When an area is closed to fishing, we expect increases in the abundance of targeted species, but changes in species not targeted by fishermen are more difficult to understand. This indirect effect of MPAs was examined in the waters off the west coast of Australia surrounding the Houtman Abrolhos Islands, where coral reefs mingle with large kelps in a transitional zone between temperate and tropical waters. In 1994, MPAs were established throughout this group of islands in order to protect reef fishes from overharvesting by commercial and recreational fisheries. In November of 2004 and March of 2005, a group of Australian scientists took to the water using underwater cameras to record the changes that occurred inside and outside four of these MPAs. They counted the number and species of each fish that swam into view of the camera, and used freeze-frames from the footage to ascertain the type and abundance of coral at each location.
Not surprisingly, species targeted by fishers were much higher inside the protected areas, including coral trout (leopard grouper), emperors, and snapper. In some cases, these abundances were eight times higher inside the protected areas. However, some fished species were less abundant inside the MPAs. Confirmed illegal fishing and movement of fishes from shallow to deeper waters for spawning confounded findings, but indirect ecological interactions within the protected areas also played a role in this perplexing discovery.
As the abundance of predatory species, which are commonly targeted by both commercial and recreational fishers, increases inside the MPAs there are often cascading changes lower on the food chain. This complex web of interactions can drive counterintuitive patterns as species respond not only to protection from fishing but also the corresponding abundances of their predator, prey, and competitors. In this case, the scientists found a greater diversity of species in shallow parts of the MPAs compared to fished areas. Of those species that were less abundant within the MPA, most were prey species. Butterfly fish, often considered an indicator of reef health, were also more common inside the MPAs.
This study highlights two important factors for determining MPA success. First, MPAs may not increase the abundance of all species, especially those lower on the food chain. In this case, one of the most heavily fished species is a dominant predator. As the predator species began to recover and increase in abundance within the MPAs, the abundance of its prey species declined. While this shift represents a restoration towards an unfished system, it serves as a reminder that not all species respond in the same way to MPA protection. Second, fishing prohibitions must be enforced in order for MPAs to be successful. While there were more fish found in MPAs than in nearby fished areas, documented illegal fishing activity decreased the abundance of several target species within the MPA. MPA protections must be enforced in order to realize their full benefits for targeted fish species.
To learn more: