| Summary: | We investigated the spatial distribution of Atka mackerel Pleurogrammus monopterygius with respect to maturity stage at Seguam Pass in the Aleutian Islands as well as their spawning locations within trawlable areas at three small-scale (10–20-nautical mile or 18.5–37.0-km) sites. Histological methods were used to determine maturity stages, and male maturity stages are described for the first time. Spawning areas were identified by catch per unit effort of demersal egg masses. At Seguam Pass, spawning was concentrated in the area adjacent to the east side of Amlia Island and between Amlia and Seguam islands. The observed spatial segregation of Atka mackerel by sex and maturity stage appears to interact with the boundaries of the trawl exclusion zone (TEZ) at Seguam Pass. During spawning, mature fish aggregate inside the closed area and immature fish and nonreproductively active mature males aggregate outside the TEZ in the area open to fishing. This increased the commercial catch selectivity of nonreproductively active males and immature fish during the September fishery. The spawning areas observed at Seguam and Tanaga passes and near Amchitka Island were within TEZs, which may serve as de facto marine protected areas for spawning Atka mackerel.
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