Size-selective mortality of Sea of Okhotsk pink salmon in the ocean in the winter and spring

The mortality of Sea of Okhotsk pink salmon in the winter and spring varies significantly from year to year, which complicates forecasts of its arrival in the following year based on data on the downstream migration of fry and surveys in the fall. The size-selective mortality of pink salmon was studied and the possibility of using the size and weight parameters of juveniles for predicting their return was evaluated through measurements of scale increments in juvenile pink salmon that were caught in the southern Sea of Okhotsk in the fall of 2007 and 2008 and in fish of these year classes that came back to spawn. In the 2007 year class, which had a low overwinter survival level in the ocean, the average scale increments for the first year of life were considerably smaller than those in adult fish that returned to the spawning grounds. In the pink salmon of 2008, which had a very high level of overwinter survival, the values of scale increments in juveniles and adults were similar. This confirms the hypothesis of a critical size and a critical period, according to which slowly growing juveniles that do not accumulate enough energy reserves for summer are eliminated in the winter to a greater extent as compared to fast-growing fish. Correlation analysis revealed a significant negative relationship between the size and weight of juvenile pink salmon and their mortality in the ocean. After conducting further and more extensive studies this will allow using the size parameters of juvenile pink salmon as one of predictors of its return for the year following the fall surveys. These results emphasize how important it is to take the size and growth rate of juvenile salmon into account when forecasting their return.