Kill ‘em or hide ‘em: where are the old female rockfish?

Principal Investigator: Susan Sogard, John Field, Mary Yoklavich, NOAA/NMFS/SWFSC/FED, Santa Cruz, CA

Co-Principal Investigators:

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Background: A major controversy inhibiting accurate assessment of rockfishes (Sebastes spp.) is the apparent disproportionate absence of older females relative to males, suggesting that either females have a higher natural mortality rate or they are migrating to habitats that are not sampled sufficiently to detect them. For example, if older females migrate to deeper habitats compared to males, they may be undersampled in many surveys. For heavily exploited species, a third mechanism of fishing-induced size or age truncation can differentially remove older females if they grow to larger sizes than males. Assumption of the wrong mechanism underlying low abundance of older female rockfish can have substantial consequences for stock assessment.

This project has two primary objectives. One is to thoroughly examine existing datasets for evidence of differential habitat use by older female vs. male rockfish and to compare sex ratios with age and habitat, primarily depth. The second is to directly test for sex and age differences in habitat use for yellowtail and canary rockfish using hook and line sampling. These targeted collections will allow us to directly test the hypothesis that older females disproportionately prefer deeper and more complex habitats. Improved understanding of any age-dependent patterns of habitat use by females vs. males will have immediate application in improving stock assessments of these valuable, long-lived species.

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Southwest Fisheries Science Center (SWFSC)


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