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(An internal fisheries briefing by Natural Resources Wales)

Angling Catch and Release success.


The two main environmental factors affecting catch and release success of salmonids is exposure to air and water temperature.

Below 18oC, and if handled with care, there is generally little effect on the behaviour of released salmon or on subsequent survival, which is high.

As water temperatures increase above 18oC (64°F) survival of released salmon declines markedly, although it is notable that salmon are less inclined to take a lure or fly.

For some fishermen, 21°C (70°F) has become a round figure that represents the "don't fish" limit.

These are merely guidelines and water temperature is not the only determining factor of dissolved oxygen (speed of current also plays a factor, for example). Nevertheless 20°C represents a limit above which, unless there is any evidence to the contrary, we would commend salmon and trout fishermen to cease fishing.

Remember – all people fishing on C&R principles should; -:

  • Keep fish in the water at all times – air exposure should be limited to less than 20 seconds

  • Only use methods that are commensurate with good catch and release principles, ie new byelaws require barbless hooks and no large trebles to facilitate ease of unhooking and returning salmon and sea trout

  • Ensure the fish has recovered before allowing it to swim away

In stillwaters the temperature threshold will be higher and vary according to species present. Fish generally cease feeding as temperatures rise and this can therefore be a good indication of thermal stress.

Protecting Wales’ rivers and the wildlife that relies on them is one of the most important jobs that we do. Smaller watercourses can dry up in the fairly exceptional hot, dry conditions such as those we are experiencing across Wales at the moment, and people including anglers are rightly concerned about the wellbeing of fish in our smaller streams and rivers. Small stillwaters also become warm and this can add to seasonal stress on cyprinids leading in some cases to mortalities

In certain situations, like large pollution incidents, we might attempt to rescue fish that are at risk of being killed, however this isn’t usually the case when it comes to extreme low flows and high temperatures.

Low flows and high temperatures are more natural events, although both may become more prevalent in future. In the recent low rainfall and heat wave, smaller watercourses are sometimes restricted to isolated deeper pools and in one or two cases this has unfortunately led to fish dying. Lower flows also leads to less dilution of discharges to rivers, although permitting arrangements do take this into account


Salmon and trout are cold water species. We would normally expect that salmonids start to experience stress when water temperatures rise above 20oC. The risk of mortality progressively increases and at water temperatures exceeding about 25oC (Solomon & Lightfoot, 2008), fish may die, although this can vary if there are other threats to water quality. Fish will always seek cooler refuges in deeper pools in warming conditions. Stillwater fish are less sensitive but in certain conditions warming temperatures can lead to problems.


Additional Info:

  • Attempting to capture and move fish has little effect on how many fish are killed during a drought, as many die from the additional stress of being caught and translocated

  • Bigger fish can sometimes be successfully moved but the smaller streams in question are usually only home to smaller fish that wouldn’t survive

  • Moving any surviving fish to larger rivers can lead to overpopulation and strain existing fish populations

  • Similarly, problems in stillwaters means that there is simply nowhere to move the fish. Aerators may help to reduce mortalities in some cases.

  • We will take a case by case approach to incidents and assess all the potential benefits against the risks.



Report all Incidents:

We encourage all anglers and members of the public to report any observations of fish in distress:


Call us

To report an environmental incident call 0300 065 3000, 24 hours day.

Choose option 1 for Welsh and 2 for English. 

Information we need

To enable us to deal with your report as quickly as possible, please provide the following details:

  • Where is/was the incident taking place? Please provide a grid reference if possible, for example, NT 252 735. Otherwise, you should provide accurate directions / descriptions of the location

  • What is/was happening?

  • Who is/was responsible for the incident?

  • When did the incident take place?

Types of incidents we deal with in relation to our fisheries (not exclusive)

  • damage or danger to the natural environment

  • pollution of water or land

  • poaching or illegal fishing

  • fish in distress or dead fish

  • illegal abstraction (removal of water) from watercourses

  • suspected alterations to watercourses made without our consent (eg dredging, straightening, impounding or works to the bank or bed)

  • unusual drops in river flow

Sources of information:

Angling Trust - How do water temperatures affect fish and fishing ?

Catch and Release updated guides:

GB non-native species secretariat - Guidance for anglers

Institute of Fisheries Management - Managing Fisheries in Prolonged Dry Weather


Solomon, D. J., Lightfoot, G. W. The thermal biology of brown trout and Atlantic salmon, Environment Agency (2008).

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