Lake-dwelling fish are restricted to their watery homes, so they can't escape from seasonal temperature swings. Since fish are cold-blooded, they are sensitive to changes in temperature. So it's likely that their muscle performance – and therefore their swimming ability – is affected by hot and cold spells. But when Norman Day and Patrick Butler studied brown trout, they were surprised to find that the fish swam at the same top speed in both winter and summer. They decided to investigate how trout achieve this(p. 2683).

When temperatures soar in summer or drop to frosty levels in winter,cold-blooded animals resort to thermal compensation; they make physiological and biochemical adjustments to keep their bodies going about their daily business. But does thermal compensation kick in if fish find themselves in warm water in winter, or are forced to cope with chilly water in summer?

To find out, Day and Butler collected brown trout from a fish farm, but soon discovered that they are not very cooperative. `Adult brown trout are very aggressive and initially were a real challenge to keep in the lab,' Day says. Once Day and Butler had worked out how to stop the fish from attacking each other, they were ready to see how trout cope with a thermal challenge. They acclimated some brown trout to normal seasonal temperatures (keeping fish at 5°C in winter and fish at 15°C in summer) and acclimated others to reversed-seasonal temperatures (keeping fish at 15°C in winter and fish at 5°C in summer). To see how this affects swimming ability, they measured each fish's speed as it swam against an increasing current. They were surprised to find that seasonally-acclimated fish swam faster than trout acclimated to reversed-seasonal temperatures. Clearly, exposing trout to reversed-seasonal temperatures wreaks havoc on their swimming prowess.

Eager to explain this, Day and Butler examined trout tissue samples for morphological and biochemical clues that might reveal why brown trout don't adjust to reversed-seasonal temperatures. From previous studies on trout, they knew that ammonia build-up in white muscle reduces swimming ability. But when they measured ammonia and another waste product (lactate) in the trout's white muscle, they found that fish swimming at reversed-seasonal temperatures actually had lower levels of both waste products than fish swimming at seasonal temperatures. Resting fish also had lower ammonia levels in their white muscle at reversed-seasonal temperatures than at seasonal temperatures,in winter at least. So a waste build-up can't be the reason for their lethargic swimming. But the lower level of waste products does suggest that trout may swim poorly at reversed-seasonal temperatures because they don't use their white muscle very much.

Day and Butler were even more intrigued to find that there were clear differences in muscle morphology and biochemistry between fish acclimated to 5°C in summer and those acclimated to 5°C in winter. Since both groups were acclimated to the same temperature, these differences cannot be due to temperature alone. There must be other `seasonal' factors at work, and Day and Butler list photoperiod, geomagnetism and the internal biological clock as possible suspects. It's clear that seasonality has complex physiological ramifications, so Day and Butler have their work cut out for them.

Day, N. and Butler, P. J. (
2005
). The effects of acclimation to reversed seasonal temperatures on the swimming performance of adult brown trout, Salmo trutta.
J. Exp. Biol.
208
,
2683
-2692.