Growth of Underyearling Sockeye Salmon (Oncorhynchus nerka) under Constant and Cyclic Temperatures in Relation to Live Zooplankton Ration Size

Growth rates of Babine Lake underyearling sockeye salmon (Oncorhynchus nerka) fed rations of zooplankton of 14–84 cal/kcal sockeye∙d−1 (1 cal = 4.1868 J; 1.3–7.8% of dry body wt) were determined at constant temperatures of 6.2, 11.3, 15.3, and 15.9 °C, and under a cyclic temperature regime similar to that encountered during their diel vertical migrations. Growth of fish fed rations of 43–76 cal/kcal∙d−1 (4.0–6.9% of dry body wt) was equal or greater under cyclic temperatures than constant temperatures. At the lowest and highest rations growth was greater, respectively, under the constant low temperature and the constant high temperature than under cyclic temperatures. Gross growth efficiency, ranging from < 1 to 31%, was affected by temperature and ration in much the same way as growth. Young sockeye ingesting moderate rations comparable to those probably available in the relatively warm surface layers of Babine Lake, and undergoing diel migrations from deep cold waters to the surface, and return, are likely to grow more rapidly and efficiently than fish maintained at constant temperatures. The selective value of vertical migrations in thermally stratified waters may have an energetic basis as suggested by some other workers.Key words: sockeye salmon, growth rate, conversion efficiency, diel vertical migration, temperature, zooplankton ration, diel cyclic temperature