MPAs safeguard against irreversible losses from overfishing

  A team of scientists from the University of Guam Marine Laboratory and other international research institutions has gathered some of the most detailed evidence yet that Guam’s marine protected areas (MPAs) maintain and build reproductive biomass a heavily exploited species of reef fish. The study is reported in the latest issue of the prominent scientific journal The Public Library of Science One (PLoS One).

Combining empirical data and mathematical modeling, the team monitored a population of mafuti (Lethrinus harak), a popular food fish, to develop a detailed understanding of the fish’s population structure. A novel aspect of the study that lends confidence in the results is that the data was inputted into a customized analytical model which was entirely based on empirically derived and locally pertinent parameters.

The study helps to resolve a long-running debate in Guam and worldwide about whether MPAs can serve as a “living larder” by periodically opening the reserves to fishing, or whether succumbing to such a temptation might lead to a fishery’s collapse via a process known as “recruitment overfishing,” in which an adult fish population is depleted to a level where it no longer has the reproductive capacity to replenish itself. The researchers found that even briefly opening MPAs to fishing led to long-term declines in the ability of this species to replenish its numbers, despite many more years of subsequent closure.

“We found that even in a fish with a relatively short life span like mafuti, the MPAs are much slower to recover its largest individuals,” said lead author Brett Taylor, a University of Guam alumnus and Ph.D. candidate at James Cook University, Australia. “Our study provides compelling evidence that fish populations respond sensitively and negatively to temporary opening of MPAs for fishing.”

Co-author UOG professor Jen McIlwain (now at Curtain University, Australia) added, “In fact, opening MPAs even briefly to fishing, results in preferential mortality of the larger and older fishes most responsible for maintaining the population, making recovery all the harder. This, in turn, makes it difficult for an MPA to do its main job of replenishing nearby areas where fishing is permitted.”

This study was conducted at four enclosed reef-flat sites including two marine reserves and two comparable fished sites on Guam. These include Piti Marine Preserve and East Agaña Bay and Achang Marine Preserve and Rios Bay.

The study, “Marine Reserves and Reproductive Biomass: A Case Study of a Heavily Targeted Reef Fish” by Brett M. Taylor, Jennifer L. McIlwain and Alexander M. Kerr, was funded by the U.S. Fish and Wildlife Sportfish Restoration Program and facilitated by the Guam Department of Agriculture and the University of Guam Western Pacific Coral Reef Institute and is freely available at the PLoS One website at: http://dx.plos.org/10.1371/journal.pone.0039599. (UOG)