Proximity of Pacific cod to the sea floor: Using archival tags to estimate fish availability to research bottom trawls |
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| Institution: | 1. Department of Medical Physics and Applied Radiation Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L8;2. Bruce Power, PO Box 1540, 177 Tie Road, R.R.2, Tiverton, Ontario, Canada N0G 2T0;3. Department of Biology, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2;4. Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1;5. Northern Ontario School of Medicine, 935 Ramsey Lake Road, Sudbury, Ontario, Canada P3E 2C6;1. Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA;2. The Manta Trust, Catemwood House, Corscombe, Dorchester, Dorset, DT2 0NT, UK;3. Pelagios Kakunja A.C., Sinaloa 1540, Las Garzas, 23070, La Paz, Baja California Sur, Mexico;4. Fins Attached, 19675 Still Glen Drive, Colorado Springs, CO 80908, USA;5. Pacific Manta Research Group, University of California Davis Bodega Marine Laboratory, 2099 Westshore Rd., Bodega Bay, CA 94923, USA;6. Santa Rosa Junior College, 1501 Mendocino Ave, Santa Rosa, CA 95401, USA |
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| Abstract: | The percentage of Pacific cod available to Alaskan bottom trawl surveys was estimated from the proximity of tagged cod to the seafloor. Archival tags recorded time and depth data at 15- or 30-min intervals. The distance of a cod above the bottom was computed by subtracting tag depth from bottom depth, estimated as the maximum depth recorded during each 24-h day. These estimates of bottom depth are based on the assumption that cod approach the seafloor at least once a day, and do not undergo migrations up or down bottom gradients. To ensure that migrations over variable bottom gradients did not confound our estimates of bottom depth, we narrowed our analysis to 11 tagged cod that were recaptured in areas of flat bathymetry, and only analyzed data recorded within one month prior to recapture (N = 29,462 depth recordings). Pacific cod maintained short distances above the seafloor that often differed between day and night. Over 95% of the daytime tag recordings occurred within 10 m of the seafloor. Average effective headrope heights for survey bottom trawls currently used in the eastern Bering Sea and Alaska Gulf/Aleutian Islands groundfish surveys are approximately 2.5 and 7 m, respectively. In the absence of any behavior responses to an approaching trawl, we would expect 47.3% of the cod within the water column are available to the trawl used on the eastern Bering Sea survey and 91.6% are available to the trawl used on the Gulf/Aleutian Islands surveys. This study demonstrated that Pacific cod are highly demersal, and current values of trawl survey catchability (Q) used in current stock assessments are consistent with estimates of cod availability to the trawl gear. |
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