Sex-Specific Natural Mortality of Arrowtooth Flounder in Alaska: Implications of a Skewed Sex Ratio on Exploitation and Management

Abstract

Conservative fisheries management in Alaskan waters typically limits harvest to maintain a target biomass of female spawners (40% of the female biomass observed in the absence of fishing). This harvest calculation does not consider the male population level for a species. If natural mortality rates (M) differ between sexes and if females outlive males, then a reduction in males relative to females could diminish the reproductive potential of the stock. This problem was examined in the present study by estimating separate sex-specific M-values for the arrowtooth flounder Atheresthes stomias in Alaska, where females are almost always found at higher proportions than males during trawl surveys. Age data indicated that females outlive males, and estimates of M ranged from 0.10 to 0.33 for females and from 0.16 to 0.51 for males. Based on four estimation methods, male M was consistently higher than female M. Simulated harvest scenarios in which male M and selectivity were varied indicated that increasing male M and lowering the age of males at first capture skewed the sex ratio even more toward females; in the most extreme harvest scenario, the percentage of females in the population was as high as 89% (Gulf of Alaska) and 96% (Bering Sea). Because reproductive success under a skewed sex ratio is probably linked to reproductive behavior, we developed a simple conceptual model to consider three reproductive strategies that differed in the number of females (1–3) that could be paired with a single male. Although a population's reproductive potential was much more resilient to changes in sex ratio for 1:2 or 1:3 (male : female) pairings (reduced by only 15% when the population was 70% female), the fishing mortality rate (F) that corresponded to a reduction in egg fertilization was much higher than the target F (i.e., that maintained biomass at 40% of the unfished biomass) and exceeded overfishing levels when the population was over 80% female.