Analysis of fish population size distributions confirms cessation of fishing in marine protected areas

Will White, Oregon State, Mark Yamane, Oregon State University, OR, Kerry J. Nickols, CSU Northridge, Northridge, CA and Jennifer E Caselle, Marine Science Institute, University of CA Santa Barbara, Santa Barbara, CA

Background/Question/Methods
The number of protected areas that restrict or prohibit harvest of wild populations is growing. In general, protected areas are expected to increase the abundance of previously-harvested species. Whether a protected area achieves this expectation is typically evaluated by assessing trends in abundance after implementation. However, the underlying assumption that harvest has actually ceased is rarely tested directly. Determining whether illegal harvest (poaching) has continued in a protected area is important to planning enforcement and adaptive management. Here, we estimated harvest rates for four kelp forest fish species inside marine protected areas (MPAs) and at non-MPA reference sites in the California Channel Islands, from 2003 (when MPAs were implemented) to 2017. We estimated harvest by fitting a size-structured population model to survey data.
Results/Conclusions
Overall, harvest rates were effectively zero in MPAs but much higher in non-MPA sites. This indicates successful adherence to MPA regulations, and possible displacement of fishing effort to reference sites. However, some poaching was detected in two MPA sites, highlighting the importance of assessing this quantity. This modeling approach could provide a tool to complement the long-term management of MPA networks, particularly given the difficulty of acquiring harvest rate data at the spatial scale of individual MPAs.