Responses of Stream Zoobenthos to Enrichment of Gravel Substrates with Cereal Grain and Soybean

In four separate replicated experiments, cereal grain or soybean was added at a rate of 500 mL/m2 to standard gravel in parallel troughs in a stream. The invertebrates that colonized the gravel were collected, after 32–65 d, in a net of mesh 350 μm; this retained about 19% of the fauna numerically. Numbers and weights were estimated after instantaneous subdivision, and differences tested by nested ANOVA. The enriched troughs contained 1.5–2.9 times the weight of invertebrates of the controls. Increases in numbers ranged from 1.4 to 1.9 times, and were owing to Chironomidae and Oligochaeta. Ephemeroptera showed a significant increase in only 1 trial. Responses of other groups of insects could not be tested on account of their low abundance. A trial with 1000 mL of grain/m2 did not produce a proportional increase in fauna. No significant differences were found in the numbers and weights of invertebrates drifting from the control and treatment troughs. We conclude that grain and soybean can be used in the field culture of salmonid food organisms.