Abstract. — The vertical distri- 

 bution of walleye pollock eggs and 

 larvae in Shelikof Strait, Gulf of 

 Alaska, was investigated by using 

 data from 36 Multiple Opening- 

 Closing Net and Environmental 

 Sensing System (MOCNESS) tows 

 taken in April and May, 1986-88. 

 Most eggs were found below 150 m 

 to near bottom (~300 m) but pro- 

 gressively shallower later in the 

 season. Eggs in middle stages of 

 development were found at shal- 

 lower depths than were younger or 

 older eggs. The vertical distribu- 

 tion of eggs was positively related 

 to observed differences in seawa- 

 ter temperature but showed no re- 

 lationship to density. Larvae hatch 

 at incubation depth and quickly 

 rise to the upper 50 m of the water 

 column where they remain during 

 larval development. Larger larvae 

 (-7-10 mm standard length [SL]) 

 undergo limited diel vertical migra- 

 tion within the upper 50 m. They 

 are deepest during the day, shal- 

 lowest at dusk, slightly deeper at 

 night, and even deeper at dawn. 

 Their mean depths of occurrence 

 were between 21 and 37 m at all 

 times. At these depths, prey (cope- 

 pod nauplii ) generally were at den- 

 sities sufficient for larval pollock 

 growth as determined in laboratory 

 studies. Pronounced thermoclines 

 and pycnoclines were present in 

 the part of the water column inhab- 

 ited by the larvae in late May. Lar- 

 vae appear to remain below the 

 upper mixed layer during periods 

 of increased turbulence, but at 

 depths during daytime where light 

 was sufficient for feeding and 

 where prey densities were adequate. 



The vertical distribution of eggs and 

 larvae of walleye pollock, 

 Theragra chalcogramma, in 

 Shelikof Strait, Gulf of Alaska* 



Arthur W. Kendall Jr. 



Alaska Fisheries Science Center, National Marine Fisheries Service. NOAA 

 7600 Sand Point Way NE. Seattle. Washington 98 1 I 5 



Lewis S. Incze 



Bigelow Laboratory for Ocean Sciences 

 West Boothbay Harbor, Maine 04575 



Peter B. Ortner 

 Shailer R. Cummings 



Atlantic Oceanographic and Meteorological Laboratory 

 4301 Rickenbacker Causeway, Miami, Florida 33 1 49 



Patricia K. Brown 



Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA 

 7600 Sand Point Way NE, Seattle, Washington 981 I 5 



The vertical distributions and move- 

 ments of planktonic organisms must 

 be studied to understand their 

 population dynamics. Often indi- 

 vidual populations undergo vertical 

 migrations and inhabit different 

 advective and thermal regimes on 

 a daily cycle (e.g. Hardy, 1936; 

 Enright, 1977). The entire plank- 

 tonic community may migrate simi- 

 larly, or different organisms may co- 

 occur on a diel cycle. The reasons 

 for diel vertical migrations of ich- 

 thyoplankton may be related to 

 feeding-digestion cycles, enhanced 

 predator avoidance, or directed 

 transport (Norcross and Shaw, 

 1984; Lampert, 1989). 



Planktonic eggs of most marine 

 fishes occur in the upper water col- 

 umn, although there are exceptions 

 (Ahlstrom, 1959; Coombs etal., 1981). 

 Neilson and Perry (1990) reviewed 

 literature on vertical migration in 



fishes, particularly larvae of marine 

 fishes, and attempted to identify 

 common patterns and underlying 

 causal mechanisms. They concluded 

 that changes in depth distribution 

 of larvae, while possibly under en- 

 dogenous control, seem to be medi- 

 ated by a number of environmental 

 factors. While light and gravity 

 dominate, other factors such as hy- 

 drography, food, tidal currents, and 

 turbulence may also be important 

 (Neilson and Perry, 1990; Lough 

 and Potter, 1993). Planktonic larvae 

 of fishes are found generally at shal- 

 lower depths at night than during 

 the day (Kendall and Naplin, 1981) 

 although the opposite pattern also 

 occurs (Boehlert et al., 1985; 

 Yamashita et al., 1985; Sogard et 

 al., 1987). In some species inconsis- 

 tent patterns have been found among 

 studies, possibly indicating the con- 

 founding effects of several biotic and 



Manuscript accepted 31 January 1994. 

 Fishery Bulletin 92:540-554 1 1994). 



540 



Contribution 0177 of the Fisheries-Oceanography Coordinated Investigations. 



