Fish are dying in Hawke's Bay streams that are heating up in the relentless summer sun.

Hawke's Bay Regional Council scientist Dan Fake said as river levels get low and water temperatures rise freshwater fish can be stressed.

"Many of our native freshwater fish prefer temperatures around the 20C, so when in-stream temperatures are pushing 29C like they have been in the past couple of weeks, we start seeing cases of infection in fish and even fish dying," Fake said.

"Trout also don't do well in warm temperatures, and it's not uncommon to find sick or dead trout at this time of year," he said.

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Low river levels can also cause river mouth blockages. When a swell moves gravel there sometimes isn't enough water flow in the river to remove it.

This occurred at the Esk River mouth last week where yellow-eyed mullet were found dead.

Yellow-eyed mullet fish are not typically freshwater fish but are commonly found in rivers such as the Esk River mouth.

"In smaller systems this can result in fish deaths as the backed-up water becomes stagnant, heats up and dissolved oxygen concentrations drop," Hawke's Bay Regional Council Schemes team leader Antony Rewcastle said.

River mouth openings can be a difficult task as their success depends on the river level tide and swell conditions among other factors, Rewcastle said.

"If the sea conditions aren't right or if there isn't enough flow in the rivers, the mouth can block up within a day or two," he said.

Residents are encouraged to contact the council on 06 835 9200 if they see blockages or distressed fish.

Hawke's Bay Fish and Game manager Jesse Friedlander said anglers should fish early or later in the day when temperatures are cooler and avoid fishing at all on the hottest days.

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"If you're intending to release your trout; keep it in the water and avoid touching it if possible. Net the hooked trout quickly to avoid the fish becoming exhausted and never drag a fish up on to hot river rocks or sand," he said.