Low-Data Methods for Managing Fisheries Around Marine Reserves Using Fishery-Dependent and Fishery-Independent Length-Frequency Data

When a no-take marine reserve is established, fish densities rebuild slowly, so that the comparison of densities between fished and unfished areas may not be informative about fishing mortality rates in the fished zone. Conversely, the difference in length frequencies between fished and unfished regions can provide information about fishing and natural mortality within a few years of the formation of a marine reserve. We applied a Bayesian length-based cohort analysis model to length-frequency data from fishery independent surveys inside and outside a marine reserve, to estimate F and M. Length-frequency data from the fishery were used to infer fishery selectivity. This method has the potential to be biased if there is movement of fish between the fished and unfished reserves. Therefore, we tested the model using simulated data under several movement scenarios.  The model was then applied to data from the PISCO intertidal survey in and around the Channel Islands Marine Reserve, California, for California sheephead, kelp bass, blue rockfish, and kelp rockfish. Results indicate that fishery independent marine reserve monitoring data can provide useful estimates of natural and fishing mortality rates. The method would be effective for data-poor fisheries lacking catch time series.