Spatial management for protogynous sex-changing fishes: a general framework for coastal systems

Most models of fish population dynamics ignore differences between male and female fish. Yet many harvested species are protogynous hermaphrodites (older females change sex into males), so size-selective fishing will disproportionately remove males. This shifts the sex ratio, potentially disrupting reproduction. Some modeling studies have investigated management strategies for protogynous fishes, but there is no general theory explaining how spatial fishery management should account for protogyny. We developed a spatially explicit model of a generic protogynous fish population to examine the factors affecting the population persistence of coastal (i.e. non-migratory) protogynous populations under spatial management. We varied both (1) biological factors-the cues triggering sex change and the mating function (the relationship between sex ratio and fertilization success) and (2) management factors-configuration of no-take reserves and fishery management outside reserve boundaries. We found that the number and size of reserves required for persistence depended strongly on the sex change cue and the shape of the mating function. Populations with less flexible sex change and requiring more males for fertilization needed more and larger reserves. Unfortunately, empirical mating functions are poorly known for most species and a worthy target of future research. Additionally, persistence of populations with less flexible sex change was impaired by fishery regulations that concentrated on male size classes outside of reserves. Finally, we found that increases in the sex ratio (proportion male) inside reserves are not a reliable indicator of reserve success. These results can be used to design spatial management plans and to set expectations for management assessments.