Estimating sea cucumber abundance and exploitation rates using removal methods

Abstract

Removal methods were used to estimate key fishery parameters, abundance and exploitation rate for five species of tropical sea cucumbers harvested by Indonesian fishers at Scott Reef, north-western Australia. Detailed catch records were kept by the traditional fishers over a period of 58 days as needed for this method, whereas effort was estimated from aerial surveillance. Concurrently, ~1007 artificial sea cucumber surrogates, were distributed and rewards were paid for recovered surrogates. Both datasets were analysed using the Huggins closed-population procedure in program MARK to obtain maximum-likelihood estimates. This procedure allowed inclusion of effort and tide covariates and an initial search phase followed by an exploitation phase. We accounted for extreme over-dispersion which is a common problem in fishery removal data. Our results strongly suggested that some surrogates became unavailable to the fishers. However, results from both datasets demonstrated strong evidence of extreme rates of exploitation on the shallow, drying reef-top habitat. Closed-removal or depletion methods are shown to be a viable method to estimate abundance and exploitation rate for sea cucumbers harvested with intense fishing pressure during a short fishing season.