Several natural and man-made variables can affect the health of fish in our rivers, but during hot weather and low-water conditions, high water temperatures and low dissolved oxygen levels are perhaps the most relevant. 

It is impractical for us to measure dissolved oxygen and pollutant levels daily. We can only be guided by water temperature, water levels and data from Natural Resources Wales. The latter can come too late. We should be concerned about fish welfare when water temperature reaches 20°C, with an absolute maximum of 23°C for fishing. When the water temperature exceeds 23 degrees, we should stop fishing in the rivers. 

If there is excessively low water level, prolonged higher water temperatures or any sign of heavy growth of green algae, brown (dead) algae, or other pollution, we should stop fishing at 22 degrees water temperature. Also, during any of the above, if there are any signs of dead or dying fish (not just one fish), we should stop fishing immediately.

Recap. Water temperature at 20 deg C or above, be concerned and look for any other factors. At 23 deg C, stop fishing irrespective of any other factors.