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Status of Stocks & Multispecies Assessment Program

Release of Pacific Cod with Geolocating Archival Tags in the Gulf of Alaska

The SSMA Fisheries Interaction Team and other SSMA staff are conducting research to determine the utility of electronic archival tags for studying movement in Pacific cod. In collaboration with the AFSC's Midwater Assessment and Conservation Engineering (MACE) program, Pacific cod were successfully tagged and released aboard the NOAA ship Oscar Dyson during a hydroacoustic survey conducted 20 February 19 March in the Shumagin Island and Prince William Sound areas.

figure 5, Lotek archival tags
Figure 5.  Lotek archival tags implanted into body cavity of a Pacific cod.  Photo by Susanne McDermott.

figure 6, Darin Jones
Figure 6.  AFSC scientist Darin Jones ready to release a tagged Pacific cod.  Photo by Susanne McDermott.

figure 7, light meter sled
Figure 7.  Configuration of the light meter sled.

Pacific cod are an important ecosystem component in the Gulf of Alaska and Bering Sea and support a lucrative commercial fishery. Existing data suggest that Pacific cod migrate annually from summer feeding grounds to winter spawning grounds where they form dense aggregations. Temporal and spatial aspects of this yearly movement pattern are poorly understood.

Archival electronic tags containing environmental sensors (e.g., temperature and depth) can be attached to fish to investigate individual behaviors. These tags are usually recovered through commercial fisheries. Tags that collect light data can be used to estimate the geographic position of fish by using the timing of sunrise, sunset, and local noon to estimate latitude and longitude. Light-sensing tags have been used to predict the geographic location of fish in Alaska waters, but the spatial resolution of the data can vary at different latitudes and with different tag types. A new generation of archival tags (Lotek Wireless Inc., LAT series) has recently been developed and promises to increase the accuracy and precision of geolocation estimates. In addition, these tags are small enough to be implanted in medium-sized fish (40-60 cm) and are therefore a promising tool to record the large-scale movement of many commercially exploited species in Alaska.

The Lotek LAT tags have light, depth, and temperature sensors and store data from these sensors at user-determined time intervals. A processor on the tag produces a daily estimate of position that includes an estimate of the associated error. These tags were tested on fixed moorings in the field in 2009 and also tested on captive Pacific cod in collaboration with the AFSC's Auke Bay Laboratories and Kingfisher Marine Research in Juneau, Alaska.

The AFSC recently purchased LAT tags for implantation in wild Pacific cod. The annual MACE survey in the Gulf of Alaska provided a convenient platform for releasing these tags (Fig. 4). Pacific cod were captured during survey trawls. Live fish were immediately transferred to seawater tanks and monitored for survival. Of 55 fish placed in the tanks, 33 survived, resulting in a 60% survival rate. Mortality was most likely caused by barotrauma (damage resulting from an inflated swimbladder). Twenty-two of the survivors received archival tags, which were implanted into the abdominal cavity with the light stalk protruding from the fish (Figs. 5 and 6). Only one fish died after the tag was implanted, most likely from handling trauma during capture.

In addition to tagging Pacific cod, walleye pollock were tagged with conventional T-bar spaghetti tags and released (Fig. 4). This work served as a pilot project to test the feasibility of tagging pollock caught with the MACE survey trawl gear. Of the 84 pollock captured, 65 survived (resulting in a 78% survival rate) and were tagged. This study demonstrated that pollock had good survival after capture in small survey trawls with knotless 0.5 inch mesh liners. Tagging of pollock during the MACE surveys might therefore prove to be a feasible undertaking in the future.

In a third aspect of the study, several of the Lotek tags were deployed on the trawl gear with two other types of light-sensing tags (Wildlife Computers MK9 and Lotek LTD tags) to compare the performance of the light sensors at depth and under different conditions of water clarity (Fig. 7). Similar comparisons have been conducted in the past, but those tests took place in the tropical ocean, which is typically much clearer than high-latitude waters. The results of this comparison will be used to understand the utility of different tags in Alaska and may guide development of tags designed specifically for northern regions.

The study described here is part of ongoing efforts by the AFSC to understand large-scale movements of Pacific cod and other species in Alaska waters. The successful tagging of walleye pollock on this cruise suggests we may be able to apply similar techniques to the most abundant commercial species in Alaska. The technological development of electronic tags will continue to improve, increasing our ability to glean information from individual tagged fish.

By Susanne McDermott and Olav Ormseth

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