Effects of oyster depuration gear on eelgrass (Zostera marina L.) in a low density aquaculture site in Long Island Sound.(Report)

ABSTRACT Oyster (Crassostrea virginica) aquaculture has a long history and tradition in Long Island Sound (Connecticut, USA). Although most of the producers practice traditional on-bottom aquaculture, there are a growing number of individuals utilizing bottom gear for cultivation and depuration. The use of this gear presents a potential conflict in eastern Long Island Sound where the last remaining populations of eelgrass (Zostera marina L.) exist. Shellfish aquaculture activity has been identified as a potential source for negative impacts to eelgrass populations. However, bivalve aquaculture has also been shown to provide an equivalent or greater degree of ecosystem services as submerged aquatic vegetation. The effects of short-term oyster depuration activity were gauged by comparing eelgrass reference sites and experimental plots (eelgrass areas containing oyster depuration cages with and without oysters) in triplicate. Changes in sheath length of the eelgrass 1 m from the cages were used as a proxy for growth rate. The aquaculture gear had no effect on this measure of growth rate of eelgrass in any of the deployments. Sediment characteristics (sediment chlorophyll, sediment % organics) in the cage footprint and 1m from the cages also failed to show an effect of the depuration cages on the local environment. Video monitoring of the footprints and local area indicated little physical damage to the eelgrass beds as a result of the short deployment of the aquaculture gear. The water column at all three sites was vertically well mixed and no effect of the cages on water column light and other characteristics was detectable. The results of this study indicated that at the current level of activity, short-term depuration of oysters has minimal effect on eelgrass growth, water quality and the sediment characteristics measured. However, if depuration activity expands in terms of the amount of gear and/or individual operations, it may result in measurable effects. Understanding the interactions between shellfish aquaculture activity and the marine environment is necessary for sustainable growth of the industry.

KEY WORDS: shellfish aquaculture, depuration, Crassostrea virginica, Zostera marina, effects, Long Island Sound

INTRODUCTION

Long Island Sound (LIS), which is bordered to the north by the coast of Connecticut and to the south by Long Island, NY, USA, supports a large commercial oyster (Crassostrea virginica) aquaculture industry. Bottom cultivation remains the most common farming method; however, a growing number of individuals are utilizing bottom gear such as cages and bags for cultivation (Getchis et al. 2008). These gear types are also being used frequently for depuration of market-sized product. Because there are limited areas suitable for direct harvest, many producers grow-out single layers of oysters in "restricted" or "conditional" waters. The animals are then concentrated in cages for depuration in clean "certified" waters. Growers seek "certified" areas that are shallow and protected. These areas also happen to be prime habitat for eelgrass (Zostera marina L.). Although the depuration period is brief, generally lasting two to three weeks, the use of this gear presents a potential conflict between the management of aquaculture and the remaining populations of eelgrass in eastern LIS.

Eelgrass is the dominant vascular plant of northern estuaries of the east and west coasts of the United States, but populations have declined precipitously throughout its range coincident with anthropogenic disturbance (Short & Burdick 1996, Duarte 2002, Keser et al. 2003, Orth et al. 2006, Burkholder et al. 2007, Micheli et al. 2008). Eelgrass beds provide critical ecological functions such as stabilizing fine sediments, recycling of nutrients into biologically available forms, and providing habitat essential to a myriad of marine organisms including juvenile fish, shellfish, and crustaceans, among others (Hemminga & Duarte 2000, McGlathery et al. 2007, Hasegawa et al. 2008). Eelgrass is considered an indicator of an unimpaired and biologically diverse system and much attention has been focused on identifying and minimizing potential causes of human disturbance, particularly fishing and aquaculture activities, to eelgrass populations in LIS (Connecticut Department of Environmental Protection & Connecticut Department of Agriculture 2007).

Shellfish aquaculture has long been considered a benign use of the coastal environment. Though recently, bivalve aquaculture activity has been identified as a potential source for negative effects particularly to submerged aquatic vegetation (Pregnall 1993, Everett et al. 1995, Rumrill & Poulton 2004, Wisehart et al. 2007) and at high densities, can impact water quality (Kaspar et al. 1985, Nizzoli et al. 2005) and sediment characteristics (Christensen et al. 2003, Bendell-Young 2006, Richardson et al. 2008, Vinther et al. 2008).

However, shellfish beds and aquaculture activity have also been shown to provide ecosystem services similar to submerged aquatic vegetation that include the provision of habitat (Dealteris et al. 2004, Hosack et al. 2006, Ferraro & Cole 2007, Powers et al. 2007) and recycling of nutrients (Dame et al. 1991, Peterson & Heck 1999, 2001) which can in turn improve water quality (Lindahl et al. 2005). Additionally, at low to moderate densities, interactions between eelgrass and filter-feeding shellfish have been shown to facilitate eelgrass growth (Reusch et al. 1994, Bos & Van Katwijk 2007, Wall et al. 2008).