Can marine reserves protect migratory species?

Most marine reserves are optimized for reef fish. These are fish that are born, live, reproduce, and ultimately die in a small area – sometimes on just a single reef. Where there is connectivity across a large area, it’s usually while the fish is in its larval stage. Once it matures, it stays put.

It’s a fitting strategy for conserving fish that live on coral reefs, rocky reefs, or in kelp forests, but does it do much to help those species that are more migratory? These are animals, like the Gulf of Mexico’s gag grouper, that spend their childhoods in one place, a nursery habitat like a mangrove, estuary, or kelp forest, and then migrate to live out their adult years in an adult habitat, like a reef or along the continental shelf.

What would a marine reserve optimized for these migratory species look like, at least in theory? That’s the question that University of North Carolina, Wilmington marine biologist J. Wilson White took up in a recent issue of the journal Biology Letters.

He started by using a mathematical model to describe a theoretical population of fishes that spend some amount of time in a nursery before migrating to an adult habitat after maturation. He simplified his model by restricting maturing juveniles from a given nursery to migrate to its own paired adult habitat. However, mature adults could move about between individual adult habitats in some cases.

White then subjected his theoretical fishes to the pressures of three different types of theoretical fishing. Under one regime, the fishery targeted only the adult habitat. Under a second regime, the fishery targeted only the nursery. Under the third, the fishery targeted both habitats. Each theoretical fishing regime was simulated both with and without reserves.

He found in most cases that reserves were critical for maintaining fisheries at sustainable levels. In other words, they were necessary to guard against overexploitation.

When fishing was allowed in the adult habitat, White’s model mimicked the “reef fish” model. Instituting reserves allowed the fish to be harvested at a high yield; without reserves, the fishery collapsed. However, the fishery collapsed even at high yields in the adults had high levels of connectivity between adult habitats. Reserves were only useful when the adults didn’t move around all that much…making them, in some ways, just like reef fish.

When fishing was restricted to the nurseries, only non-reproductive juveniles could be targeted. Reserves in this case also allowed for high fishing yields without overexploitation. This is because juveniles do not disperse among individual nursery habitats. That means that when a nursery gets fished, it is not restocked by juveniles from neighboring habitats. Instead, it gets restocked when larvae migrate from the adult habitats to their nurseries. When fishing was allowed in both nurseries and adult habitats, reserves became critical in both nurseries and adult habitats. Placing reserves into nurseries or adult habitats alone did nothing to prevent overexploitation. In fact, the results were indistinguishable from scenarios in which there was no reserve at all!

Together, White’s analysis leaves us with three things. First, he shows that no-take reserves could benefit migrating fish in some scenarios but not in others. Second, it reveals what types of data are necessary – adult and larval connectivity patterns – to collect from a given species, which can then be used in order to determine the most effective reserve design to protect that species. Finally, it suggests that reserves can act as a buffer against overexploitation in some circumstances, but it is not useful to enhance a fishery. To allow a fishery to recover from exploitation would require just leaving it alone. – Jason G. Goldman | 06 February 2015

Source: White J.W. (2015). Marine reserve design theory for species with ontogenetic migration, Biology Letters, 11 (1) 20140511-20140511. DOI: http://dx.doi.org/10.1098/rsbl.2014.0511

Header image: shutterstock.com

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