Effects of a large fishing closure on benthic communities in the western Gulf of Maine: recovery from the effects of gillnets and otter trawls

Fishery Bulletin, July, 2009 by Raymond E. Grizzle, Larry G. Ward, Larry A. Mayer, Mashkoor A. Malik, Andrew B. Cooper, Holly A. Abeels, Jennifer K. Greene, Melissa A. Brodeur, Andrew A. Rosenberg

Abstract--The recovery of benthic communities inside the western Gulf of Maine fishing closure area was evaluated by comparing invertebrate assemblages at sites inside and outside of the closure four to six years after the closure was established. The major restriction imposed by the closure was a year-round prohibition of bottom gillnets and otter trawls. A total of 163 seafloor sites (~half inside and half outside the closure) within a 515-[km.sup.2] study area were sampled with some combination of Shipek grab, Wildco box corer, or underwater video. Bottom types ranged from mud (silt and clay) to boulders, and the effects of the closure on univariate measures (total density, biomass, taxonomic richness) of benthos varied widely among sediment types. For sites with predominantly mud sediments, there were mixed effects on inside and outside infauna and no effect on epifauna. For sites with mainly sand sediments, there were higher density, biomass, and taxonomic richness for infauna inside the closure, but no significant effects on epifauna. For sites dominated by gravel (which included boulders in some areas), there were no effects on infauna but strong effects on epifaunal density and taxonomic richness. For fishing gear, the data indicated that infauna recovered in sand from the impacts of otter trawls operated inside the closure but that they did not recover in mud, and that epifauna recovered on gravel bottoms from the impact of gillnets used inside the closure. The magnitudes of impact and recovery, however, cannot be inferred directly from our data because of a confounding factor of different fishing intensities outside the closure for a direct comparison of preclosure and postclosure data. The overall negative impact of trawls is likely underestimated by our data, whereas the negative impact of gillnets is likely overestimated.

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The Western Gulf of Maine (WGOM) closure area was implemented by the National Marine Fisheries Service on 1 May 1998 as part of an overall effort to rebuild overfished New England groundfish stocks such as cod (Gadus morhua), haddock (Melanogrammus aeglefinus), other gadids, and flatfish. The WGOM closure is one of the largest year-round closures in the United States, designed to protect habitat and help in the recovery of overfished species. The major restriction imposed by the closure was year-round prohibition of commercial fishing gear that is capable of capturing groundfish, principally otter trawls and gillnets. A variety of other fishing gears (e.g., lobster pots, recreational hook-and-line gear), however, have been deployed in the area since its establishment. The WGOM closed area fits the definition of a marine protected area (MPA) because it provides some level of protection of habitat and resources (NRC, 2002). In many areas, MPAs have been useful management tools but the causal mechanisms for their effectiveness in protecting habitat and meeting other management objectives differ widely, largely because MPAs vary widely in design and other characteristics (Fogarty and Murawski, 2005; Shipley, 2004). Hence, there is a need to assess individual MPAs in the context of environmental and other characteristics, as well as in regard to the particular combination of restrictions placed on each area. For assessing the data presented here, the closure was considered as an area that was potentially recovering from the impacts of gillnets and otter trawls.

There have been no published studies on the effects of the WGOM closure on seafloor habitats, and none designed specifically to address the effects of the closure on groundfish stocks. Recent analyses of commercial trawl data (Murawski et al., 2004, 2005) have shown no appreciable effect of the closure on stock rebuilding in the region, though there has been substantial recovery of some groundfish stocks since the closure was implemented. From studies of MPAs in other areas, for example, New England (e.g., Collie et al., 2005), it seems reasonable to expect that a closure of the magnitude of the WGOM area would have a measurable effect on habitat recovery and fish stock rebounds. Its role in these respects, however, remains to be demonstrated.

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In this article, we present data from grab and box core samples of sediments and infauna, underwater video surveys of benthic fauna, and general seafloor conditions four to six years after the closure of WGOM, all of which indicate that the closure has resulted in substantial recovery of some bottom habitat types. We offer hypothetical causes for the recovery in the context of gear removal, and we discuss the implications for ecosystem-level management of the closure.

Materials and methods

Study area

The study was restricted to a 515-[km.sup.2] area that comprised a section of the central western WGOM closed area and a section outside the closed area (Fig. 1). The overall closure area, which covers much of Jeffreys Ledge, is about 30 km wide (east-west) and 110 km long (north-south) and is located off the southern Maine, New Hampshire, and northern Massachusetts coasts. The location of the study area was chosen mainly because it has similar environmental characteristics both inside the closure and outside the closure.