Atka Mackerel Giving Up Secrets
Hard-to-study Atka mackerel are beginning to give up their secrets to determined NOAA Fisheries scientists. Alaska Fisheries Science Center researchers have trawled for Atka mackerel, monitored their horizontal movements with traditional spaghetti tags, and have observed their mating behavior with cameras. Scientists are also attaching electronic depth and temperature-recording tags as well as non-electronic 'spaghetti' tags to individual fish.
"Our tagging research shows that Atka mackerel undergo very patterned vertical movements, quite unique compared to other Alaskan fish species. Atka mackerel seem inactive at night, staying on or very close to the bottom when light levels are low. Only during daylight hours do they migrate up from the bottom," said NOAA scientist Dan Nichol. "Daytime movements correlate with light intensity, time of day and also sea current velocity."
The daily up and down movement of Atka mackerel is one area of interest. Another is their horizontal movement between areas in the ocean. Steller sea lions prey on Atka mackerel. Large areas of the ocean near the Aleutian Islands are closed to trawl fishing in order to protect the Steller sea lion's food sources. If Atka mackerel stay close to home, the closed areas would hold Atka mackerel for the endangered sea lions. If the mackerel move into open fishing areas, then the closures would not be as effective.
From 2000 to 2003, scientists tagged, released and then recovered Atka mackerel at Seguam Pass and Tanaga Pass and near Amchitka Island, according to NOAA Fisheries researcher Susanne McDermott. Scientists saw no large-scale movement of Atka mackerel near Seguam Pass and very little between Amchitka and Tanaga. Scientists consider the Atka mackerel populations closed within their local aggregations.
Detailed study results suggest that the protected areas in Seguam and Tanaga Passes, where Atka mackerel biomass is relatively high and movement is relatively low, may be effective at preserving local foraging areas for Steller sea lions. The protected area at the south end of Amchitka, where biomass is low compared to other areas and movement is high, may be less effective. These differences in movement may be due to differences in the distribution of Atka mackerel habitat.
Atka mackerel are difficult to detect acoustically with echosounders because--unlike many fish species--they lack gas-filled swimbladders which clearly reflect sound.
Nichol and his co-investigator, Dave Somerton, tagged 117 Atka mackerel with electronic "data storage tags" in the summer of 2000 near Seguam Pass. Between seven and nine weeks later, nine tagged Atka mackerel were recovered by fishermen and researchers. The tags had worked well, recording the depth of the fish and the temperature of the water.
Two of the fish -- males in bright yellow spawning colors -- had stayed near the bottom for the majority of their time at liberty. Considering the time of year that these fish remained on the bottom (summer through fall), they were likely displaying nest-guarding behavior. Atka mackerel are unusual in that males fiercely and continuously guard a nest of eggs, which take somewhere around 43 days to hatch. Researchers have found that egg cannibalism is prevalent among the male spawning adult population.
The other fish tags gave Nichol and Somerton the Atka mackerel's typical daily pattern of movement -up in the morning when the light grows strong enough, and then, at somewhat differing times, back to the same depth and, presumably, the same area.
Nichol believes that Atka mackerel are visual hunters, and rise when there is enough light to see, catch and feed on small marine creatures -- euphausiids and copepods.
Back at the NOAA research lab in Kodiak, Alaska, Nichol tagged two Atka mackerel and 10 kelp greenling and kept them in a tank for 'control' on the wild tagging. The fish remained in the tank for six months without showing any apparent effects from the tags.
NOAA Fisheries Scientist Dan Cooper has been studying the reproductive life history of Atka mackerel. Preliminary results show that Atka mackerel segregate locally not only by sex but also by reproductive stage. Spawning locations at Seguam Pass, Tanaga and Amchitka appear to be most prevalent inside the protected areas and are unlikely to be disturbed by commercial fishing.
Early survey results indicate that the Atka mackerel biomass in the Aleutian Islands and southern Bering Sea regions was increasing between 1980 and 1991, reaching a peak of approximately 725,000 metric tons. The next two surveys showed biomass estimates declining by 17% and 39% in 1994 and 1997, respectively, followed by steady increases. Between 1997 and 2004, biomass estimates more than doubled from 366,000 tons to 887,000 tons.
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