AbStfclCt. Microzooplankton 



retained by a 41-um mesh was 

 sampled along a 50-km transect in 

 the Shelikof Strait between 

 Kodiak Island and the Alaska Pen- 

 insula. We sampled once each year 

 during spring (April-May) 1985- 

 1989 using Niskin bottles closed at 

 10-m depth intervals. Sampling 

 was conducted near the time and 

 place of peak hatching of walleye 

 pollock (Theragra chalcogramma) 

 larvae. We examined horizontal 

 and vertical patterns of abundance 

 of potential prey organisms, espe- 

 cially copepod nauplii, and de- 

 scribed these patterns with respect 

 to the oceanography of the Strait. 

 Hydrography, nutrients, chloro- 

 phyll-a and net zooplankton data 

 also were collected and were used 

 to help interpret the microzoo- 

 plankton patterns. Copepod nau- 

 plii composed from 46 to 82% of all 

 organisms in the formalin-pre- 

 served samples. Eggs (3-35%), ro- 

 tifers (up to 14%) and loricate 

 tintinnids (up to 11%) were the 

 next most abundant taxa. The 

 abundance of microzooplankton 

 varied greatly across the Strait 

 and, for copepod nauplii, had 

 maxima associated with the 

 Alaska Coastal Current. A meso- 

 scale feature in the coastal current 

 appeared to influence the distribu- 

 tion of microzooplankton and may 

 affect feeding conditions for larval 

 walleye pollock. Significant differ- 

 ences in abundance of copepod 

 eggs and nauplii were detected 

 between some transects. The inte- 

 grated, 0-60 m depth, across-strait 

 average abundance of copepod 

 nauplii varied from a low of 5.8 x 

 10 3 nr 2 (sampled in 1985) to a 

 high of 17.6 x 10 3 nr 2 (1987). The 

 maximum concentration found in 

 these same transects varied from 

 18 to 144 L' 1 , respectively. Be- 

 tween 60 and 70% of the nauplii 

 sampled were of a size (>125 urn 

 total length) composing approxi- 

 mately 98% of the naupliar diet of 

 larval walleye pollock in spring. 



Distribution and abundance of 

 copepod nauplii and other small 

 (40-300 jim) zooplankton during 

 spring \n Shelikof Strait, Alaska* 



Lewis S. Incze 



Bigelow Laboratory for Ocean Sciences 

 West Boothbay Harbor. ME 04575 



Terri Ainaire 



Bigelow Laboratory for Ocean Sciences 

 West Boothbay Harbor, ME 04575 



Manuscript accepted 17 September 1993 

 Fishery Bulletin 92:67-78 (1994) 



The high mortality rate of marine 

 fish larvae is attributed to high 

 rates of predation (Moller, 1984; 

 Bailey and Houde, 1989), sensitiv- 

 ity to feeding conditions (Thei- 

 lacker and Watanabe, 1989) and 

 interactions between these factors 

 (Houde, 1987; Purcell and Grover, 

 1990). The larvae of temperate 

 fishes often occur during spring, 

 when planktonic production is in 

 early stages of its annual cycle and 

 is easily disrupted or delayed by 

 adverse conditions. Also, larvae 

 have small search volumes and 

 generally small energy reserves 

 (Bailey and Houde, 1989). Thus, a 

 spatial or temporal "match" or 

 "mismatch" between the demand 

 for larval food and its availability 

 seems intuitively likely and has 

 been the subject of much research 

 (e.g., Lasker, 1981; Buckley and 

 Lough, 1987; Cushing, 1990). The 

 quest to quantify feeding relation- 

 ships has led to continuing efforts 

 to reduce container effects in ex- 

 perimental studies (Gamble and 

 Fuiman, 1987; McKenzie et al., 

 1990), to improve the sensitivity of 

 physiological measurements (e.g., 

 Buckley et al., 1990), to understand 

 the small-scale distribution of prey 

 in the field (Owen, 1989), and to 

 understand the role of mixing in 

 enhancing or retarding interactions 



between predator and prey 

 (Rothschild and Osborne, 1988; 

 Davis et al., 1991). In the ocean, 

 feeding takes place in a complex 

 spatial array of biological and 

 physical conditions. Any study of 

 rate-influencing processes that af- 

 fect larvae must take into account 

 the distribution of these conditions 

 in order to understand effects at 

 the population level. 



In this paper we examine the 

 springtime community of small 

 zooplankton, primarily copepod 

 nauplii, that may be prey for larval 

 walleye pollock, Theragra chal- 

 cogramma, in Shelikof Strait, 

 Alaska (Fig. 1), and we report on 

 the distribution and abundance of 

 these organisms with respect to 

 oceanographic conditions. A large 

 population of walleye pollock 

 spawns in the Strait in late March 

 and early April, forming dense ag- 

 gregations of planktonic eggs in the 

 deepest part of the sea valley be- 

 tween Kodiak Island and the 

 Alaska Peninsula. Hatching occurs 

 from middle or late April through 

 early May (Kendall et al., 1987; 

 Incze et al., 1989; Yoklavitch and 

 Bailey, 1990). While the eggs re- 

 main mostly below 150 m, larvae 



* Bigelow Laboratory Contribution No. 93- 

 006. Fisheries Oceanography Coordinated 

 Investigations Contribution No. 0186. 



6 7 



