KENDALL ET AL.: GROWTH OF LARVAL WALLEYE POLLOCK 



Feeding of 

 Walleye Pollock Larvae 



The diet composition of larval walleye pollock 

 in Shelikof Strait is similar to that described for 

 walleye pollock larvae collected in the southeast- 

 ern Bering Sea (Clarke 1978) and Uchiura Bay, 

 Hokkaido, Japan (Kamba 1977). Copepod nauplii 

 and copepodids of Pseudocalanus spp. were the 

 dominant food items in the guts of 6-20 mm lar- 

 vae in all these studies. As in this study, copepod 

 eggs were also abundant food items. It is difficult 

 in any of these studies to judge if the eggs were 

 captured as individual food items or along with 

 adult female copepods. 



Feeding by larvae in the Gulf of Alaska is 

 highest during daylight hours, as observed in 

 other studies (Kamba 1977; Clarke 1978). Clarke 

 (1978) reported that the few collections made at 

 sunrise had larvae with the lowest feeding inci- 

 dences. Kamba (1977) also reported that the low- 

 est feeding incidences and the lowest abundance 

 of food in the gut occurred near sunrise. 



The high densities of larvae in the Shelikof 

 Strait seem to have little effect on their food 

 habits. Oithona spp. are abundant in the Bering 

 Sea, Gulf of Alaska, and Uchiura Bay, Japan, and 

 are intermediate in size between Pseudocalanus 

 spp. copepodids and copepod nauplii. Oithona are 

 an important component of the diet of pollock lar- 

 vae in the Bering Sea, accounting for more than 

 25% of the total number of food items for larvae 

 between 11.8 and 17.7 mm (Clarke 1978), but are 

 rare in guts of larvae collected near Hokkaido 

 (Kamba 1977), and represent <167c of the food 

 items for all size groups in the present study. 

 Kamba (1977) cited the low incidence of occur- 

 rence of this food item in larvae collected in Uchi- 

 ura Bay as evidence of selective feeding by 

 walleye pollock larvae. 



The zooplankton species composition in the 

 oceanic and outer shelf regions of the Bering Sea 

 (Cooney and Coyle 1981; Smith and Vidal 1984) is 

 similar to that described for the northern Gulf of 

 Alaska and Ocean Station P (Le Brasseur 1965; 

 Damkaer 1977; Fulton 1983; Miller et al. 1984). 

 The Shelikof Strait species composition is similar 

 to these areas. Our zooplankton sampling did not 

 include copepod nauplii so we cannot assess their 

 abundance. The size distribution of copepod nau- 

 plii ingested, however, indicates that Pseudo- 

 calanus spp. and Oithona spp. are the probable 

 sources of the copepod nauplii ingested by larval 

 walleye pollock in Shelikof Strait. 



Daily production of copepod nauplii at a single 

 station in the Bering Sea has been estimated to be 

 27,094 m^^, of which more than 95% was Pseudo- 

 calanus spp. (Dagg et al. 1984). The abundance of 

 Pseudocalanus females ranged from 9.9 to 258.9 

 m''^ (x = 87.7). The mean abundance of Pseudo- 

 calanus females in Shelikof Strait was 244 m"^, 

 or 2.6 times greater than the mean abundance in 

 the Bering Sea. Assuming the same rate of daily 

 production, about 69,000 nauplii m"^ would be 

 produced in Shelikof Strait. Mean abundance of 

 walleye pollock larvae where Dagg et al. (1984) 

 performed their study was 6.3 larvae m"^, 

 whereas at the diel station in Shelikof Strait the 

 abundance was 156 m^, about 25 times greater 

 than in the Bering Sea study. If these larvae ate 

 nauplii at the same rate as those in the Bering 

 Sea, 18.3 per day, they would eat about 24% of the 

 production, as opposed to the <1% in the Bering 

 Sea. Other factors such as the relationship be- 

 tween size of larvae and daily ration need to be 

 investigated before more precise estimates of the 

 impact of larval feeding and the possibility of food 

 limitation can be made. It appears that enough 

 nauplii were being produced to preclude density 

 dependent food restrictions at the larval densities 

 observed in the present study. 



Growth of 

 Walleye Pollock Larvae 



Growth rates were similar in areas of both high 

 and low density (Table 5, Fig. 13). It cannot be 

 determined from our study whether density- 

 dependent factors modified larval growth; growth 

 variations could be produced by patchy distribu- 

 tions of prey. Walleye pollock larvae have been 

 shown to grow faster in the laboratory at higher 

 food densities (Bailey and Stehr 1986), further, 

 where lower or constant larval densities interact 

 with variable prey density, field studies have 

 shown variability in growth (Govoni et al. 1985). 

 Without knowledge of prey availability at each 

 location, however, it is difficult to discern if high 

 densities of prey coincide with dense patches of 

 larvae. The relatively low growth rates found at 

 two adjacent stations outside the patch (Stations 

 6, 12; Table 6) might indicate an area of less than 

 adequate prey availability. 



Growth rates for fishes can be influenced by 

 environmental factors such as temperature, as 

 well as availability of adequate food supplies 

 (Boehlert and Yoklavich 1983). Within species. 



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