Changes in Water Circulation

The presence of fish cages, like any other structure placed in a moving fluid, affects the velocity and direction of water movement. The magnitude of the effect depends on the size and mechanical properties of the structure, which for aquaculture equipment is related to the design characteristics and the degree of fouling. The velocity drop within fish cages varies from 35-81% of the external current velocity, depending on stocking density, net mesh size and structure and the growth of fouling organisms.

Local changes in water circulation are, therefore, expected, although predicting the magnitude and extent of effects is impossible because of the extremely variable hydrographic regimes around the coast. Possible water quality impacts could include preventing the dispersal of metabolic wastes, reducing oxygen supply and affecting the distribution of planktonic organisms. These potential effects have their most severe impact on the farmed stock. Given maintenance of good conditions for the farmed stock and selection of sites with good water exchange, the impacts of water quality changes due to poor water circulation per se are considered minimal. The introduction of cages, moorings and other structures also offers new substrates for the attachment of organisms which may affect water quality through respiration or excretion or alter the structure of local food chains.