Zinc Poisoning in a Trout Hatchery

Abstract

Natural acid waters will dissolve zinc from galvanized-iron pipes. In some acid waters with a very low mineral content 0.01 p.p.m. dissolved zinc is toxic to brown and rainbow trout ova and alevins. All stages in the life history of rainbow trout are more susceptible to zinc poisoning than corresponding stages of brown trout. The lethal concentration normally decreases with the age of the fish, but considerable variations have been recorded amongst ova, aievins, fry, and fingerlings. Survivors from batches of fish subjected to dissolved zinc are less susceptible to further toxic concentrations than are specimens that have not previously been in solutions of zinc. Under certain conditions mortality amongst trout fingerlings, due to einc poisoning, will occur after they have been in a zinc vessel for 15 min. The effects of zinc poisoning may not be immediately apparent. When removed from zinc-contaminated to relatively zinc-free water trout may appear normal but death may occur within 48 hr. In well-aerated water, little or no flow is necessary for the development of trout ova, which will develop normally in distilled water. Aluminium paint does not normally protect an underlying metal from the action of acid water. Zinc is an unsuitable material for use in a hatchery with an unbuffered water supply. The lethal temperatures for acclimatized brown (Salmo trutta L.) and rainbow (Salmo gairdnerii Richardson) trout fingerlings in Victoria are 20–30°C. For adult fish the temperatures are 27–29°C. A self-cleaning hatchery inlet screen and a spray jet for circular ponds are described. In hot cliaates the temperature of water entering a pond through a spray jet is less than that of water entering through a normal radial-arm jet.