Feb 1998

The aquaculture industry has - and for the foreseeable future is likely to have - some reliance on chemotherapeutants for prevention of damage or loss of stock by certain pathogens and parasites. These chemotherapeutants are given as injections, bath treatments or as infeed preparations and wastes from unused drug from baths, or that contained in fish excreta or waste food may enter the environment.

Attempts to modify conventional cages to contain and collect waste feed have had limited success. The use of simple collection devices beneath cages has proved cumbersome and the effectiveness of containing the wastes is questionable. Stock transfer to well-boats allows the containment and removal of wastes but the capital costs are likely to be prohibitively expensive for all but the largest producers. There are also many practical operational constraints, including increased stress to fish during transfer. Current approaches to minimising chemotherapeutants use involve monitoring infestation levels and targeting treatment to when it is likely to be most effective. As a result, demand for treatment and containment equipment is likely to be highly seasonal in nature, making leasing arrangements problematic.

Feeding systems primarily developed to minimise feed losses also minimise the amount of in-feed drugs used during treatment, although most fish farmers feed by hand during treatment.

Although land-based svstems currently offer the greatest potential for containment and treatment of wastes following chemotherapeutant use, the systems are not viable for commercial salmon production under present economic conditions.

Novel floating closed cages offer promise with regard to chemotherapeutant waste recovery. The capital costs are less than those of land-based facilities and it mt y be possible to fit the impermeable bags to existing cage structures. The ability to control the quality of water entering the cage environment could be beneficial. Water pumped from deeper areas is likely to be more stable than surface waters in terms of salinity and temperature. More importantly, a controlled water intake may allow avoidance of harmful organisms, such as sea-lice and other parasites, toxic algae and jellyfish, as well as pollutants in surface waters, thus reducing treatment requirements. At the present time, however, floating closed cages remain untried in a commercial environment and the technology requires further consideration. Currently available systems, under trial in Canada and Norway, discharge untreated wastes into the sea, via an outlet pipe, although there is potential for treatment before discharge.

Aquaculture chemotherapeutants, sea cages, salmon, waste treatment

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