Assessing long-term changes in sex ratios of Pacific herring in Prince William Sound, Alaska

Abstract

The Pacific herring (Clupea pallasii) population in Prince William Sound, Alaska suffered a sudden collapse in 1993 and has failed to recover. No consensus has been reached for the causes of the collapse, but hypotheses span a range of ecological, environmental, and anthropogenic drivers. During the course of investigations into changes in PWS herring and salmon population dynamics and fisheries we discovered that the observed percentage of male herring in spawning biomass estimates appeared to increase since the mid-1980s. Using available data (1983–2015) we analysed sex ratios of herring caught in close proximity to the spring spawn using various gear types. In recent years, the overall sex ratio of Pacific herring in PWS has been skewed towards males (>60%), but only if samples from cast nets are included, which generally catch spawning fish. In contrast, purse seine samples, generally collected days to weeks prior to the spawn, had relatively equal sex ratios. Age also affected the sex ratio, with a higher proportion of older fish being females. Using an age-structured simulation model, we illustrate that overestimating the number of males in the population can lead to biases in estimated model parameters (such as vulnerability at age) and as a result, biases in the inferred age composition of the population, and other derived quantities. After accounting for differences in sampling gears to improve estimated sex ratios, we recommend that future work focus attention on separating spatial and temporal differences in sex ratios of this otherwise much-studied population.