Discharges from sewage treatment plants into rivers may act as a barrier to migrating fish. European eels (Anguilla anguilla) seem to deliberately avoid plumes of treated wastewater, in some cases delaying their journeys by several days.
Sewage treatment creates a complex chemical cocktail 鈥� 鈥渢housands and thousands of chemicals, and we don鈥檛 know the exact composition鈥�, says at Wageningen University in the Netherlands.
When studying how hydropower or pumping stations delayed or killed migrating eels, his team noticed that the fish sometimes slowed down when the water flowed freely. They wondered if poor water quality might be to blame.
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To explore this possibility, Winter and his colleagues collected 44 European eels over two migration seasons in the Eems canal near Groningen in the Netherlands. The scientists tagged the animals and followed their paths upstream and downstream of a plume of treated wastewater as they made their way towards the sea.
About 60 per cent of the eels steered clear of the wastewater, changing their swimming paths and delaying their journeys by hours to days. A few of the eels turned back and didn鈥檛 return to the site during the study.
Wastewater can differ from the water in the channel it flows into in terms of temperature, salt concentration or oxygen levels. It isn鈥檛 clear exactly what the eels were responding to, but they may want to avoid sudden changes, says at the Swedish University of Agricultural Sciences in Umea.
More research is needed to learn how fish interact with wastewater in the wild, she says. Winter鈥檚 team is now digging into a database of eel-tracking studies done in other European waterways to look for links between effluent discharge and delayed migration.
Such setbacks could be costly for the eels, which make an epic journey across the Atlantic Ocean to breed. 鈥淭hey have to travel 6000 kilometres with their body fat as a fuel,鈥� says Winter. Female eels, especially, need sufficient reserves to produce eggs at the journey鈥檚 end. Taking a longer route that avoids plumes could eat up precious energy, and delayed animals may miss the majority of fellow migrants.
Treatment plants could take simple steps to reduce the potential impact of effluent, such as diverting the inflow to one side of the waterway, allowing fish to easily bypass it, says Winter.
鈥淏oth the American eel and the European eel are doing very poorly,鈥� he says, and any actions we can take to mitigate the threats they face could make a difference. 鈥淚t might help change the tide for eels.鈥�
PLoS One
