Evaluating Closed Season Options with Simulation for a Tropical Shrimp Fishery

R. A. Watson, V. R. Restrepo, (1995). ICES Marine Science Symposia 199, 391-398.

We used simulation modelling to find seasonal closures which maximized either yield per recruit (Y/R) or relative value per recruit (V/R) for several tropical shrimp fisheries. Each case examined represented a choice of (1) a single versus a multicohort population, (2) the reduction of annual fishing effort levels through closures versus its redistribution to the fishing season, and (3) a range of fishing mortality levels. Under most conditions, seasonal closures which maximized Y/R also maximized V/R, though simulated gains in the latter were smaller. Timing of seasonal closures was more critical for fisheries of single-cohort populations than multicohort populations. Multicohort- based fisheries required shorter seasonal closures to maximize V/R than did those based on single-cohort populations. Assumptions about the disposition of fishing effort normally expended during the period of a seasonal closure greatly affected the best closure choice. Predicted best closures were of a longer duration when annual fishing effort was simply redistributed rather than reduced by closures. Greatest improvements in Y/R (30 to 40% ) were obtained for fisheries based on singlecohort populations when closure effort was redistributed. Predicted increases in Y/R for multicohort-based fisheries never exceeded 7%. The duration of best closures increased with increasing values of fishing mortality when the annual fishing effort was reduced by closures. This trend was reversed when annual fishing effort was conserved. As fishing mortality increased, potential gains in Y/R or V/R improved at the cost of equilibrium egg production.

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