Long-term changes of abundance and population structure for sockeye salmon in Lake Dalneye (southeastern Kamchatka)

Abstract

Effects of interannual fluctuations of sockeye salmon escapement to the spawning grounds on dynamics of sexual, age and length-weight structure of the population are demonstrated on generalized data of long-term observations in Lake Dalneye in 1939–2020. Mature sockeye specimens were caught in the Dalnaya River during their anadromous migration to the lake and analyzed by the authors, personally; archive and earlier published data were used, as well. The escapement was evaluated by visual assessment of the sockeye migrants entered to Lake Dalneye and confirmed by counting of mature individuals on the littoral spawning grounds in the lake. Number of dwarf sockeye salmon was estimated using the earlier found correlation between their stock and forage zooplankton biomass (Kurenkov, 1991; Pogodaev, Kurenkov, 2012). The zooplankton biomass was assessed on samples of zooplankton collected monthly at a stationary station in the central part of the lake (once a month during the ice period and up to three times per month during the ice-free period) from the layer 0–50/55 m by vertical towing of Juday plankton net (mouth diameter 18 cm). The numbers of anadromous sockeye males and females had ratio 1 : 1 in years with the maximum spawning stock, but < 1 : 1 when their returns to the lake decreased. Besides, the males were smaller in such years and the dwarfs (matured without migration to the sea) and jacks (matured with short marine period) were able to escape commercial fishing gears. Age of the spawners had a tendency to increase with decreasing of spawning runs and, conversely, more abundant runs were formed by younger spawners. Long-term pressure from fishery and selection of larger individuals caused the body length and weight decreasing for mature sockeye salmon, shortening of their period of marine feeding, and increasing of the smallsized males portion in the population.