Resource Assessment & Conservation Engineering (RACE) Division

Groundfish Assessment Program

Atka Mackerel Natural History Studies

Atka mackerel (Pleurogrammus monopterygius) spawn demersally in rocky areas, and nests comprised of egg clutches are defended by guardian males. Reproductively mature male Atka mackerel aggregate at specific nesting sites along the Alaskan continental shelf. Aggregations of nesting males, the developing embryos in the nests that males guard, and the nesting habitat itself are all vulnerable to the effects of bottom trawling. The potential impact of trawl fishing on Atka mackerel populations cannot be assessed without first understanding how the spatial and temporal aspects of their reproduction overlap with the commercial fishery.

The geographic distribution, depth range, and description of Atka mackerel nesting and spawning habitat were investigated in Alaska waters from 1998 to 2004. Scuba diving and in situ and towed underwater video cameras were used to locate and document Atka mackerel nesting sites and reproductive behavior.

Results from this study extend the geographic range of nesting sites from the Kamchatka Peninsula to the Gulf of Alaska and extended the lower depth limit for nesting and spawning from 32 m to 143 m. There was no apparent concentration of nesting sites in nearshore coastal areas as was surmised by other investigations. Nesting sites were widespread on the continental shelf across the Aleutian archipelago and into the western Gulf of Alaska. Nesting habitat invariably had rocky substrate and current, and water temperatures for nesting sites ranged from 3.9° to 10.5°C. Water temperatures within nesting sites varied little and did not appear to be limiting the upper or lower depth boundaries of nesting.

The temporality of the Atka mackerel spawning and nesting season in Alaska is currently being investigated using a towed video camera, time lapse camera, archival tags, and egg samples brought up in trawls. Using the time lapse camera and data from one archival tag, it was established that male Atka mackerel begin to aggregate at nesting sites in mid-June. In Kamchatka, nesting was found to start at the same time and spawning to last until September. Incubation for Atka mackerel eggs was determined as 40-45 days; hence, it was inferred that nesting season off Kamchatka lasted until early October.

Histological analysis of Atka mackerel ovaries by AFSC scientists indicate spawning lasts through October in Alaska waters, however, the ending time for nesting season remains unclear. As late as October, aggregations of nest guarding males were observed in Alaska waters with a towed video camera, and egg masses were brought up in trawl tows done through a nesting site. No effort has been made later into the year to see if aggregations of males or egg masses are present in November and December.

Recent laboratory incubation experiments of fertilized eggs obtained from the field and from fish in captivity at the Alaska SeaLife Center in Seward indicate that incubation of eggs lasts from about 1 to 3 months depending on temperature (at 10°C and 4°C, respectively). If eggs are being deposited in nests in October, it is likely that males are still guarding incubating eggs at nesting sites through November or December. The towed video camera will be used at a known nesting site near Dutch Harbor, Alaska, in late November or early December 2005 to see if aggregations of males are still guarding incubating eggs.

Other means besides histology and underwater video are being used to determine the end of the spawning and hatching periods. Incubation rates from laboratory experiments will be used to stage over 100 egg clutches brought up from trawl tows made through nesting sites. Eggs will be staged according to their embryological development. Historical temperature data from the areas near the nesting site where eggs were collected will be used to estimate the range of spawn and hatch dates for the egg samples.

By Robert Lauth


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AFSC Quarterly
Research Reports July-Sept 2005