74 



Fishery Bulletin 92[ I). 1994 



57°30 



;-»T llll 



Alaska 

 Peninsula 



155°30' 



154 D 30 



Longitude ( W ) 



Figure 5 



Contours of 0-150 m dynamic height in western Shelikof 

 Strait during April 1988. Solid circles show locations of CTD 

 stations. The study transect is the farthest northeast sec- 

 tion. Open circles denote those transect stations with the 

 highest microzooplankton standing stocks (cf. Figs. 4, 6). A 

 dynamic high (H) and low (L) are labelled; arrows show 

 inferred flow. 



the genus that prohibit any simple correction for 

 different mesh collections: see Frost, 1987.) Within 

 these limitations, data for 1985 and 1986 (333 pm) 

 were statistically different (Wilcoxon signed rank 

 test, P=0.076), whereas the multi-year comparison 

 for early spring samplings (1986, 1988, 1989: 150 

 pm mesh) showed no statistically significant differ- 

 ences (Quade test, a= 0.05). Among early spring 

 values, there were no statistically significant differ- 

 ences in abundance of Metridia spp.. 



Discussion 



The method of sampling and preservation used in 

 this study under-represented smaller components of 

 the microzooplankton (James, 1991) but was ad- 

 equate to capture the majority of prey items of lar- 

 val walleye pollock based on prey sizes reported 

 from earlier studies of Clarke (1984: Bering Sea), 

 Nishiyama and Hirano (1983, 1985: Bering Sea), 

 Dagg et al. (1984: Bering Sea); and Kendall et al. 

 (1987: Shelikof Strait). For small larvae of 5-10 mm 

 standard length (SL) in those studies, copepod nau- 

 plii composed the majority of items found in larval 

 stomachs. They also made up the bulk of estimated 



volume or carbon content of prey when these 

 values were calculated (Incze et al., 1984; 

 Nishiyama and Hirano, 1983). The 10-m 

 vertical resolution of our sampling method 

 almost certainly failed to detect the highest 

 concentrations of prey available to larval 

 walleye pollock under some conditions, such 

 as in small patches (Owen, 1989), but prob- 

 ably reflects adequately the average abun- 

 dances found at different depths in the wa- 

 ter column, in different sections across the 

 Strait and in different transects. 



Size-frequency distributions of sampled 

 nauplii and dimensions of the sampling 

 mesh suggest that there was virtually com- 

 plete retention of nauplii with total length 

 > 125 pm. In most cases these measure- 

 ments were carapace ("prosome") lengths. 

 Unpublished data from stomach content 

 studies (Canino, 1992 9 ) show that ca. 98% 

 of the nauplii consumed by larval walleye 

 pollock collected during our cruise in May 

 1989 had carapace length > 125 pm. Be- 

 tween 60 and 70% of the nauplii in our 

 samples were of this size (Fig. 8). 



Concentrations and integrated abun- 

 dances of nauplii differed across Shelikof 

 Strait in patterns that appear to be related 

 to circulation features. Our data indicated 

 that standing stocks and maximum concen- 

 trations of copepod nauplii in spring were greatest 

 in the ACC, which is also where greatest chloro- 

 phyll-o concentrations occurred (latter data for 1988, 

 1989; cf. Figs. 4, 6, 7). The lowest naupliar concen- 

 trations of the early spring samplings occurred in 

 1985, which had the weakest stratification. In gen- 

 eral, nauplii were most abundant at 20-m depth 

 except in 1988, when maximum concentrations oc- 

 curred at 30-m depth in the deeper mixed perimeter 

 of the anticyclonic feature. The lowest standing 

 stock of nauplii coincided with the latest apparent 

 phytoplankton bloom in 1985, but we cannot deter- 

 mine if lower individual copepod egg production 

 rates or lower standing stocks of copepods were re- 

 sponsible because we lack adequate collections ( 150— 

 pm mesh) of Pseudocalanus spp. in 1985. Alterna- 

 tively, the low naupliar standing stocks could have 

 been due to higher predation, but our data show 

 that springtime populations of predators were gen- 

 erally low and were similar among years. 



Our data suggest that the distribution of copepod 

 nauplii and some other microzooplankton across 



9 M. Canino. 1992. Natl. Mar. Fish. Serv., Alaska Fisheries Sci- 

 ence Center, Seattle, WA, unpubl. data. 



