic 

 or semipelagic life style, a diet of zooplankton, 

 winter or spring spawning activity, and feed- 

 ing migrations that take place during spring 

 and summer. 



64°N 



62°N 



60°N 



58°N 



56° N 



- 64 'N 



_ 62- N 



- 60"N 



-\-- 58°N 



- 56°N 



175'W 



170 W 



165 -W 



1 60"W 



Figure 5 



Bottom water temperature contours during the bottom trawl 

 survey in the coldest year (1999 — upper map I and warmest 

 year (1996 — lower map). 



Why is temperature important? 



Temperature may affect the proportion of the 

 stock that is in the standard EBS survey area. 

 Ianelli et al., 3 using population modeling, esti- 

 mated that fewer pollock were detected during 

 the BT survey in the EBS with increasing tem- 

 perature, and fewer pollock would indicate that 

 pollock are probably leaving the survey area 

 during seasonal migrations. We conclude that 

 a significant part of the EBS pollock popula- 

 tion migrates into the Navarin-Anadyr area, 

 which can have an impact on the way the EBS 

 stock is managed. We should account for land- 

 ings of pollock in the Navarin-Anadyr area, 

 estimate how much of these landings include 

 pollock from the EBS stock, and use this esti- 

 mate in determining the EBS total allowable 

 catch. Further research is needed to quantify 

 the proportion of the EBS stock migrating 

 into the Russian fishing zone and to estimate 

 the number of pollock caught there. Stokes 5 

 suggested that the biomass estimates from 

 the NBS are in the range of 0.5-1.0 million 



' See next page for footnote text. 



