Endocrine disruption in wild freshwater fish
Endocrine disruption has been reported in freshwater fish populations around the world. This phenomenon ranges from subtle changes in the physiology and sexual behavior of fish to permanently altered sexual differentiation and impairment of fertility. Despite widespread reports of endocrine disruption in fish (and this is well characterized at the individual level), few studies have demonstrated population-level consequences as a result of exposure to endocrine-disrupting chemicals (EDCs). An exception to this is in Lake Ontario Lake trout where precipitous declines in the population have been linked with periods of high exposure to organochlorine chemicals (known EDCs). Recently, it has been established that roach (Rutilus rutilus) exposed to treated sewage effluent (that contains complex mixtures of EDCs) in UK rivers, have a reduced reproductive capacity. This, in turn, may have population-level consequences. Evidence for a link between exposure to effluents from kraft mill (BKME) and sewage treatment works (STWs) and altered reproductive function in freshwater fish is compelling. In most cases, however, a causal link between a specific chemical and a physiological effect has not been established. Indeed, identifying specific chemical(s) responsible for adverse effects observed in the wild is difficult, given that tens of thousands of man-made chemicals enter the aquatic environment and that mixtures of chemicals can have combination (e.g., additive) effects. Some EDCs are known to act at a number of different body targets to affect a variety of physiological processes, further complicating the identification of the causative agent(s). Endocrine disruption appears to be particularly widespread in freshwater fish populations. There is little evidence, however, to suggest fish are more susceptible to EDCs relative to other wildlife. Notwithstanding this, there are some features of the endocrine physiology of fish that may be particularly susceptible to the effects of EDCs, including the processes of sex-determination and smoltification (in salmonids). Furthermore, their aquatic existence means that fish can be bathed constantly in a solution containing pollutants. In addition, uptake of chemicals readily occurs via the gills and skin, as well as via the diet (the major exposure route for most EDCs in terrestrial animals). The exposure of fish early life stages to the cocktail of EDCs present in some aquatic environments may be of particular concern, given that this is an especially vulnerable period in their development. The challenge, from the point of view of ecological risk assessment, is to determine effects of EDCs on freshwater fish populations and freshwater ecosystems. In order to meet this challenge, high-quality data are required on the population biology of freshwater fish, on the effects of EDCs on their various life history characteristics, and comprehensive and appropriate population models. Basic information on the population biology of most species of wild freshwater fish is, however, extremely limited, and needs significant improvement for use in deriving a sound understanding of how EDCs affect fish population sustainability. Notwithstanding this, we need to start to undertake possible/probable predictions of population level effects of EDCs using data derived from the effects found in individual fish. Furthermore, information on the geographical extent of endocrine disruption in freshwater fish is vital for understanding the impact of EDCs in fish populations. This can be derived using published statistical associations between endocrine disruption in individual fish and pollutant concentration in receiving waters. Simplistic population models, based on the effects of EDCs on the reproductive success of individual fish can also used to model the likely population responses to EDCs. Wherever there is sufficient evidence for endocrine disruption in freshwater fish and the need for remediation has been established, then there is a need to focus on how these problems can be alleviated. Where industrial chemicals are identified as causative agents, a practical program of tighter regulation for their discharge and/or a switch to alternative chemicals (which do not act as EDCs) is needed. There are recent examples where such strategies have been adopted, and these have been successful in reducing the impacts of EDCs from point source discharges on freshwater fish. Where EDCs are of natural origin (e.g., sex steroid hormones from human and animal waste), however, remediation is a more difficult task. Regulation of the release of these chemicals can probably be achieved only by improvements in treatment processes and/or the implementation of systems that specifically remove and degrade them before their discharge into the aquatic environment.