A method for evaluating effects of toxic chemicals on the productivity of freshwater ecosystems.

The finding that trout apparently consumed more invertebrates than were produced in the Horokiwi stream has been described as "Allen's paradox." One explanation for this is that invertebrate sampling techniques underestimate the standing crop. If so, trout growth might be a relatively sensitive indicator of the effects of toxic chemicals on the productivity of experimental ponds. In a replicated pond experiment, methyl parathion was applied at concentrations toxic to invertebrates but not to fish. Three 50-m3 ponds were treated at 40 micrograms liter-1 and three at 10 micrograms liter-1, and there were three untreated controls. Each pond was stocked with eight small, individually marked rainbow trout. The fish were removed by electrofishing 3 weeks after treatment. Mean growth rate of fish in control ponds was 6.3% per day, 4.3% per day in ponds treated with 10 micrograms liter-1, and 3.7% per day in ponds treated with 40 micrograms liter-1. Effects were significant at the 1% level. The standing crop of invertebrates was apparently insufficient to support the growth of the fish, an indication that the active, predatory rainbow trout is more efficient at sampling invertebrates than standard limnological procedures.