Incze and Ainaire: Distribution and abundance of copepod nauplii 



75 



naupui (So. r 1 ) 

 1985 (R=0-18;CI=2) 



Distance (km) 



1986 (R=1-56;CI=4) 



3 y \0 JO ' 



1987 (R=1-144;CI=10) 



1988 (R=0-26; Cl=2) 



10 » 



Figure 6 



Contour plots of naupliar concentrations (no. Lr 1 ) 

 across Shelikof Strait during spring. Numbers in 

 parentheses after the year (upper left of each plot) 

 show the range (R) of data and the contour inter- 

 val (CI) used in plotting. Transects are viewed look- 

 ing westward. 



Shelikof Strait were subject to the influence of 

 baroclinic instabilities. The timing and rotational 

 sense of these instabilities therefore may have a 

 large influence not only on the distribution of wall- 

 eye pollock larvae themselves (Reed et al., 1989; 

 Incze et al., 1990; Vastano et al., 1992), but also on 

 the feeding conditions they experience. For example, 

 the feature sampled in 1988 covered a substantial 



Chlorophyll - a (ug I 1 ) 



Distance (km) 



20 JO 



T" 



Figure 7 



Chlorophyll-a distributions across Shelikof Strait, 

 May 1989, looking westward (data may be compared 

 with nutrient and hydrographic structure in Fig. 2 

 and naupliar concentrations in Fig. 6). 



portion of the main spawning and hatching area. 

 Although we do not have extended observations of 

 this feature, Vastano et al. (1992) showed that eddy- 

 like features may remain near the hatching area for 

 as long as two weeks, a substantial portion of the 

 hatching period (Yoklavitch and Bailey, 1990). If 

 walleye pollock larvae migrate vertically into the 

 center of a dynamic high after hatching, then the 

 amount of time that passes before they are advected 

 into better feeding conditions (in this case at the 

 periphery of the high ) may be important to early 

 larval feeding and growth. 



The average integrated abundance of copepod 

 nauplii across the Strait was different for the vari- 

 ous transects. The maximum values that were seen 

 in 1987 probably can be attributed to the compara- 

 tively late sampling of that year. However, among 

 the four years with similar timing of transect sam- 

 pling, there remained statistically significant differ- 

 ences that may have been important to hatching 

 walleye pollock larvae (see Canino et al., 1991, for 

 feeding conditions and larval RNA/DNA ratios). 

 Since hatching takes place over a relatively short 

 time period (Yoklavitch and Bailey, 1990), the phas- 

 ing of hatching and upper layer conditions may play 

 an important role in establishing the larval year 

 class. Unfortunately, we do not know how long the 

 observed conditions persisted in each year relative 

 to the population hatching time or to other require- 

 ments of the early feeding period in larval develop- 

 ment. Advection (Incze et al., 1989) and short-term 

 fluctuations in mesoscale circulation (Vastano et al., 

 1992) may cause conditions in the Strait to change 

 quickly, requiring more frequent sampling and im- 

 proved techniques to rapidly assess prey distributions. 



Nauplii that were most abundant in the diet of 

 larval pollock must have come from copepods large 

 enough to be retained by mesh sizes used on the 



