A fifth of male fish in English rivers now have both female and male or intersex characteristics, according to Exeter University research, which blames hormone-disrupting chemicals that are flushed away and enter wastewater effluents that pass into rivers and streams. These include ingredients in the contraceptive pill, and by-products of cleaning agents, plastics, and cosmetics.
Charles Tyler, a leading fish physiologist and eco-toxicologist, has investigated roach living in UK rivers. He found intersex characteristics in fish at 44 of the 51 sites he studied. Overall, 23%of male fish could be classified as intersex, where the gonad contains both male and female tissues. This means they displayed ‘female’ behaviour and some even produced eggs. Some males had reduced sperm quality and displayed less aggressive and competitive behaviour, associated with attracting females.
In lab studies, Tyler’s team has shown that moderately to severely intersex fish are less able to breed competitively. In wild populations, ‘although we saw a definite trend for a reduced effective population breeding size – that is, fewer males contributing the next generation,’ he reports. ‘This did not turn out to be statistically different; however, it was incredibly close.’
‘We do not yet know if the populations have adapted to oestrogens; this is what we are looking at currently,’ he continues, adding that other studies have shown that exposure to certain environmental oestrogens can have transgenerational effects. The problem is that little is known about any other health effects, he says. ‘Can fish adapt to oestrogenic chemicals and what are the fitness consequences of such adaptations? Such an understanding should help provide insight into population resilience.’
Researchers in this area have also found that some of these chemicals can have much wider health effects on fish. ‘For example, [Thomas Brodin’s team in Sweden found that] antidepressant drugs reduce the natural shyness of some fish species, including the way they react to predators,’ Tyler says.
Tyler’s team used specially created transgenic fish to see responses to these chemicals in their bodies of fish in real time. Oestrogens found in some plastics, for example, have been observed to affect the valves in the heart.
Tyler presented his findings at the 50th Anniversary Symposium of the Fisheries Society in the British Isles at Exeter University in July.