Are we overthinking management zoning for the Tasmanian southern rock lobster fishery? A size of maturity approach

Abstract

Crustacean fisheries, which are influenced by environmental drivers, can benefit from spatial management solutions. More size limit zones or other controls afford finer tuning of management solutions, but can also increase complexity in fishing operations. Defining management boundaries, and the feasibility of that compliance can be a balancing act between numerous stakeholders from commercial and recreational fishers, environmental groups and the science which informs such decisions. In Tasmania, the Southern Rock Lobster, SRL (Jasus edwardsii), exhibits dramatic growth variation across its range from the north to south of the fishery. A known proxy for growth is the Onset of Size of Maturity (SoM), which we have calculated for SRL at specific locations around Tasmania using a General Additive Model. Five assessment methods (objective functions: absolute difference and squared difference, both with and without equal zone weighting, and directionally penalised absolute difference) were used to identify possible management boundaries. Objective function inputs were the same for each method, and comprised of 100000 permutations of sampled SoM and spatial boundary locations for assessed regimes of 1–5 spatial boundaries. Statistical methods were applied to make zonation optimal for biology based on SoM. The results show that number of spatial zones required to yield “good” results did not require overly complex management solutions, nor unwieldy enforcement. Specifically, an investigation into two distinct spatial boundaries yielded solutions in which a critical, narrow boundary was complemented by a more flexibly located, secondary boundary for results of similar quality. The recommendations provided are in ranges of feasible locations, and likely to satisfy all stakeholders.