Estimating the effects of 17α-ethinylestradiol on populations of the fathead minnow Pimephales promelas:: Are conventional toxicological endpoints adequate?

Environmental benchmarks have recently been proposed for several steroids including the synthetic steroid, 17alpha-ethinylestradiol (EE2). These benchmarks are based on extrapolation from studies involving long-term exposure of various fish species to EE2. One of the critical studies was a complete life-cycle experiment performed with the fathead minnow Pimephales promelas over a 289 day exposure period (2). The lowest observed effect concentration (LOEC) and the no observed effect concentration (NOEC) for gonad histology were 4 and 1 ng L-1, respectively. This was because no testicular tissue could be found in any fish exposed to 4 ng L-1. In the present paper, the survival and reproduction data from that study are reanalyzed to determine the effects of EE2 on the intrinsic rate of population growth (r = In (lambda)), a parameter of demographic importance. We estimate critical threshold concentrations with respect to r and compare these with those previously derived from conventional toxicity test summaries. Further, we assess the influence of individual variability on threshold estimates using a combination of bootstrap and regression approaches, together with a suite of perturbation analyses. These yield E(r)C100 values (the concentration estimated to reduce intrinsic growth rate to zero) of 3.11 ng L-1 (linear model) and 3.41 ng L-1 (quadratic model), comparable with a maximum acceptable toxicant concentration (MATC) of 2 ng L-1 for feminization of exposed fish calculated by Laenge et al. (2). Our results indicate that reduction in population growth rate with increasing concentration occurred more through EE2 acting to reduce fertility than survival rates. The significance of these summary statistics when deriving environmental benchmarks for steroid estrogens is discussed in the context of affording protection to populations following long-term exposure.