Handling temperature, and when not to fish at all
Explainer · Last reviewed 2026-08-14
Above a certain water temperature the arithmetic of catch and release stops working, because the fish needs more oxygen to recover and there is less of it available. The honest response is to stop, and it is rarely said.
Safety
Fish welfare — Can cause serious injury or death
In warm water a fish needs more oxygen to recover from capture while less is available, and post-release mortality rises steeply. A fish released looking healthy may die hours later, and in extreme conditions a substantial proportion of released fish do not survive.
What reduces it: Fish the cooler part of the day and the cooler water; shorten fights with adequate tackle; keep the fish in the water; and stop fishing where water temperatures approach the upper part of the target species' tolerance. Observe any voluntary or mandatory high-temperature suspension in force.
Worse when:
- Cold-water species such as salmonids, which are affected at temperatures many species tolerate easily
- Shallow stillwaters and low flows, where warming is fastest and refuge is least
- Long fights and any air exposure, both of which compound with temperature
WARM WATER PRODUCES A DOUBLE PROBLEM AND THE TWO HALVES MULTIPLY.
LESS OXYGEN IS AVAILABLE. The solubility of oxygen in water falls as temperature rises, so a warm water body simply holds less of it.
THE FISH NEEDS MORE. Metabolic rate rises with temperature in an ectotherm, so oxygen demand goes up at exactly the point where supply goes down. A fish in warm water is closer to its limit before anything happens to it.
SO THE SAME FIGHT COSTS MORE. A capture event that a fish would recover from comfortably in cool water may exceed its capacity in warm water, and post-release mortality rises steeply with temperature across a wide range of fisheries and species. This is one of the more consistent findings in the catch-and-release literature, and the shape matters: it is not a gentle gradient but a curve that steepens near the top of a species' tolerance.
WHAT ELSE GOES WRONG AT THE SAME TIME. Recovery takes longer, so the fish is vulnerable for longer. Air exposure is more damaging. Handling injuries become more consequential because infection risk rises. And in stillwaters, warm surface layers may sit above deoxygenated deep water, so the fish has nowhere better to go.
WHAT THE THRESHOLD IS depends entirely on the species and the water, and stating a single number would be false precision. Cold-water species — salmonids especially — are affected at temperatures that warm-water species tolerate without difficulty. What is general is the DIRECTION and the fact that the last part of a species' tolerance range is where the risk concentrates.
HOW TO ACT ON IT WITHOUT A NUMBER.
KNOW WHERE THE SPECIES YOU ARE FISHING FOR SITS. The species records carry temperature association, and a fish being caught at the top of its range is a fish at risk.
WATCH THE WATER RATHER THAN THE AIR. Air temperature is a poor proxy; a shallow stillwater in a heatwave and a deep spring-fed river in the same weather are different situations.
FISH THE COOL PART OF THE DAY, or the cooler water, and shorten fights.
AND ACCEPT THE CONCLUSION THAT FOLLOWS. There are conditions in which the responsible thing is not to fish. Many fisheries and authorities impose voluntary or mandatory suspensions during high temperatures and low flows, particularly on salmonid rivers, and those exist because the arithmetic above is well understood. Where no such measure is in force, the arithmetic has not changed.
THIS IS THE PART OF WELFARE GUIDANCE MOST OFTEN OMITTED, because every other piece of advice tells the reader how to fish better and this one tells them not to. It is included for that reason.
What the decision looks like, and why this record publishes no number
A reader arriving here wants a temperature above which they should stop. This record does not give one, and the reason is not caution for its own sake.
THE THRESHOLD IS NOT A PROPERTY OF TEMPERATURE ALONE. What kills a released fish is the gap between the oxygen it needs to repay the debt of a fight and the oxygen available to it. Temperature sets both sides of that — warmer water holds less oxygen and simultaneously raises the fish's demand — but so do the time of day, how long the water has been warm, whether there is any flow, how much weed is respiring overnight, how deep the water is, and how long the fight and the handling lasted. Two waters at the same temperature can sit on opposite sides of the line.
AND IT DIFFERS ENORMOUSLY BY SPECIES. A cold-water salmonid is in trouble at temperatures a carp is comfortable in, and the literature is explicit that species-specific guidance is needed precisely because a single number does not transfer. A figure published here would be read as permission on the waters where it is wrong.
WHAT REPLACES THE NUMBER IS A SET OF OBSERVATIONS, and they are available to anybody standing at the water.
FISH VISIBLY AT THE SURFACE, or gathered at an inflow, a weir, an aerator or the windward end — that is not feeding behaviour, it is fish going to where the oxygen is.
A FISH THAT WILL NOT SWIM AWAY. If a released fish needs holding upright, or drifts on its side, or turns over after swimming a few metres, the water is already past the point where the fishing is defensible. One such fish is the signal; waiting for a second is not a plan.
NO FLOW, NO WIND, NO NIGHT COOLING. Still water at the end of a long hot spell that has not dropped overnight is the specific case, because it is the one where the oxygen minimum arrives just before dawn and the water is warmest in the afternoon — both ends of the day are bad for different reasons.
AN OFFICIAL POSITION. Many fisheries, clubs and authorities impose their own hot-weather restrictions, and where one exists it is a far better answer than anything inferable from the bank.
AND THE DECISION ITSELF IS ORDINARY. Not fishing is a normal outcome and it is the correct one often enough that it should not feel like a defeat. The intermediate options are real and worth taking first: fish at dawn instead of noon, fish deeper or moving water, shorten fights with heavier tackle rather than lighter, keep every fish in the water, stop using a keepnet, and go home when the first fish is slow to swim off.
