FISHERY BULLETIN: VOL. 85, NO. 3 



Table 6. — Concentrations and lengths of pollock larvae collected 

 24-28 May 1981, 1982, and 1983 In the patch resulting from the 

 Shelikof Strait spawning (1981 and 1982 data from Dunn et al. text 

 fn. 5). 



1, 2). However, the abundance of larvae in 1981 

 was about 2,000 m^ while in 1983 it was only 

 about 150 m"". Mean length in 1981 was about 

 7.5 mm while in 1983 it was 11.2 mm. Since the 

 spawning dates were the same, this would indi- 

 cate a much slower growth rate (about 0.09 mm 

 d-^) in 1981 than in 1983 (0.21 mm dM. In 1982 

 there were fewer larvae (20 m"'^) and they were 

 distributed further southwest than in the other 

 2 years (Dunn et al. fn. 5). These larvae were not 

 different in length from those in 1981 (7.7 mm). 

 The position of the larvae in 1982 suggests a 

 much faster drift than in 1981 and 1983. 



Although most of the spawning occurs in the 

 deep trench (>200 m) in Shelikof Strait, the lar- 

 vae are in the upper part of the water column. 

 Southwest of Shelikof Strait the trench runs 

 south between the Semidi Islands and Chirikof 

 Island (Fig. 2). Early larvae drift in the Alaska 

 Coastal Current which flows southwest, parallel 

 to the Alaska Peninsula (Schumacher and Reed 

 1980). At the time of our surveys in May 1981 and 

 1983, larvae had drifted to the area between 

 Sutwik Island and the Semidi Islands. In 1982 

 they were further to the southwest in water over 

 the deeper trough and the continental shelf. 



Vertical Distribution of 

 Walleye Pollock Larvae 



Vertical distribution of walleye pollock larvae 

 has been addressed in detail in several other stud- 

 ies (Kamba 1977; Cooney et al. 1978; Walline 

 1981^1; Dagg et al. 1984). Although the areas of 



study, procedures, and gear have varied, a consis- 

 tent pattern of diel-vertical distribution emerges 

 from these studies and is supported by the present 

 study. Haryu (1980) summarized his and earlier 

 work by stating "larvae inhabit the mid-layer 

 rather than the surface layer and perform diurnal 

 vertical migration in search of food." Most larvae 

 have been found between 10 and 60 m, and within 

 this depth range, some larvae generally move to 

 shallower depths at night. Vertical movement is 

 not pronounced in any of the studies but is evi- 

 dent by comparing proportions of larvae at vari- 

 ous sampling depths at different times of day. In 

 general it appears that larvae <15 mm are most 

 concentrated vertically at 10-15 m at twilight, 

 both in the evening and morning. During night- 

 time and daytime the larvae are more dispersed 

 vertically, and during daytime their distribution 

 is deeper than at night. Samples confined to day 

 and night periods do not show the crepuscular 

 nature of the distribution. This pattern is seen in 

 the present study but is less pronounced than in 

 some others, possibly because we conducted our 

 sampling only in the vertical range of high con- 

 centration. Larvae larger than about 15 mm ap- 

 pear able to avoid plankton nets to some extent, 

 particularly during daytime. The available data, 

 however, suggest that these larger larvae remain 

 concentrated in a shallow depth stratum (5-15 m) 

 except at night when they are more dispersed ver- 

 tically and may rise closer to the surface (Walline 

 fn. 11). 



Fish larvae of most other species that have 

 been studied also migrate upward in the water 

 column at night (Kendall and Naplin 1981). Some 

 species undergo a much more pronounced vertical 

 migration than is apparent with walleye pollock 

 larvae and may cross much greater temperature 

 gradients than observed here. Similar to walleye 

 pollock, larvae of other fish species are visual 

 feeders, and their vertical movements are proba- 

 bly associated with a diel feeding periodicity. 

 Walleye pollock larvae may move to shallower 

 depths at night to allow more feeding in reduced 

 light. They then may spread downward in the 

 water column during daytime in response to in- 

 creased light penetration and the distribution of 

 their prey. Too little is known about predation on 

 fish larvae to assess the importance of vertical 

 movements on predator avoidance (see Incze et al. 

 1984). 



iiWalline, P. D. 1981. Hatching dates of walleye pollock 

 iTheragra chalcogramma) and vertical distribution of ichthy- 

 oplankton from the eastern Bering Sea, June-July 1979. U.S. 



Dep. Commer., Natl. Mar. Fish. Serv., NOAA, NWAFC Proc. 

 Rep, 81-05, 12 p. 



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