Research Document - 2006/011

Effects of Shellfish Aquaculture on Fish Habitat

By McKindsey, C.W., M.R. Anderson, P. Barnes, S. Courtenay, T. Landry, and M. Skinner

Abstract

The purpose of this document is to provide information about the role of bivalve culture in the ecosystem to enable habitat management to make informed ecosystem-based management decisions with respect to the bivalve culture industry. We do this by considering the roles of bivalves in the ecosystem under natural conditions, describing culture methods and conditions used in Canada and elsewhere, and subsequently evaluating whether these roles are mimicked under aquaculture conditions.

To date, much research (and regulation) has focused on sedimentation processes and their influence on biogeochemical and biological processes in sediments below suspended bivalve culture operations. Similarly, much effort has been directed to developing production carrying capacity models. Such models typically focus on water column (phytoplankton, zooplankton, nutrients and detritus) and benthic (nutrients) processes to ensure that harvests may be maximized.

These approaches are quite well developed and go some distance towards allowing for ecosystem-based management. That being said, they also provide a somewhat truncated and negative view of the role of bivalve culture in the ecosystem. In this document, we develop the model that many ecosystem services provided by bivalve culture are in fact positive and may largely compensate for the more negative effects that are often considered. We concentrate on macrofauna (macroinvertebrates and fishes) and species directly associated with bivalves in culture and suggest that these organisms should be considered more often and formally when decisions are made with respect to bivalve culture operations. We also highlight some issues that we feel need be better addressed so that true ecosystem-based management may be practiced.

A review of the literature shows that bivalves are very important in many ways in the way they interact with the environment. They may exert considerable influence on planktonic communities and nutrient cycling. Through a series of mechanisms, they also greatly promote the diversity and productivity of the assemblages associated with them and may have cascading effects on the ecosystem as a whole.

The main species cultured in eastern Canada are the native mussel (Mytilus edulis and Mytilus trossulus) and the American oyster (Crassostrea virginica). On the West Coast, the main cultivated species are the Pacific oyster (Crassostrea gigas) and the manila clam (Venerupis philippinarum), both of which are not native to that area. Mussels are mostly farmed in longline systems whereas oysters in eastern Canada are mostly in bottom or off-bottom culture. On the West Coast, oysters are culture both on the bottom and off-bottom as well as increasingly in suspended culture. Spat on the East Coast is almost all form wild set whereas the reverse is true on the West Coast.

On the whole, it was found that bivalves in culture played much of the same roles as do bivalves under natural conditions. That being said, the greater concentration of bivalves in culture does lead to some negative effects on the ecosystem due to increased organic loading in the vicinity of the farms and to harvesting in bottom and off-bottom culture. On the other hand, bivalve culture operations also function more or less as do artificial reefs. In suspended bivalve culture, the abundance of fouling organisms and large mobile species that are associated with these fouling organisms and the abundance of macroinvertebrates and fishes directly under culture operations was, when evaluated, consistently great. Some work has shown that the presence of these species may compensate for any losses directly below suspended culture operations. Similar increases in associated species have also been observed in off-bottom culture operations. Interactions between bivalve culture and birds and marine mammals are variable.

A number of methods are discussed to address sampling strategies for the suite of organisms that we feel should be included in the evaluation of the influence of bivalve culture in the ecosystem. We also briefly outline issues concerning aquatic invasive species in bivalve culture. We finish up by highlighting certain knowledge gaps and make recommendations for future research.

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