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. Testwater volumes and replenishment were high enough in these experiments that decreased toxicant loading at higher densities did not appear to be a factor contributing to these results.