TOWARDS STRATEGIC USE OF FUNGICIDES AGAINST SAPROLEGNIA-PARASITICA IN SALMONID FISH HATCHERIES

The life-cycle of Saprolegnia parasitica was considered in relation to the use of protective fungicides against it in salmonid fish hatcheries. Malachite green oxalate was the example fungicide, and all experiments were at 10-degrees-C. If the fungicide is to be used as a continuous addition to the water, as a protectant, then target stages of the fungus are identified as zoospores and secondary zoospore cysts; the latter, although physically sedimentary, have a remarkably high degree of buoyancy in the water and hence pose a constant threat to the fish. Continuous addition to the water, to give 0.25 mg l-1 malachite green oxalate, would be expected to kill zoospores and zoospore cysts, but 0.05 mg l-1, while allowing zoospore cyst germination, was a strong inhibitor to fungal growth and, aided by natural defences of the fish, might well succeed in disease prevention. If the fungicide is to be used as an intermittent addition to the water (every 24 h), then target stages of the fungus are small mycelia on the fish in addition to zoospores and zoospore cysts in the water. An experimental model was made to consider the vulnerability of these small mycelia, growing on the fish over the preceding 24 h, to malachite green oxalate. If mycelia are of narrow diameter, as expected, then 0.25 mg l-1 malachite green oxalate, applied for 15 min, should control their growth and protect the fish.