Fish Hatchery Management - fisheries & aquaculture

HAI CHF],RY RFTQIJIRLMEN I Sl3Heauy MetalsThere is a wide range of reported values for the toxicity of heavy metals tofish. Concentrations that will kill 50'li, of various species of fish in 96 hoursrange from 90 to 40,900 parts per billion (ppb) for zinc,46 to 10,000 ppbfor copper, and 470 to 9,000 ppb for cadmium. Generaily, trout and salmonare more susceptible to heavy metals than most other fishes; minuteamounts of zinc leached from galvanized hatchery pipes can cause heavylosses among trout fry, for example. Heauy metals such as copper, lead, {nc,cadmium and mercurl should be aaoided in fish hatcher] water supplies, as shouldgalvanized steel, copper, and brass fittings in water pipe, especially inhatcheries served by poorly buffered water.SalinityAll salts in a solution change the physical and chemical nature of waterand exert osmotic pressure. Some have physiological or toxic effects aswell. In both marine and freshwater fishes, adaptations to salinity arenecessary. Marine fishes tend to lose water to the environment by diffusionout of their bodies. Consequently, they actively drink water and get rid ofthe excess salt by way of special salt-excreting cells. Freshwater fishes takein water and very actively excrete large amounts of water in the form ofurine from the kidneys.Salinity and dissolved solids are made up mainly of carbonates, bicarbonates,chlorides, sulphates, phosphates, and possibly nitrates of calcium,magnesium, sodium, and potassium, with traces of iron, manganese andother substances.Saline seepage lakes and many impounded waters situated in aridregions with low precipitation and high rates of evaporation have dissolvedsolids in the range of 5,000-12,000 parts per million. Fish production insaline waters is limited to a considerable extent by the threshold of toleranceto the naturally occurring salt. Rainbow trout, as an example, generallytolerate up to 7,000 parts per million total dissolved solids. Survival,growth and food efficiency were excellent for rainbow trout reared inbrackish water at an average temperature of 56"F. The trout were convertedfrom fresh water to i30 parts per thousand over a 9-day period andwere reared to market size at this salinity.Mineral deficiencies in the water may cause excessive mortality, particularlyamong newly hatched fry. Chemical enrichment of water with calciumchloride has been used to inhibit white spot disease in fry. Brook trout canabsorb calcium, cobalt, and phosphorous ions directly from the water.