Conscientious anglers take extra care when landing fish in the winter. They preach short fight times and even shorter photo ops in the frigid air — and for good reason. Fish populations often face an increased risk of a massive die-off, or recruitment bottleneck, in the winter as the dissolved oxygen in a system plummets. Many fish struggle as a result of their sluggish metabolisms, sluggish both in finding food and avoiding being food. But when should anglers really be more careful with their fish handling: the finger-numbing wintertime or the dog days of summer?


To answer that question, fisheries researchers assess the metabolic rates of individual fish, or the rates at which they consume oxygen. At rest, a fish sits comfortably at its standard metabolic rate. When active, the same fish can fall anywhere between its standard metabolic rate and its maximum metabolic rate, which is its greatest sustainable rate of exercise. The difference between the two rates is that fish’s aerobic scope. A fish with a greater aerobic scope has a greater capacity for prolonged activity, which means that their ability for burst swimming and sustained swimming is higher. Aerobic scope can fluctuate in response to ambient water temperature.


Fish also rely on anaerobic metabolism, which does not require oxygen, for burst swimming. Anaerobic metabolism results in a buildup of lactic acids and metabolic ions in a fish’s tissue and bloodstream. Lactic acid is the substance that makes your muscles sore after working out and can cause the same stiffness in fish. After heightened activity, like fighting on the end of an angler’s line, fish need time to recover and allow their systems to return to normal. While they recover, fish have a reduced ability to avoid being eaten or blown into dangerous situations. Think running a marathon and then being chased by a bear – adrenaline can only get you so far.


A recent study from the University of Illinois at Urbana-Champaign, published in Environmental Biology of Fish in 2018, studied how bluegill respond to activity at low temperatures. Researchers exercised individuals by pinching their tails to make them swim away, either for 30 seconds or until exhaustion in 41˚F water. Fish were deemed exhausted when they no longer swam away from the tail pinching. After exercise, the fish were exposed to room temperature air for either 30 seconds or four full minutes to further stress their cardiovascular and respiratory systems.


At 41˚F, none of the varying exercise or air exposure treatments had any effect on the fish’s metabolic rates. Their standard metabolic rates, maximum metabolic rates, and aerobic scopes stayed constant regardless of how long they were exercised or exposed to air. Previous studies on crappie, trout, walleye, and pike in 75˚F water showed the opposite; fish became more stressed and less able to cope with activity the longer they were exercised and exposed to air.


Although it can take hours for a fish to completely recover – for its cortisol, lactate, oxygen, and glycogen levels to return to normal – it is clear that most North American game fish are less vulnerable to physiological stress at colder temperatures. It’s also worth noting that hook-site injuries may be more likely to become infected in warmer waters.


Minimizing the stress a fish experiences is more critical in the summer months than in the winter, especially with cold water species like trout and salmon. As long as your drag is set correctly, you’re quick on the hookset, and you have a nice net handy, though, there’s no reason that a fish you release in August won’t be around in January.


Originally published in Outdoor News.

Winter et al. 2018. “Metabolic response of bluegill to exercise at lower water temperature: implications for angling conservation”. Environmental Biology of Fish 101:1657-1667.