Estimation of short-term tagging mortality of adult Atlantic cod (Gadus morhua)

We investigated short-term tagging mortality in Atlantic cod (Gadus morhua) tagged at the base of the first dorsal fin with t-bar anchor tags (Floy FD-68BC). Overall mortality of tagged and untagged cod held in submersible enclosures was low, with 11.8% of 719 tagged and 10.3% of 146 untagged cod dead or in poor condition after retention periods of 5-10 days; only two fish lost a tag. The effects of length at capture (range 45-97 cm), gear type (hand-line, otter-trawl, cod trap), and season (spring and summer/fall) on survival of tagged cod was also examined using a generalized linear model. Only season was found to be a significant (P < 0.0001) factor. Mortality of tagged cod was highest in summer and fall (22%, range 21-25%) when water temperatures were warmer (5.6-7.9degreesC) or showed wide fluctuations, and mortality was lowest in spring (3%, range 0-10%) when water temperatures were consistently cold (-1.0 to 3.0 degreesC). Overall survival of three batches of cod (n = 200) caught by otter-trawl at depths of 110-179 m, tagged, and held in tanks of ambient seawater (1.0-2.0degreesC) on a research vessel for 5 days was 91.5%. Relative survival of tagged cod caught with otter-trawl, cod trap, or line-trawls compared to tagged cod caught with hand-lines was also examined and found to be not significantly different, at least at depths <100 m. In many of our field studies, typically from 25 to 45.5% of the initial number of tagged cod have been recaptured to date; thus at least that percentage of tagged individuals must have survived. These percentages increase to between 45 and 95% when natural mortality, tag loss, and reporting rates are also taken into account. Overall, the results indicate high survival of tagged adult cod, particularly when water temperatures are consistently cold and capture depths are <180 m; survival of adult cod caught for tagging in deeper water (>200 m) requires further investigation. (C) 2003 Elsevier B.V. All rights reserved.