Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 



229 



However, the reverse pattern ('Type II' migration), 

 although less frequently documented, has been ob- 

 served for larvae of several fish species, including 

 many of the taxa we examined. For example, 

 Boehlert et al. (1985) observed larval G. 

 macrocephalus at lower depths at night than dur- 

 ing the day off the Oregon coast. Walline 5 found that 

 Bathymaster spp. in the Bering Sea generally mi- 

 grated downward at night. Larvae of A. hexapterus 

 collected in bays around Kodiak Island were concen- 

 trated from 10 to 30 m during the day but were 

 found at lower depths at night (Rogers et al. 6 ), and 

 larvae of a congener (A. personatus) collected off Ja- 

 pan also exhibited reverse migration (Yamashita et 

 al., 1985). Rogers et al. 6 and Pritchett and Haldor- 



son (1989) found that rock sole (P. bilineatus), as 

 well as larvae of several other taxa, showed reverse 

 diel migrations during the spring. 



We believe that sampling bias could not have re- 

 sulted in the observed reverse distributions. Eggs of 

 H. elassodon, as expected, showed no differences by 

 time of day in our study and walleye pollock larvae 

 in these same collections exhibited a normal diel mi- 

 gration pattern (Type I), occurring mainly in the 30- 

 45 m range during daytime and above 30 m at night 

 (Kendall et al. 1 ; see also Kendall et al., 1987). Net 

 avoidance, although suggested by the higher night 

 catches overall as well as the larger mean size of 

 larvae collected at night, is not a plausible expla- 

 nation for the observed diel pattern. Light-aided 

 daytime avoidance would be ex- 

 pected to influence the catch of lar- 

 vae in the surface strata more than 

 those in deeper strata, thus leading 

 to underestimates of near-surface 

 daytime abundances and the mag- 

 nitude of reverse migration. 



The prevalence of the reverse 

 diel migration pattern in our 

 study suggests an adaptive role 

 for this behavior. Temperature 

 gradients are relatively minor 

 (<1°C) over the upper 50-60 m 

 where most of the migration oc- 

 curs (Fig. 7), and the majority of 

 the larvae appear to be above the 

 seasonal thermocline at all times 

 of the day. Thus, we see no possi- 

 bility of temperature-mediated 

 energetic advantage related to 

 migration at any time of the day. 

 Similarly, observed density gradi- 

 ents are not pronounced (<0.5 o t 

 units) within this surface layer 

 (Fig. 7; Kendall et al. 1 ) and there 

 appears to be no physical mecha- 

 nism that would aggregate either 



5 Walline, P. D. 1981. Hatching dates of 

 walleye pollock (Theragra ehalco- 

 gramma) and vertical distribution of 

 ichthyoplankton from the eastern 

 Bering Sea, June-July 1979. NWAFC 

 Processed Rep. 81-05, 22 p. 



6 Rogers, D. E., D. J. Rabin, B. J. Rogers, 

 K. J. Garrison, and M. E. Wangerin. 

 1979. Seasonal composition and food 

 web relationships of marine organisms 

 in the nearshore zone of Kodiak Island 

 including ichthyoplankton, mero- 

 plankton (shellfish), zooplankton and 

 fish. Univ. Washington, Fish. Res. Inst. 

 Rep. FRI-UW-7925, 291 p. 



