Length based assessment of spawning potential ratio in data-poor fisheries for blue swimming crab (Portunus spp.) in Sri Lanka and Indonesia: Implications for sustainable management

Abstract

Quantitative assessment and management of small-scale fisheries is a persistent problem for fisheries. Crustaceans are particularly challenging for conventional techniques because their lack of permanent hard body parts makes ageing difficult. While a growing body of literature is aimed at developing alternative approaches to small-scale fisheries assessment there are concerns about the indiscriminate application of generic methodologies and the need to address the specific circumstances of fisheries. The length-based assessment of spawning potential ratio (LBSPR) is appealing because it has simple data requirements, a well-developed theoretical foundation, and estimates Spawning Potential Ratio (SPR) an internationally recognized indicator of stock status. With the aim of establishing basic standards and providing structured discussion of the issues raised by its application, this study documents its application to the regionally important blue swimming crab (BSC) (Portunus spp.) fisheries, in Sri Lanka and Indonesia. Our study demonstrates the methods technical feasibility and cost-effectiveness for small-scale BSC fisheries and by extension other small-scale fisheries. In Sri Lanka and Indonesia, these LBSPR assessments are successfully informing discussions about sustainability; focusing discussion on managing size selectivity, one of the few management controls available to fisheries managers in many small-scale fisheries. In Indonesia BSC are first caught at around the size of maturity by trawling and with baited traps without escape gaps, reducing SPR to unsustainable levels. Despite similarly high fishing pressure, in Sri Lanka the larger size of first capture with bottom set nets preserves a sustainable level of SPR. Supported by these assessments, in 2018, that fishery attained a provisional sustainability rating from Seafood Watch. We suggest how LBSPR assessment might be used to adaptively manage size selectivity within the harvest strategy being developed in Indonesia. It is hoped this documentation of the methodology will assist other small-scale fisheries to use this technique similarly.