A SIMPLE-MODEL OF THE BLUE-CRAB, CALLINECTES-SAPIDUS, SPAWNING MIGRATION IN CHESAPEAKE-BAY

The blue crab, Callinectes sapidus, supports large commercial and recr eational fisheries along the southern and eastern coasts of the United States. Females of the Chesapeake Bay stock, the most important comme rcially, spawn in summer near the mouth of Chesapeake Bay. Estimates o f crab population density on the spawning grounds, based on 18 cruises during the 1986 and 1987 spawning seasons, were published in 1990. In an attempt to put those estimates into a conceptual framework, I have developed a simple model of the blue crab spawning migration in Chesa peake Bay. When fit to the data from 1986 and 1987, the model represen ted the within-year patterns of population density reasonably well. Th e model is based on a scaled normal probability function and requires estimation of only four parameters; these provide year-specific estima tes of the female spawning stock size, the average residence time of a female on the spawning grounds, and two quantities related to migrato ry timing. For the two years examined, the estimates of spawning popul ation size were strongly negatively correlated to the estimates of res idence time, and each estimate had a high coefficient of variation. Th us, in the absence of external information about residence time, the m odel could not furnish precise estimates of the spawning-stock size. A s a plausible working hypothesis, I assumed that the mean residence ti mes in 1986 and 1987 were identical and between 4 and 21 days. Under t his assumption, parameter estimates from the model suggest an approxim ate 60% decline in spawning-stock size from 1986 to 1987.