Kendall and Nakatani: Early life history of Theragra chalcogramma in Shelikof Strait and Funka Bay 



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However, in 1983 larvae averaged 11.23mm in late 

 May (Kendall et al. 1987). No interannual differences 

 in larval growth rates were discerned for larvae col- 

 lected in Shelikof Strait in late May 1983, 1985, 1986, 

 or 1987. Because larvae were larger in late May 1983 

 than in 1985, 1986, or 1987, they may have been sur- 

 vivors of an earlier spawning than those observed in 

 the other years (Yoklavich and Bailey 1989). 



Early juveniles 



Young-of-the-year juvenile walleye pollock (18-73 mm) 

 have been sampled extensively in Funka Bay using mid- 

 water and bottom trawl nets (Nakatani and Maeda 

 1987). The juveniles are about 34 mm in late May, 

 36 mm in June, and 55-80 mm in late July. In June, 

 juveniles (22-66 mm) are found mainly at 25-30 m at 

 night and at 10-15 m during the day (Nakatani and 

 Maeda 1987). The juveniles move deeper in the water 

 column in May and June, and by late July most are on 

 the bottom (Nakatani and Maeda 1987). In July, larger 

 fish are caught in bottom trawls while smaller fish are 

 still in the water column. As the juveniles grow and 

 move toward deeper water and the bottom, they also 

 move from inside the bay toward the entrance (in June) 

 and to the shelf (100-300 m) just outside the bay (by 

 August) (Nakatani and Maeda 1987). 



Some variation in size-at-date of juveniles among 

 years has been observed (Fukuchi 1976, Nakatani and 

 Maeda 1987), which may be due to interannual differ- 

 ences in growth rates, or differences in hatch dates of 

 surviving juveniles. 



Food organisms changed during juvenile growth with 

 Neocalanus plumchrus being most important in fish 



>27mm long in midwater. Juveniles collected on the 

 bottom fed on large-sized copepodids of Neocalanus 

 cristatus and Eucalanus bungii, Euphausia pacifica 

 (a euphausid), and Parathemisto japonica (an amphi- 

 pod) (Nakatani and Maeda 1987). 



Young-of-the-year juveniles from the Shelikof Strait 

 spawning were sampled with a Methot midwater frame 

 trawl (Methot 1986) in June and July 1987 (Hinckley 

 et al. 1989), and by small-mesh midwater and bottom 

 trawl surveys in late-summer of several years (Bailey 

 and Spring, in review). Data from these studies have 

 not yet been completely analyzed. However, in June 

 and July the early juveniles (mainly 20-30 mm) were 

 found on the shelf along the Alaska Peninsula. As with 

 the eggs and larvae, they formed a large discrete patch 

 surrounded by a large area with lower abundances. 

 From their pattern of distribution, it appears that at 

 this size and time of year they still inhabit midwater 

 depths and are not schooling (Hinckley et al. 1989). Fish 

 were found to feed mainly on various life stages of 

 Pseudocalanus spp., smaller fish ate primarily nauplii 

 and copepodids, while adults became more important 

 in larger fish. Differences in diet between fish sampled 

 at different locations indicated that the food organisms 

 were patchily distributed (Grover 1990). 



Sampling in late-summer has concentrated mainly on 

 the bays around Kodiak Island and along the Alaska 

 Peninsula. Considerable interannual variation in sam- 

 pling and pattern of distribution of juveniles character- 

 ized these surveys. In 1987, when the sampling area 

 in late-summer included the shelf west of the Shumagin 

 Islands, a concentrated patch of juveniles was found 

 that was likely the product of the Shelikof Strait spawn- 

 ing, i.e., the eggs and larvae that had been followed 

 through the spring during their drift to the southwest 

 from Shelikof Strait (Bailey and Spring, in review). 



Year-class determinants 



Studies of walleye pollock early life history in both 

 Funka Bay and Shelikof Strait have been designed to 

 determine causes of year-class fluctuations. The basic 

 premise is that these fluctuations result from events 

 during early life history and have little relation to the 

 abundance or other characteristics of the spawning 

 population. The influence of hydrography and its ef- 

 fect on larval food supply has been the most intensive- 

 ly studied factor in both areas, but predation has also 

 been considered in Shelikof Strait research. 



In Funka Bay, walleye pollock early life history 

 seems to be closely tied to the timing and extent of 

 the influx of Oyashio Water (Nakatani 1984). This 

 cold, low-salinity water carries Psevdocalanics minutus 

 into the bay where they produce nauplii, the primary 

 diet of small larvae in nearsurface waters. Walleye 



