Scientific Models Demonstrate Recovery of a Reef Fish in Two South African MPAs

Photo copyright Wilhelm van Zyl

Along the southern coast of South Africa, great white sharks perform mesmerizing acrobatic feats in the hopes of snagging a tasty sea lion for dinner. But beneath the waves, less aggressive fish flit from one rocky reef to the next. Artisanal fishers still catch many of these species using handlines, one of the oldest forms of fishing technology. The Red Roman, whose scientific name Chrysoblephus laticeps means “golden-eyed broad head”, was one of the 10 most important species in the South African handline fishery and is now considered a “collapsed” fishery. The abundance of this brightly colored red-orange fish has declined so much that it is no longer economically worthwhile to fish for it.

Using powerful new computer models, South African scientists simulated how this species would respond to protection over time. They looked at two rocky reef MPAs; the Castle Rock MPA and the Goukamma MPA, which is 8 times larger. The computer models account for individual variation in size and behavior to predict how the species will fare inside and outside of these protected areas. The Red Roman is an interesting species to study because it changes sex over time, switching from female to male around 7 years of age. This obviously affects recovery since fishing tends to target larger individuals, often skewing sex ratios and leading to sperm limitation.

The models clearly demonstrate the toll fishing can take on Red Roman populations. After less than a decade of fishing, fish density declined to less than half of its original level. More importantly, the models showed that male and female Red Romans respond differently to protection within each of the MPAs, and that the larger Goukamma MPA resulted in more complete recovery than the smaller MPA at Castle Rock. Female fish recovered quickly, but the abundance of male fish increased more slowly and did not reach pre-fishing levels. This sex-specific response to the fishing mortality also affected the sex ratio of females to males. Prior to fishing, females were more abundant than males, but this discrepancy doubled after the onset of fishing. Fishing therefore created an unbalanced population that, without protection, might not have been able to persist.

Many reef fishes change sex during their lifetime, and these fascinating changes can have profound implications on how well protection strategies work. This South African study highlights the importance of considering how different sexes contribute to recovery, and suggests that it will take time for males to rebound from heavy fishing pressure. Young Red Roman are all female, and it will take at least 7 years to produce older males capable of contributing to the next generation. At least for some species, fully realizing the benefits of protection within an MPA will take time. Sex matters, but in this case size matters as well. There was clear evidence that the larger MPA performed better in the long term, suggesting that where possible larger MPAs can offer greater opportunities for recovery.

For more information: Kerwath, SE. 2008. The effect of marine protected areas on an exploited population of sex-changing temperate reef fish: an individual-based model. African Journal of Marine Science 30(2):337-350.