Research Document - 2010/024

Aquaculture-related physical alterations of habitat structure as ecosystem stressors

By C.W. McKindsey

Abstract

Effects related to the addition or modification of physical structure due to bivalve and finfish cage aquaculture were reviewed. The on-bottom physical structures include anchoring devices for suspended and off-bottom bivalve farms or fish cage farms. In the case of on-bottom culture of oysters, it also includes the oysters themselves. For in-bottom clam culture, it includes the gravel that may be added to culture sites. Vertical structure added to the water column includes ropes and cage/net structures as well as buoys, etc. This infrastructure adds physical structure for colonisation by a large suite of hard-bottom associated species that may not otherwise find suitable habitat in a given area (e.g., muddy bottoms or water column). These have a variety of direct and cascading effects on the surrounding ecosystem. These structures also modify current regimes which may influence various ecosystem processes. Cage and netting structures may trap a variety of large organisms but data on this effect is rare. Modification of the physical environment via harvesting may have greater or lesser effects, depending on the methods used. In general, effects related to the addition or modification of physical structure are not well studied and most effects have not been quantified. Moreover, discussion of effects in the scientific literature is largely based on extrapolations from other systems. Shading adjustments may result from the physical structures used in all forms of culture operations blocking the sunlight. Clearing of waters by fouling organisms (and farmed bivalves) and increased turbidity due to waste from finfish culture sites may also alter light conditions in the water column and on the bottom. A number of specific knowledge gaps were identified, including the need for basic quantified descriptive studies, scale-dependent carrying capacity studies, the establishment of dose-dependent response curves to different stressors, the identification of appropriate indices for determining impacts, and all aspects related to finfish cage culture in freshwater.

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