With Met Éireann forecasting a heat wave for large parts of the country into this weekend, combined with lower-than-average rainfall, Inland Fisheries Ireland (IFI) is warning that there is a high risk of fish kills due to thermal stress and reduced oxygen levels in lakes and rivers.

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 responsible for the protection and conservation of freshwater fish and habitats is concerned that water levels in many rivers are low and approaching drought conditions.

Therefore, it is appealing to anglers, the farming community and the general public 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 also being asked to voluntarily stop using ‘keep nets’ while high temperature warnings from Met Éireann are in place, as these nets may cause unintentional distress to fish.

In addition, anglers that practice ‘catch and release’ fishing are asked not to fish during the heat wave where possible, as this may put undue pressure on fish populations.

Dr Gregory Forde, head of operations at IFI 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.

‘During the current heat wave, air and water temperatures are approaching dangerous levels and fish kills may be unavoidable’

“Once the water temperature exceeds 20C, fish species such as salmon and trout suffer ‘thermal stress’. During the current heat wave, 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 practicing 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.”

Last month, IFI and the Office of Public Works’ data buoy recorded a surface water temperature of 21.84C at Lough Sheelin in Co Cavan, coinciding with July’s hottest air temperature of 30.75C. 

IFI notes that significant thermal stress can occur in brown trout and other cold water fish species at temperatures at or above 20C.

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.

“We’re encouraging the public to report any sightings of fish suffering thermal stress to our 24-hour confidential hotline on 0818 34 74 24 so we can respond as quickly as possible to help rescue or relocate these fish.

“It is also a good time to remind all those using pesticides that these should be used only as a last resort, always in accordance with product instructions and always respecting statutory ‘no use’ zones, being mindful at all times of proximity to water bodies such as ditches, streams, ponds, rivers, lakes and springs. Even a very small amount of pesticide can be highly toxic to the aquatic environment.”

‘Many of our fish species will find survival difficult in these warm conditions without the added pressure of angling stress’

Elsewhere, the Loughs Agency is advising anglers across both the Foyle and Carlingford catchments to help preserve salmon and trout stocks in local rivers following prolonged periods of hot weather.

These extreme weather conditions have led to low river flows, high water temperatures and low oxygen levels in many bodies of water.

The guidance comes after only 43% of the average rainfall fell in Northern Ireland last month, although some rivers in the Foyle catchment also experienced severe flooding. Each one of these significant weather events put stress on fish.

The agency’s head of science Dr Sarah McLean is hopeful that anglers will proceed with caution on the rivers, particularly when fishing in the current warm temperatures.

“Many of our fish species will find survival difficult in these warm conditions without the added pressure of angling stress,” she said.

“Even catch and release poses a risk in these conditions as low dissolved oxygen in the water can result in poor fish recovery rates and inadvertent mortalities.

“It is also worth remembering that high water temperatures and low dissolved oxygen levels may also leave fish more susceptible to parasites and disease, so any fish caught will also require additional care when handling.”

There are several measures anglers in Foyle and Carlingford can take to help protect fish during the hot weather:

• Consider taking the water temperature before you fish and avoid fishing at times when water temperatures are high. Water temperature will be coolest in the early morning.
• Where possible, keep fish in the water during catch and release.
• Seek advice from fishery or angling clubs where appropriate.
• Avoid targeting larger fish or sensitive species.
• Keep nets should not be used by coarse anglers during warm weather.
• Limit handling time.
• Where possible and safe, release fish into deeper, faster flowing water. 

Anglers should report distressed or dead fish in the Foyle or Carlingford catchments directly and promptly to the Loughs Agency at +44 (0)28 71 342100 or [email protected].