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Paige Roberts - Fish & Invertebrate Ecology

University of Miami, Florida

Estimating population size, tag retention, and mortality of adult blue crabs

The blue crab (Callinectes sapidus) spawning stock in the Chesapeake Bay has experienced a sharp and persistent decline during the last twenty years. In an attempt to increase the spawning population, hatchery-based stock enhancement is being implemented. To fully understand the potential effectiveness of stock enhancement to augment wild stocks, it is necessary to estimate the current size of the wild population within rivers where enhancement is occurring. We estimated the wild adult population size in the Rhode River using mark-recapture methods. Crabs were captured by SERC-operated or commercial trotline, tagged with an individually numbered tag wired to the carapace, then released. Tagging occurred over five consecutive days in September, 2007 during which time we tagged 3,704 crabs and recaptured 69 (1.9%). Using individual recapture histories, we estimated the population size to be approximately 87,514±10,751 adult individuals using program MARK. However, this estimate assumes no tag loss and no tagging-related mortality. To test the validity of these assumptions we also conducted two consecutive experiments in which tagged crabs captured by SERC (Experiment 1, July-Sept.) and two commercial trotliners (Experiment 2, Sept.-Nov.) were held in tanks for 58-60 days. Mechanical tag loss was minimal in both experiments with a rate of 0.00067±5.67x10-5 tags lost per day and a mean time to tag loss of 31.3±4.3 days. Mortality was significantly different between the experiments with 14.1% mortality in experiment 1 and 68.3% mortality in experiment 2 with observed differences likely resulting from varying conditions among the tagging boats. The most likely causes of the high mortality in experiment 2 were related to (1) the use of a mechanical dipper to take crabs off the trotline and (2) holding crabs in bushel baskets. More research is necessary to investigate other causes of mortality and to fully understand the impacts of these capture methods on the blue crab fishery.