Trout Fishing by ºC & When to go to the Pub… Please take a minute to read this…it’s important.
We have recently experienced several hot summers with very poor flow in many rivers and the virtual disappearance of some in the South and East. With poor flow in hot weather the water temperature can rise very considerably.
Water temperature is often talked about but no where near frequently enough taken. It’s bang up to date and exactly for the spot that you’re fishing. Can you think of another piece of absolutely reliable information so easy to acquire and update ?
Why take the temperature of the water? A brief background:
Unlike us mammals who keep our core body temperatures at a steady 37ºC the trout’s body temperature is pretty much determined by the temperature of the water. This has advantages and disadvantages. Importantly the trout does not expend much valuable energy keeping its self warm as mammals do. This explains at least partially why they don’t need the thick layers of insulating, energy rich, fat common to marine mammals (whales, dolphins, seals etc).
As a result, the trout has to have a body which functions perfectly normally over a temperature range way beyond that which a human can survive.
One reason mammals hold their body temperature at 37ºC is to allow tight control of the metabolic rate at which their cells perform the thousands of different, complex enzyme controlled processes necessary for mammalian life. Biological enzyme systems work faster as the they get warmer, in fact they roughly double in speed for every 10ºC rise in temperature. Double the metabolic rate and the amount of oxygen being used to power it doubles.
The trout also has to deal with another factor which alters with the water temperature. That is the amount of oxygen held in the water. A little surprisingly perhaps, this falls as the water temperature rises. To make matters worse all the other oxygen dependent organisms, both visible and microscopic, are also mopping up the oxygen faster and faster as the temperature rises.
The trout at 5ºC has a metabolic rate half that of the trout at 15ºC. The cool trout needs less oxygen at a time when the water offers more. The cooler trout can and certainly does fight harder, for longer and recover quicker than the warmer trout.
In essence, rising water temperature, for trout, is similar to being made to walk up an increasingly steep hill and at the same time having to take smaller and smaller breaths. Clearly there must be a limit.
So when, you ask, does it start to become a problem for the fish? It turns out that at about 18ºC trout will start to move about abnormally and lose interest in food (recognised signs of metabolic stress). At 20ºC the trout will be in difficulty and stop feeding completely. Beyond this temperature there is a fish mortality rate which accelerates with time and temperature.
The warmer the trout, the more it will be interested in searching for cool, slower (but not too slow) well oxygenated water rather than feeding.
What of the the anglers responsibility in all this? A trout caught at 18ºC is already exhausted at the moment the hook is set. So bring it quickly to the net (it’s not going to be difficult). Don’t lift it out of the water at all…no matter how much you want to take that trophy photo, it is a very “short of breath” fish. Any collapse of its delicate gill structure caused by lifting it out of the water could put it beyond recovery. Keep the fish facing up stream all the time. Cradle it gently in the water until it gets enough energy back to swim off positively up stream. This can take a surprisingly long time, be patient.
Sticking your hand in the water is a very poor way to judge the temperature. Consider for example when you jump into a swimming pool, it might feel cold at first but much warmer after a few moments. Treat yourself and the fish, to a thermometer. Stick it in the middle of the flow until it has stopped changing temperature… It only takes a minute….literally.
If it’s very cold ( a useful piece of risk assessment ) put your thermals on!
As the water approaches 15ºC look for the fish more in the cooler areas where they will expend least energy. Be extra gentle with the fish. If they don’t seem to put up a reasonable fight, re-check the temperature and ask yourself if they are too heat exhausted to be responsibly caught? At 18ºC check you know where the best pub is. Beyond 18ºC it’s probably time you were heading for it.
The environmentally aware decision not to fish, in such circumstances, is something that a responsible angler should take pride in pointing out. It shows you not only to be conscientious and knowledgable but to have the wisdom to act appropriately. That can only be good for fishing…. >>)))c> RW.
To the scientists and academics: I apologise for some over simplifications but I hope you find that the end justifies the means… RW