Patterns of Growth of the Pink Salmon Oncorhynchus gorbuscha in Year-Classes with Different Survival Rates during the Marine Life-History Phase

AbstractA decrease in the rate of growth in fish, which is one of the most important characteristics of stock dynamics, can result in a decrease in their abundance, as is widely accepted in classical fisheries ichthyology. A study of growth patterns in pink salmon year-classes with different survival rates during the marine period of their life history has been carried out to elucidate the causes of the sharp reduction in the abundance of this species that occurred recently on the eastern coast of Sakhalin Island. The retrospective analysis of growth rates was based on measurements of scales from pink salmon that returned to spawn on the southeastern Sakhalin coast in 2005–2018. An analysis of the relationship between the growth parameters (body length increase, variability, and skewness of the size structure) with the survival rates of the respective year-classes has shown that these parameters can be used as indicators of survival rate reduction only for feeding juveniles, i.e., during the early marine phase of pink salmon life history. Thus, the hypothesis about a “critical size” for juvenile salmon that, if not reached by the end of the summer–autumn feeding season, significantly decreases its chances to survive the overwintering period, has not been confirmed. The data show that the estimates of size-selective mortality of juvenile pink differ from the actual values not only because of the probable incorrectness of comparing actually observed body lengths (unrepresentative sampling) and those calculated using scales. The unreliability of estimates may also come from the fact that variations in the calculated size structure of surviving fish to some extent reflect the similar processes that occur initially in the entire year-classes.