Nonlinear Relation Between Smolts and Adults in Babine Lake Sockeye Salmon (Oncorhynchus nerka) and Implications for Other Salmon Populations

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Abstract
Data on sampling variability in smolt abundance for Babine Lake sockeye salmon (O. nerka) are combined with a previous analysis to calculate a more accurate estimate of the degree of nonlinearity in the relation between smolts and adults. Results indicate significant within-stock nonlinear mortality, large variability in mortality which tends to obscure any smolt-to-adult relation, or both. Analysis of age structure data identifies the first 15 mo. of marine life as the period when most of this nonlinear or relation-masking mortality occurs. The amount of smolt measurement error below which other salmon stocks are classed was calculated as having nonlinear marine survival. A distinct separation between even and odd brood year marine survival of Babine Lake sockeye suggests interactions with pink salmon. Juvenile pink salmon (O. gorbuscha) abundances correlate positively with residuals in Babine Lake sockeye survival for the same seaward migration year. A depensatory mortality effect which occurs later in the marine life stage than the possible within-population compensatory effect is suggested. Depensation is plausible because the size of pink fry equals that of sockeye smolts by Aug. in coastal waters permitting mutual swamping of predators.