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Creating Homes for Fish: Can Oyster Aquaculture Provide New Habitat?By Graham Forrester Oyster aquaculture has undergone a tremendous resurgence in Rhode Island over the past decade. Favorable environmental conditions in Narragansett Bay and Rhode Island’s coastal salt ponds support the growth of high-quality shellfish and suggest the potential for further growth of this valuable industry. However, part of the responsible development of any industry means checking for environmental side effects that might arise alongside the economic benefits. Because of their impressive ability as water filterers and purifiers, the likely effect of oysters on water quality is almost certainly positive. Some of the other possible environmental effects of oyster culture are, however, not as easy to guess. Concerns have been raised that the grow-out phase of oyster culture, which occurs in shallow inshore areas, might impair the value of these habitats as critical nurseries for finfish. This phase of the aquaculture process involves placing juvenile seed oysters in mesh bags, which are then placed on tiered racks on sandy/muddy bottom areas. Racks are briefly removed from the water every three to six months for cleaning, and to harvest legal-sized oysters, but otherwise remain on the bottom continually. The coffee table–sized racks are full of nooks and crannies and become home to small barnacles, shrimps, crabs, and various other little creatures. We wondered whether this abundance of food and shelter might make the racks good habitat for finfishes—especially commercially and recreationally important fish like black seabass, tautog, and scup, which prefer hard-bottom habitats. With funding from the Rhode Island Aquaculture Initiative, colleagues Robert Rheault, Moonstone Oysters president; Jessica Tallman, University of Rhode Island (URI) natural resources science graduate student; and I decided to examine this potentially beneficial side effect of oyster culture more carefully by tracking these fish on the oyster racks at three leased sites in Narragansett Bay. We caught and released fish regularly at the lease sites for two years. Each fish captured was given a unique tag and, by building up a set of records on where and how often tagged fish were recaptured, we were able to calculate the population size and survival rate of fish at the grow-out sites. By measuring each fish as it was captured, and then remeasuring all the fish recaptured at a later date, we were also able to tell how fast the fish were growing. To put these measurements from oyster racks in context, we also tracked fish living on five rocky reefs—which represented prime natural habitat for these fish species—and on the artificial reef that was built in response to the M/V World Prodigy oil spill. Our results were encouraging for oyster aquaculture. There were subtle differences between the oyster grow-out sites, the natural reefs, and the custom-built artificial reef. Scup and tautog were more common at oyster grow-out sites than on natural rocky reefs or the artificial reef, whereas cunner was most abundant on natural reefs. Black sea bass was equally abundant in all three habitats. Fish grew slightly faster on natural rocky reefs, but survived slightly better at the aquaculture sites. These subtle differences are not surprising—each type of habitat provides a slightly different kind of home for the fishes. Like people choosing between Cape Cod, colonial, and raised ranch houses, different fish prefer different sorts of homes. But subtle differences between house styles, and these fish habitats, are less important than the fact that all can provide a good home. Oyster grow-out racks provide good quality habitat for fishes typically associated with hard-bottom habitats, which represents an unintended, but still beneficial, side effect of oyster aquaculture. For further reading: Tallman, J.C. and G.E. Forrester. 2007. Oyster grow-out cages function as artificial reefs for temperate fishes. Transactions of the American Fisheries Society 136:790–799. —Graham Forrester is a URI Natural Resources Science Professor.
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