232 



Fishery Bulletin 92(2), 1994 



Water temperature and density 



DAWN 



NIGHT 



DAY DUSK 



Sigma-t 



2S 2 253 254 26.5 266 25.2 25 3 254 25 5 25 6 25 1 25 2 253 25 4 25 5 25.2 25.3 254 25 5 25 6 



sigma-t 



temp 





3 4 6 60 30 4 5 6 30 4 5 6 3.0 4 5 6 



Temperature (°C) 



Figure 7 



Diel vertical profiles of temperature and density (o t ). Data are means of at least four casts within 

 each time interval and were collected at 1 m depth intervals. 



component of the diet of many larval fishes includ- 

 ing walleye pollock (Kendall et al., 1987), were the 

 most abundant microzooplankton category found in 

 Shelikof Strait, mostly in the upper 30 m during 

 May 1986 and 1987 (Incze and Ainaire 8 ). During diel 

 Series 4, copepod nauplii had overall mean depths 

 between 20 and 34 m but showed no obvious diel 

 pattern in depth distribution (Kendall et al. 1 ). Al- 

 though feeding at a different time of day from wall- 

 eye pollock might reduce interference competition 

 (i.e. behavioral interactions) with the dominant spe- 

 cies, it is highly unlikely, based on typical larval fish 

 and copepod naupliar densities, that prey resources 

 could ever be depleted by larval fish (Cushing, 1983; 

 MacKenzie et al., 1990). Moreover, if food were lim- 

 iting, then it would be advantageous for all larvae 

 to stay in the layer of maximum food concentration 

 throughout the diel period to maximize total intake. 

 Thus, we do not see a trophic benefit accruing from 

 a reverse migration pattern for these larvae. 



If feeding by these larvae is periodic and depen- 

 dent on some minimum light level, then the verti- 

 cal distribution pattern can be partially explained 

 by larval feeding response. Assuming light levels 

 were limiting feeding at depths below 30 m, then it 

 would be necessary for larvae to ascend to a shal- 

 lower depth during the daytime when light is at a 

 maximum. Following the cessation of feeding at 

 dusk, larvae would be expected to become inactive 



and passively sink to deeper levels at night. Such a 

 mechanism has been postulated for Japanese sand 

 lance (A. personatus) by Yamashita et al. ( 1985) who 

 demonstrated a nocturnal cessation of feeding in 

 this species. Although we lack data on the diel feed- 

 ing chronology of any of the taxa examined here, it 

 is possible that feeding occurs mainly in the crep- 

 uscular periods, with a temporary cessation of in- 

 gestion occuring during midday as observed in the 

 field for larval walleye pollock (Canino and Bailey 9 ). 

 The shallowest mean depth occurs at either dawn 

 or dusk for the three common species that were ex- 

 amined over the four time periods with slightly 

 greater depths occurring during midday. If larvae 

 were not feeding during the middle of the day, it 

 would be advantageous to cease swimming alto- 

 gether and sink through the water column to avoid 

 being sensed by mechanoreceptive or visual preda- 

 tors. Following a particular isolume would produce 

 a similar daytime pattern but could not account for 

 the deeper distribution at night that we observed. 

 Larval walleye pollock in the laboratory have been 

 shown to avoid high light levels (Olla and Davis, 

 1990) but they also require relatively low light lev- 

 els to initiate feeding (Paul, 1983). Unfortunately, 

 we have no data available on the light levels neces- 

 sary for feeding in the taxa we examined with which 

 we can evaluate this hypothesis. 



8 Incze, L. S., and T. Ainaire. In review. Zooplankton of Shelikof 

 Strait, Alaska. I. Micro-zooplankton prey of larval pollock, 

 Theragra chalcogramma. Submitted to Fish. Bull. 



9 Canino, M. F., and K. M. Bailey. In review. Gastric evacuation 

 of walleye pollock, Theragra chalcogramma (Pallas), larvae in 

 response to feeding. Submitted to Journal of Fish Biology. 



