Resource Assessment & Conservation Engineering 

Groundfish Assessment:
Data Storage Tags Show Vertical Movement Patterns of Atka Mackerel and Pacific Cod

The RACE Division, in collaboration with the Center’s Resource Ecology and Fisheries Management (REFM) Division and the Alaska Department of Fish and Game, has been conducting research on the vertical movement patterns of Atka mackerel (Pleurogrammus monopterygius) and Pacific cod (Gadus macrocephalus) using externally attached data storage tags. The archival tags collect depth and temperature information from 2 to 60 minute intervals.

Figure 1. Diel vertical movement pattern of Atka mackerel for a 1-week period.

Figure 2. Starting and ending times of Atka mackerel vertical movements within a 24-hour period.

	Figure 3. Maximum surface-directed (top graph) and slope-directed (bottom graph) vertical excursions of Atka mackerel as related to current velocity.

In July 2000, 117 Atka mackerel were released with data storage tags in Seguam Pass.  Thus far, 13 tags have been recovered from fish at liberty from 42 to 407 days.  All movement away from the bottom occurred approximately between sunrise and sunset.  At night, fish apparently remained on bottom (Fig. 1 above).  Due to the shorter day-length during winter, Atka mackerel spent longer periods on the bottom than at other times of the year (Fig.2  above).  During summer months (July-September), fish often returned to the same depth each night, indicating that fish likely maintained a homesite.  Daytime vertical movements were correlated with light intensity, time of day, and current velocity.  Vertical movements increased with increasing light during the morning and early afternoon, but then decreased with increasing time.  The currents in Seguam Pass, which reach velocities in excess of 4 knots, affected the magnitude or distance of vertical excursions away from the bottom.  Surface-directed excursions were reduced in magnitude during periods of greater current velocities (spring-tide periods), whereas slope-directed excursions increased in magnitude (Fig. 3).  These observations suggest that the diel behavior of Atka mackerel should be taken into account when conducting resource assessment surveys.  Current bottom-trawl surveys are conducted only during daylight hours when Atka mackerel are often above the bottom (A manuscript on  Atka mackerel behavior has been submitted to  Marine Ecology Progress Series).

Figure 4. Vertical movements of a Pacific cod off Kodiak Island, Alaska (tag #103).

In November 2001, 224 Pacific cod were released with data storage tags off Kodiak Island.  Thus far, 143 tags have been recovered (64% return rate).  The high return rate resulted from the combination of very limited horizontal movements of cod and concentrated fishing by commercial longline and pot fisherman on the location where tagged fish were released.  Based on the gradual return to the initial capture depths, Pacific cod likely had swimbladder damage from the initial capture, but all fish recovered after a period of  1 to 3 weeks (Fig. 4 above).  Vertical movement patterns have not shown near the regularity in diel pattern that Atka mackerel displayed; however, intermittent periods of diel movement did occur.  We plan a comprehensive analysis comparing vertical movement patterns between males and females, mature and immature fish, and among fish of different length and ages.

Planned data storage tag research for 2002 includes tagging of Pacific cod off Unimak Island in the eastern Bering Sea during April, tagging of walleye pollock (Theragra chalcogramma) off Kodiak Island during May, and tagging of additional Atka mackerel in Seguam and Tanaga Passes during June.

By Dan Nichol.

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Jan-March 2002