Oestrogen causes fish populations to decline

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Hormones found in domestic wastewater can cause declines in wild fish populations, a new study has revealed.

The study, which took place over a seven-year period, monitored the effects of the female sex hormone, oestrogen, in a series of lakes in Canada, to see the degree to which it affected the populations of the fathead minnow, Pimephales promelas.

In previous studies it has been found that oestrogen can have negative effects on the reproductive capabilities of the fish, but this is the first time that a full-scale study has been done an entire wild fish population.

Fathead minnowsThe study, led by fish scientist Karen Kidd, has for the first time conclusively shown that the hormone has a negative effect on wild fish populations.

The study took place in a series of experimental lakes in Ontario, Canada, which contained a healthy population of Fathead minnows.

Levels of oestrogen similar to those found in domestic wastewater were dosed into one of the lakes regularly during the study, while two other 'control' lakes were not dosed.

The fish population of the lake dosed with oestrogen was observed closely, and this data was then compared against the other lakes in the study, which were not dosed with the hormone.

'Feminised' malesNumerous studies have previously suggested that low levels of the hormone oestrogen could impact on the sustainability of wild fish populations by affecting on the fishes' reproductive capability.

Studies have demonstrated that oestrogen can cause male fish to become 'feminised' " a process through which the presence of the hormone promotes the development of female, as well as male, gonadal tissue within the fish.

This can result in a reduction in sperm production, and could even lead to the production of eggs in the feminised male fish.

Female fish have also been found to be affected by increased levels of oestrogen " with it causing a delay in the development of ovaries.

The impact of oestrogen on the reproductive development of fish in this way could consequently lead to a reduced rate of reproduction " and the prevous studies suggested that this could result in a decline in wild fish populations.

Delayed ovular developmentEarly on in the study, the feminisation of the male fish became apparent in samples taken from the lake dosed with oestrogen, and the females also demonstrated a delay in ovular development.

As a result of this, there was the predicted reduction in the number of successful spawnings carried out by the fish, and consequently there was a decline in the population of fathead minnows.

By the end of the study, the fathead minnow was close to extinction within the lake. The fathead minnow populations in the lakes that were not dosed were still sustainable at the end of the study, indicating that the oestrogen had caused the decline in the fish population.

Which fish are affected?The Fathead minnow were, however, not the only species of fish present within the lake " which also contained White suckers, Catostomus commersoni, and Pearl dace, Margariscus margarita.

Yet the minnows appeared to be the only fish that had a noticeable decline in population numbers during the study.

Kidd suggests that the decline in the fathead minnow population was more rapid than the other species because of its short life cycle of just two years, whereas the other fish present have a significantly longer life-cycle.

Therefore, with a longer-term exposure to the hormone, the effects would be the same on other fish populations.

The minnow are also a crucial food source for many predatory fish species - including Lake trout, Salvelinus namaycush), with which they shared the experimental lake.

Therefore, this decline in the fathead minnow population could directly impact upon the other fish populations within the lake as their food sources decline, which could have further effects on surrounding wildlife and habitat.

OestrogenOestrogen (or estrogen) can be found in domestic wastewater in both its natural and artificial forms - such as those found in the contraceptive pill.

The hormones enter the wastewater as they are excreted through urine, and low levels of oestrogen can make their way into rivers as the wastewater is released.

For more information see the paper: Kidd KA, Blanchfield PJ, Mills KH, Palace VP, Evans RE, Lazorchak JM and RW Flick (2007) - Collapse of a fish population after exposure to synthetic oestrogen. Proceedings of the National Academy of Sciences, May 22, 2007, Vol. 104, No. 21, pp. 8897-8901.