Effect of sublethal concentrations of four chemicals on susceptibility of juvenile rainbow trout (Oncorhynchus mykiss) to saprolegniosis

The effects of sublethal concentrations of a variety of chemicals on the susceptibility of rainbow trout (Oncorhynchus mykiss) juveniles to Saprolegnia parasitica infection was examined. Sublethal concentrations of un-ionized ammonia (0.05 mg/liter) and nitrite (0.12 mg/liter) increased fish susceptibility after 10 days of exposure to the toxin, this increase being higher for ammonia (75% and 20% morbidity) than for nitrite (20% and 0% morbidity, respectively) with inoculum doses of 1.4 x 10(6) and 9.5 x 10(5) zoospores per liter, respectively. Sublethal concentrations of copper (0.05 mg/liter) or cyanide (0.05 mg/liter) did not show enhancement of infection by S. parasitica, even though the toxin exposure was for 21 days and the inoculum doses were higher than those for the experiments with the nitrogen compounds (4 x 10(6) and 3.2 x 10(6) zoospores per liter. However, infections began to appear in control animals.