Seasonal and Geographic Variation in Condition of Juvenile Walleye Pollock in the Western Gulf of Alaska

—Body length, length-specific weight, and length-specific whole-body energy content (WBEC) of juvenile walleye pollock Theragra chalcogramma were examined for evidence of seasonal and geographic variation. Identifying the impact of winter on these measures of fish condition was of primary concern, but we also sought to determine whether condition varies among different geographic regions. The 2000 year-class of walleye pollock was sampled in the western Gulf of Alaska during four seasons (the autumn of 2000 and the winter, summer, and autumn of 2001), and the study area was divided into three regions (Kodiak, Semidi, and Shumagin). In each region, length-specific weight decreased from autumn to winter by up to 17%, subsequently increasing into the summer by 13–20%. Significant seasonal changes in length-specific WBEC varied by region but reflected the same trends as length-specific weight. Walleye pollock from the Kodiak region were in better condition than those from other regions, as evidenced by larger lengths in winter (18-mm difference) and summer, a 5.9% higher length-specific weight in autumn of 2000, and higher length-specific WBEC in autumn of 2000 (0.783 kJ/g wet weight, or 18.3%, higher) and in the following winter (0.607 kJ/g, or 14.1%, higher). The poorer condition of juvenile walleye pollock in winter may be widespread through the Gulf of Alaska, but spatial heterogeneity in habitat can also greatly influence the size and condition of juveniles. The impact of such variability on growth and mortality remains uncertain. Seasonal variation in productivity and water temperature largely determines the reproductive strategy and growth schedule of fish in temperate marine environments (Pitcher and Hart 1982). In these waters, spawning usually occurs during spring so that young fish are maximally exposed to periods of abundant food and warmer temperatures conducive to rapid growth (match–mismatch hypothesis; Cushing 1990). With the onset of winter, fish growth slows, energy reserves may be depleted, and juvenile mortality rates can increase (Sogard 1997; Hurst and Conover 1998; Gotceitas et al. 1999) due to cooler water temperatures and diminished prey fields (Shuter and Post 1990; Schultz and Conover 1999; Hurst and Conover 2001). Large fish often have higher rates of survival than do smaller individuals, owing to a decreased vulnerability to predation and a greater energy storage capability that can be relied on as growth and foraging opportunities decrease during the winter (Sogard 1997). Large body size also results in a reduced weight-specific metabolic rate, which lowers the rate of depletion of stored energy. However, size-selective mortality does not occur for all species in temperate waters (Sogard 1997), presumably due to the complex interactions between water temperature, prey availability, and predation (Garvey et al. 1998). For age-0 walleye pollock Theragra chalcogramma, survival was positively related to body size when fish were held without food in low-temperature conditions in the laboratory (Sogard and Olla 2000). Thus, accumulation of sufficient energy reserves and attainment of a relatively large body size before winter may be an important factor in walleye pollock winter * Corresponding author: [email protected] Received April 19, 2005; accepted February 6, 2006 Published online June 26, 2006 897 Transactions of the American Fisheries Society 135:897–907, 2006 Copyright by the American Fisheries Society 2006 DOI: 10.1577/T05-105.1 [Article]