Geographic Distribution, Depth Range, and Description of Atka Mackerel Pleurogrammus monopterygius Nesting Habitat in Alaska

Understanding the spatial and bathymetric extent of the reproductive habitat of Atka mackerel Pleurogrammus monopterygius is basic and fundamental information for managing and conserving the species. From 1998 to 2004, scuba diving and in situ and towed underwater cameras were used to document reproductive behavior of Atka mackerel and to map the geographic and depth ranges of their spawning and nesting habitat in Alaska. This study extended the geographic range of nesting sites from the Kamchatka Peninsula to the Gulf of Alaska, and extended the lower depth limit from 32 to 144 m. Male Atka mackerel guarding egg masses were observed during October--indicating that the duration of the nesting period in Alaska is more protracted than in the western Pacific. Results from this study also suggest that nearshore nesting sites constitute only a fraction of the nesting habitat and that there is no concerted nearshore spawning migration for Atka mackerel in Alaska. Nesting sites were widespread across the continental shelf and found over a much broader depth range than in the western Pacific. Nesting habitat was invariably associated with rocky substrates and water currents; however, smaller-scale geomorphic and oceanographic features as well as physical properties of the rocky substrate were variable between different island groups and nesting sites. Water temperatures for nesting sites ranged from 3.9 degree C to 10.5 degree C. Water temperatures within nesting sites varied little and did not appear to be limiting the upper or lower depth boundaries of nesting. Results from dive transects showed significantly fewer egg masses above 20 m water depth. Other possible factors limiting the upper or lower depth limit of nesting sites are discussed.