Fish Behaviour In Cold Water

Fish behave differently when the water temperature drops. Their propensity to move about and feed is reduced and consequently they become more difficult to catch. Anglers know this, yet still venture out when the odds are stacked heavily against them. So how can we improve our chances when the water is close to freezing point?

We asked a fishery scientist to answer three key questions that might help turn an outright shiver-fest into a heart-warming winter session.

It is cold. YOU are cold. The water is cold and even the warm drink you put in your flask before you left home is joining the party.

But you stick at it, because in the next few minutes you know – just know – you’ll have a bite. It might be the only one of the day and you’d be foolish to pack up, leave and miss it. Just another few minutes……

Paul Coulson is the Director of Operations for the Institute of Fisheries Management (IFM) and has spent his career working with and studying fish in a wide range of environments. He is a graduate of Sparsholt College and a Fellow of the IFM. So, as a keen angler himself, Paul has an advantage over most of us when it comes to fishing in challenging conditions.

He’s far too modest to claim that all this knowledge makes him an expert angler but he’s more than happy to share the science behind what’s going on beneath the surface when above it, our teeth are chattering and our knees are knocking.

Paul Coulson

What happens to fish in cold water?

“Fish are ectotherms (cold blooded) which means they’re unable to regulate their own body temperature so their physiology and thus their behaviour is dictated by the medium they’re in, which is obviously water. So cold water, equals cold fish and the impact that a drop in temperature has on the physiology of a fish is quite marked.

Fish in temperate climates have evolved to be adaptable to seasonal change but they don’t react well to a short, sharp shock. In the British Isles these days, we’ve almost lost our seasons and it can be 15 or 16 degrees in December and then the following week it’s -8 or -9. They’re not keen on that!

“Fish want the change to be gradual so they can acclimate. When change occurs they can display behavioural thermalregulation and will move themselves into an aquatic environment where they feel more comfortable. Think about the difference between a shallow pond and a deep pond with regard to water temperature. Shallow ponds can become cold all the way through but deeper ponds have bands of temperature through the depths so fish will find the thermal range in which they feel most comfortable. Hence the reason that in general, if you’re fishing still waters in the winter, it’s better to fish deep ones.”

In the winter months, it pays to fish deep. (Most of the time)

What do we need to know about fish behaviour?

“Metabolic rate is linked to body temperature so in the summer, the fish are more active and as a result they eat more to fuel this activity. They are also growing so need to take in lots of nutrients to help them pack on muscle. In the winter, their bodies close down because they’re completely regulated by the cold water they’re in. It’s almost a hibernation, some species can even burrow into soft sediments and go dormant in really cold spells.

But there’s a secondary condition we also need to be aware of here and that’s the environmental condition of those waters and how that fish responds to events like predation, overhead cover and structures within the water. If you’re a fish impacted by temperature, the last thing you then want is the additional stress of being in clear, shallow water and seeing big black shadows flying above you. So, in that scenario, it’s not just about not feeding, it’s about not getting munched. It’s not only about fish biology and physiology, it’s also about habitat management and fishery management and giving fish what they need to sustain them and make them as comfortable as possible.

“But the complication is that you couldn’t just look at a body of water and say that it’s a nice environment and has all the factors that we, as fisheries managers would consider to be appropriate because that doesn’t automatically translate into ’I bet this fishes its head off in the winter’.

“What we’re thinking of in the winter is that we want variation in depth, we want cover so the fish can feel safe and you want different habitat types so the fish can find areas that they are comfortable in depending on the conditions. But these factors don’t necessarily sit with a body of water automatically being a good fishery.

“As an example, I know of an angler recently who was fishing on a well-known stillwater which is quite deep. He broke the ice, fished shallow in the top eighteen inches to two feet of water and had over 30lb of F1 carp. For me, as a fisheries manager and fish biologist, you’re hearing that and thinking it’s bonkers!

“Given what we think we know about fish behaviour you would imagine that at the very least they’re going to be slightly perturbed at having a great big hole smashed in the surface ice but fish sometimes don’t read the books!

Sometimes frozen lakes can throw up surprises

“In that situation, a host of factors have come together and the fish just obviously felt happy there in those conditions. The angler just worked it out and in winter you have to work harder to do that. Fish will sometimes find a comfortable little niche within the water and inhabit that zone for a period. Knowing your water is incredibly important in the winter because it’s much easier to catch those same fish in the summer!”

What should we look out for when we’re planning a winter session?

“You’ve got to think about the thermal properties of water and it’s better that you understand the water you’re facing than the fish that are in it. You’re generally better off targeting running water because the fish have got to eat to maintain status. As a fish, there is a significant energy budget to hold your place in a river whereas fish in a stillwater don’t have that demand and can be almost torpid in the winter. If you ask anybody who’s netted a coarse fish lake during the winter, particularly one with carp in it, they’ll know you can often walk through a reed bed and just pick the carp up because they’re so inactive. In the cold conditions, they lose their fight or flight response because their systems are so suppressed.

“In the winter, a carp angler will think nothing of slinging out a bright single bait and nothing else. They’re not putting out kilos of pellets over the top because they’re not feeding the fish, they’re just hoping one waddles past and thinks; ‘that looks bright and smells quite nice, I’ll have that’. Carp anglers will often also fish with a tight line so they pick up ‘liners’ to establish if fish are moving about in the area. So they’ll cast around in different areas rather than cast once and just leave it there. In the winter, you don’t need to make an opportunity by feeding and drawing the fish in. It’s a case of setting a trap and hoping that a fish will come across that trap and give you a chance of a pickup. In short, fish in cold ponds have a metabolic rate that’s so reduced, they don’t need to feed.

“Sometimes you’ll have anglers on the sunny side of a pond and the water just warms up enough to turn the fish on. You can watch anglers on the other bank and they’re just sitting there getting cold. In the winter, that feeding window might only be half an hour and as they say you have to be in it to win it.

“All these cumulative factors are at play when you’re thinking about a ball of muscle that’s within a medium it can’t control.”

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