Density-Dependent Catchability and Its Potential Causes and Consequences on Connecticut River American Shad, Alosa sapidissima

Abstract

Using commercial catch per unit effort data (C/f) and relative shad population size, we assessed the hypothesis that commercial catchability coefficients for Connecticut River American shad, Alosa sapidissima, are inversely related to female shad population size. Daily female catch per gillnet drift and commercial effort data from 1978 through 1980 were also used to evaluate the nonrandom search behaviour of fishermen, as well as the extent of temporal heterogeneity of the female shad population. Commercial catchability coefficients were inversely related to female shad population size, the relationship being well described by a power function with an exponent (B) of -0.743. Commercial C/f data were asymptotically and not linearly related to changes in the female population size, indicating that commercial C/f data underestimate percent changes in the female shad population. The inverse relationship between catchability and female population size was not due to serial improvements in gear technology, but rather to the ability of commercial fishermen to differentially exploit patches of female American shad during their upriver migration.