Intense removal and non-saturating functional responses by recreational divers on spiny lobster Panulirus argus

Abstract

The effects of recreational fishing often go undetected, yet can lead to the collapse of fishery populations. Fishery managers must understand the dynamics of recreational fishers and their prey to successfully predict the outcome of management actions. We measured the impact of a 2 d exclusively recreational fishery on the Caribbean spiny lobster Panulirus argus in the Florida Keys, USA, and used a theoretical, predator-prey functional response approach to identify whether or not sport diver catch rates were density-independent or -dependent (Types I, II or III functional response), and if catch rates were saturated (i.e. reached an asymptote) at relatively high lobster densities. Herein, we describe how this predator-prey framework could be applied to fisheries management for the spiny lobster. Our approach consisted of SCUBA diver surveys of lobster distribution and abundance patterns immediately before and after the 2 d recreational 'mini-season' coupled with counts of recreational boats and divers to assess the aggregative response of divers to lobster density. During the mini-season, the density of boats along coral patch reefs and patch heads was over 900× higher than during the regular fishing season ∼3 wk later. Overall, there was a 95 and 79% decline in mean lobster densities in the patch reef and patch head habitats, respectively. Fishing effort was 10× greater on patch heads than on reefs, probably due to ∼100-fold higher lobster densities in patch-head habitats. In both patch reefs and heads, the proportion of lobsters removed by divers was density-independent (Type I functional response). Diver catch never reached saturation at natural lobster densities. Thus, recreational divers have a relatively simple predatory response to spiny lobsters, whereby catch rates increase linearly with lobster density such that catch is a reliable indicator of abundance. A major concern is that the current levels of lobster exploitation by sport divers are extremely high (∼90%); however, diver predation pressure is not expected to increase proportionally with a decline in lobster density, which could exacerbate local extinction. Management actions that reduce diver effort should have a concomitant reduction in catch. The recreational diver-lobster predator-prey construct in this study provides a useful predictive framework on which to build as the spatiotemporal scales of measurements expand, management actions are implemented, and the lobster population varies.