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AFSC/RACE/GAP/Nichol: Archival tag depth and temperature data from Atka mackerel
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Atka mackerel Pleurogrammus monopterygius were captured and tagged with depth and
temperature recording devices (archival tags) on 23 July 2000 in Seguam Pass, Aleutian
Islands, Alaska. Nine of the 117 tagged fish were recovered in Seguam Pass during
September 2000, and an additional 5 fish recovered thereafter (14 total). In addition,
413 tags were released in Tanaga Pass in 2002, with 10 recoveries. Fish were tagged
externally just below the dorsal fin. Atka mackerel displayed strong diel behavior,
with vertical movements away from the bottom occurring almost exclusively during daylight
hours and little to no movement at night. Vertical movements occurred when light levels
at 150 m were greater than 7.31 × 10?5 µmol photons m?2 s?1, or approximately between
08:00 and 23:00 h Alaska Daylight Time (ADT; GMT ? 8) during August. Daytime vertical
movements were correlated with light intensity, time of day and current velocity.
The occurrence of vertical movements tended to increase with increasing light during
the morning and early afternoon, but then decrease with increasing hour of the day
after 13:00 h ADT. The magnitude of surface-directed vertical excursions was reduced
during spring tide periods, when current velocities are highest. By comparison, the
magnitude of slope-directed excursions was greater during spring tide periods and
reduced during neap tide periods. Eight fish were at liberty for 42 to 44 d and 1
for 65 d. Two of the tagged males displayed nest guarding behavior for the majority
of their time at liberty. Depths for these 2 males (115 to 117 m) were much deeper
than previously observed for Atka mackerel spawning grounds. Given that Atka mackerel
are more likely to be on the bottom during the night and less likely during the day,
the variance of abundance estimates from bottom trawl surveys may be reduced by accounting
for these diel differences.
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