Effects of an experimental rainbow trout (Oncorhynchus mykiss) farm on invertebrate community composition

We examined the development of changes in the zoobenthos along a transect from an experimental rainbow trout (Oncorhynchus in kiss) farm in Lake 375, Experimental Lakes Area. northwestern Ontario, Canada. After 2 months, invertebrate abundance was reduced under the fish cage (2542 +/- 569 individuals . m(-2)) compared with samples collected 45 m away (16 137 +/- 2624 individuals . m(-2)). Taxa richness was also depressed, but changes in biomass were variable. Reductions in abundance and richness at high organic loading levels are consistent with earlier models developed for the marine environment of responses to organic loading in marine systems. After two production cycles, the significant principal components axis explaining 76% of total variance in abundance was correlated with distance from the cage (Spearman rank correlation, r = -0.775, p = 0.014) and with chemical variables recommended for freshwater aquaculture monitoring (Pearson's correlation coefficient, r = 0.78, 0.76, and 0.75 with p = 0.013, 0.018, and 0.020 for pore-water ammonia and sediment Cu and Zn, respectively). The effects of farming were localized, dissipating within 15 in of the cage edge. Invertebrate abundance demonstrated the most potential for incorporation into monitoring schemes at new farms. At established farms, richness may be a valuable monitoring metric.