Embryonic development of walleye pollock , Theragra chalcogramma , from Shelikof Strait , Gulf of Alaska *

Alaska Fisheries Science Center; National Marine Fisheries Service. NOM 7600 Sand Point Way N.E. , Seattle. WA 98 J ) 5-0070 Embryonic development of walleye pollock, Theragra chalcogramma, from Shelikof Strait, Gulf of Alaska * Abstract.Eggs of walleye pollock, Theragra chalcogramma, from Shelikof Strait, Alaska, were reared at three temperatures: 3.S·, 5.7·, and 7.TC. Development was divided into 21 stages. A piecewise regression model with midpoints of each stage describes the relation between time to each stage of development and temperature. Preserved eggs of each stage are described, illustrated, and photographed. Midpoint of hatch was 393 hours at 3.S·C, 303 hours at 5.7·C, and 234 hours at 7.7"C. Mean length of larvae at hatch increased linearly with temperature. We compared rate of development, time to 50% hatch, and morphological development with other studies of walleye pollock eggs. Rate of development and time to 50% hatch were similar among populations of eastern North Pacific walleye pollock. Western North Pacific walleye pollock required longer incubation times than eastern North Pacific walleye pollock. Morphological development of Shelikof Strait eggs differs from development of western North Pacific walleye pollock eggs: optic vesicles, myomeres, eye lenses, heart, and otic capsules appear earlier than in Shelikof Strait eggs. and eye pigment appears later. The differences in development may be exacerbated by the condition of the eggs in which they were examined (e.g. preserved vs. live). Developmental differences between stocks are discussed with the conclusion that model components for egg mortality and spawning biomass must be based on specimens collected in the area of interest. Walleye pollock, Theragra chalcogramma, is' the most abundant member of the family Gadidae in the subarctic Pacific Ocean and Bering Sea, supporting the largest single-species commercial fishery in the world (Megrey, 1991). In the Gulf of Alaska, Shelikof Strait is the principal spawning area (Kendall and Picquelle, 1990) and has been the site of intensive research to understand processes leading to recruitment variability of walleye pollock (Schumacher and Kendall, 1991). Age determination of fertilized eggs is a basis for investigating biotic and abiotic impacts on the earliest life-history stage and thus for understanding interannual variability in walleye pollock recruitment. Age of walleye pollock eggs has been crucial to several studies. Egg mortality and spawning biomass are estimated by modeling age-specific egg abundance over time (Picquelle and Megrey, 1993; Batesl ). Patterns in horizontal or vertical distribution and abundance of walleye pollock eggs in the western Gulf ofAlaska have been described by grouping developmental stages into broad age groups (Kendall and Kim, 1989; Kendall and Picquelle, 1990). Egg age is an independent variable in the models used to estimate egg production and mortality. Therefore, increasing the accuracy in measuring egg ages should improve estimates of these values. In past studies, walleye pollock eggs have been incubated in the laboratory to develop temperature-specific equations that estimate duration of development or age of the eggs, to describe morphological development, to observe effects of light on egg buoyancy and hatching rate, and to obtain larvae for experiments (Table 1). Although these incubation studies provide pertinent'data on ontogeny of walleye pollock, none can be used with accuracy to determine age of eggs