Effects of Rearing Density on the Growth Response of Juvenile Fathead Minnow (Pimephales promelas) under Toxicant-Induced Stress

The effects of rearing density on the apparent toxicity of pentachlorophenol and 2,4,5-trichlorophenol were investigated using 28-d growth and survival tests with larval fathead minnow (Pimephales promelas). The fish were held at rearing densities of 1, 5, or 10 fish per 1-L cage. Among individual fish, those held at the lowest density showed three times less variability in wet weight at 28 d than those at the highest density. Two-way analysis of variance of the growth data showed a significant interaction between concentration and density for both toxicants, suggesting that density affected the nature of the relationship between exposure concentration and response. The change in the concentration–response relationship was most dramatic for 2,4,5-trichlorophenol; a quadratic function for fish reared at low and medium densities changed to a linear function at the highest density. The estimated thresholds of toxicant impact on growth were twofold lower for low- versus high-density fish for both chemicals. T...