Pulp mills make more males: skewing the sex ratio in fish. (Science Selections).

The toxicity of discharges from some pulp mills has decreased due to changes in manufacturing processes. Yet effluent from cleaner mills can still contain compounds that cause measurable effects in wildlife, according to research by D. G. Joakim Larsson and Lars Forlin of Goteborg University, Sweden [EHP 110:739-742]. The pair present epidemiologic data that support a causal relationship between exposure to pulp mill effluent and male-biased sex ratios in fish that may be mediated by endocrine-disrupting substances in the effluent.

Endocrine disruptors are chemicals that interfere with the action of hormones produced by the endocrine glands. Hormones affected by endocrine disruptors can include androgens and estrogens, which are responsible for the development of male and female sex characteristics, respectively.

During the 199742000 study, the researchers studied broods of the viviparous eelpout (Zoarces viviparus), a small bottom-dwelling marine fish. Eelpout mate once a year in early autumn, and the mothers carry their young for about two months. Each fall, in late October to mid-November, eelpout embryos were collected near a kraft pulp mill on the Swedish Baltic coast that uses chlorine-free bleaching technology. Samples were taken at distances of 1.7-46.0 km from the mill. Prior to collection, the eelpout broods had been exposed to effluent during critical periods in embryonic sexual development.

In 1997, eelpout harvested 1.7 km from the mill were significantly male-biased (54.5% male) compared with the reference sites, where sex ratios were closer to 50/50. Samples taken farther from the mill had fewer males, with the male bias decreasing on a gradient as the distance from the mill increased. The pattern was similar in 1998 (57.8% and 53.9% male at the two sites closest to the mill).

Pulp production was stopped for 17 days in 1999 while the mill's facilities were modified to allow increased production. This shutdown coincided with the period when sexual differentiation usually occurs in eelpout embryos, a development that allowed Larsson and Forlin to test whether the skewed sex ratios previously found by the mill were associated with exposure to the effluent. Analysis of the broods from 1999 showed that the sex ratios of populations near the mill were normal. But by the following year, sex ratios were again male-biased (61.3% male).

The researchers also compared eelpout collected near the mill with those taken from sites near an oil refinery, a petrochemical-based factory, and a river polluted with heavy metals and organo-chlorines, as well as two sites with mixed exposures. Only the eelpout collected near the pulp mill had skewed sex ratios.

The Swedish research is consistent with a Canadian study in which mill effluent acted as an endocrine disruptor in white suckers (Catostomus commersoni). Those fish also reverted to a near-normal physiology during a mill shutdown. In both cases, the endocrine-disrupting effects were reversed in a relatively short time once exposure stopped.

Larsson and Forlin are working to identify specific endocrine disruptors in effluents from pulp and paper mills. They hope that results of this research will contribute to an evaluation of manufacturing processes to further reduce the effects of pulp mill effluents on the environment.