High water temperatures could lead to fish being killed

Also known as deoxygenation, reduced oxygen levels in a river or lake make it very difficult for fish to breathe and survive.
High water temperatures could lead to fish being killed

Kenneth Fox

Rising water temperatures and falling water levels may kill countless fish across the country this week.

As the Irish Examiner report, Inland Fisheries Ireland is warning of a “high risk” of fish kills due to thermal stress and reduced oxygen levels in lakes and rivers as Met Éireann forecasts a heatwave for much of the country.

Also known as deoxygenation, reduced oxygen levels in a river or lake make it very difficult for fish to breathe and survive.

The State agency, which is responsible for the protection and conservation of freshwater fish and habitats, is concerned water levels in many rivers are low and approaching drought conditions.

It is appealing to people to report any sightings of fish under thermal stress, which may be caused by the extreme heat combined with low water levels and other pressures.

Anglers are being asked to stop using ‘keep nets’ while high temperature warnings from Met Éireann are in place, as these nets may cause unintentional distress to fish.

Anglers who practise ‘catch-and-release’ fishing are also being asked not to fish during the heatwave where possible, as this may put undue pressure on fish populations.

Dr Gregory Forde, head of operations at Inland Fisheries Ireland explains: “Unfortunately, low water levels and high water temperatures can lead to fish kills, especially as there is less oxygen in the water to allow fish to breathe.

"Once the water temperature exceeds 20C, fish species such as salmon and trout suffer ‘thermal stress’. During the current heatwave, air and water temperatures are approaching dangerous levels and fish kills may be unavoidable.

"In some instances, moving fish in and out of the water may also prove too stressful. That’s why we’re asking anglers practising catch-and-release fishing to consider taking a break from fishing while high temperature warnings are in place. We’re also asking anglers to voluntarily stop using ‘keep nets’, until conditions become more favourable.”

In July, Inland Fisheries and the Office of Public Works’ data buoy recorded a surface water temperature of 21.84 degrees at Lough Sheelin in Co Cavan, coinciding with the hottest air temperature that month of 30.75 degrees.

Inland Fisheries Ireland notes that significant thermal stress can occur in brown trout and other cold-water fish species at temperatures at, or above, 20 degrees.

Dr Forde said: “Inland Fisheries Ireland staff are continuously monitoring water bodies, such as rivers and lakes, for any signs of fish suffering thermal stress in shallow water or drying out rivers and streams."

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