When a trap is not a trap: converging entry and exit rates and their effect on trap saturation of black sea bass (Centropristis striata)

Abstract

Abstract Bacheler, N. M., Schobernd, Z. H., Berrane, D. J., Schobernd, C. M., Mitchell, W. A., and Geraldi, N. R. 2013. When a trap is not a trap: converging entry and exit rates and their effect on trap saturation of black sea bass (Centropristis striata) – ICES Journal of Marine Science, 70: 873–882. Catch rates are often used to index the abundance of marine organisms, but catch saturation (i.e. declining catch rate as fishing time increases) can decouple catch and abundance. Researchers have struggled to account for saturation when using trap catch to infer population dynamics. We used the underwater video to document entries and exits of black sea bass (Centropristis striata) from chevron traps (n = 26) to quantify catch saturation. Black sea bass catch varied between 3 and 188 individuals for soak times of ∼90 min. Overall, 3564 black sea bass entered the traps and 1826 exited; therefore, over half (51%) of black sea bass entering traps exited before traps were retrieved. Black sea bass catch rates were non-linear and asymptotic for most (81%) trap samples, despite short soak times. Moreover, catch saturation occurred at 50 min, when the entry rate declined and the exit rate increased to a point where their confidence intervals overlapped. Several lines of evidence suggest that the level of black sea bass catch once saturation occurred may be positively related to true abundance, but additional research is needed to more fully test this hypothesis.