Scientific evidence
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
- Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Review of catch-and-release outcomes, including water temperature, dissolved oxygen, fight duration and air exposure as determinants of post-release survival.States this directlyCitation checked — the work exists and is described correctly
- A review of catch-and-release angling mortality with implications for no-take reserves — Reviews in Fish Biology and Fisheries 15:129–154, 2005 (Scientific literature)Relied on for: Review of catch-and-release angling mortality and the physiological and handling factors associated with it.States this directlyCitation checked — the work exists and is described correctly
- Do we need species-specific guidelines for catch-and-release recreational angling? — Biodiversity and Conservation 14:1195–1209, 2005 (Scientific literature)Relied on for: Analysis of whether species-specific catch-and-release guidelines are needed, and of how widely the relevant thresholds vary between species and waters.States this directlyCitation checked — the work exists and is described correctly
Where to go next
Where it matters
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Surface fishing for coarse fish
A link from a practice or a piece of equipment to the mechanism or explanation it rests on.
- Angling Trust guidance for anglers — Angling Trust (Angling or fisheries organisation)Relied on for: Guidance to anglers on welfare, handling and the fishery rules that bear on both, including bait and method restrictions.States this directlyCitation checked — the work exists and is described correctly
- Humane Harvesting of Fish: percussive stunning and percussive equipment — Humane Slaughter Association (Angling or fisheries organisation)Relied on for: Welfare of caught fish, including handling and time out of water.States this directlyNot yet checked
Related reading
- Dissolved oxygen and fishThe solubility of oxygen falls as temperature rises, which is the physical fact the whole argument rests on and the reason the threshold is a property of the water rather than of the air.
- Warm-water fishing, and the oxygen problemWarm water holds less oxygen while the fish needs more, and post-release mortality rises steeply with temperature. The conditions record explains the fishing consequence; this one draws the welfare conclusion.
What these connections rest on (2)
Scientific evidence — 2 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Dissolved oxygen and fish
A mechanism edge to the biology or physiology beneath a handling practice, resting on the catch-and-release and fisheries science cited rather than on convention.
- Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Review of catch-and-release science including air exposure, handling, temperature effects and post-release survival.States this directlyCitation checked — the work exists and is described correctly
- Do we need species-specific guidelines for catch-and-release recreational angling? — Biodiversity and Conservation 14:1195–1209, 2005 (Scientific literature)Relied on for: Synthesis of recreational angling effects on fish including physiological disturbance, recovery and the variables that dominate outcome.States this directlyCitation checked — the work exists and is described correctly
Warm-water fishing, and the oxygen problem
A mechanism edge to the biology or physiology beneath a handling practice, resting on the catch-and-release and fisheries science cited rather than on convention.
- Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Review of catch-and-release science including air exposure, handling, temperature effects and post-release survival.States this directlyCitation checked — the work exists and is described correctly
- Do we need species-specific guidelines for catch-and-release recreational angling? — Biodiversity and Conservation 14:1195–1209, 2005 (Scientific literature)Relied on for: Synthesis of recreational angling effects on fish including physiological disturbance, recovery and the variables that dominate outcome.States this directlyCitation checked — the work exists and is described correctly
How this compares
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Reviving a fish before release
Two records a reader will hold together, joined where the relationship between them is the informative part.
- Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Review of catch-and-release science including air exposure, handling, temperature effects and post-release survival.States this directlyCitation checked — the work exists and is described correctly
- Do we need species-specific guidelines for catch-and-release recreational angling? — Biodiversity and Conservation 14:1195–1209, 2005 (Scientific literature)Relied on for: Synthesis of recreational angling effects on fish including physiological disturbance, recovery and the variables that dominate outcome.States this directlyCitation checked — the work exists and is described correctly
Rules and access
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Close seasons
A link to a rule CONCEPT rather than to a rule. The record already renders the regulatory-variance notice by its own declaration; this edge says WHICH rules bear on it, which turns a generic warning into a route to the body that holds the current answer.
- Humane Harvesting of Fish: percussive stunning and percussive equipment — Humane Slaughter Association (Angling or fisheries organisation)Relied on for: Humane dispatch and handling of caught fish, and the standards and expectations applied to it.States this directlyNot yet checked
- Angling Trust guidance for anglers — Angling Trust (Angling or fisheries organisation)Relied on for: Guidance to anglers on welfare, handling and the fishery rules that bear on both.States this directlyCitation checked — the work exists and is described correctly
Check the rules before you fish
Fishing laws, bylaws, access rules, seasons, bait restrictions and permitted methods vary by location and can change. Check the current rules from the relevant authority and the water you intend to fish before fishing.
Check with:
- Current national or regional law for where you are fishing
- Current local bylaws
- The relevant fisheries authority
- The landowner, club or fishery whose water it is
- Any restrictions specific to the species or the method you intend to use
Sources for this page
- Understanding the Complexity of Catch-and-Release in Recreational Fishing — Reviews in Fisheries Science 15:75–167, 2007 (Scientific literature)Relied on for: Review of catch-and-release science including air exposure, handling, temperature effects and post-release survival.States this directlyCitation checked — the work exists and is described correctly
- A review of catch-and-release angling mortality with implications for no-take reserves — Reviews in Fish Biology and Fisheries 15:129–154, 2005 (Scientific literature)Relied on for: Review of catch-and-release mortality across fisheries, including the variables that dominate outcome and their relative weight.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Temperature tolerance, distribution, habitat association and life history recorded for the species concerned.States this directlyPassage read and verified
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