As sea temperatures rise, stocks of some fish species maydecline while others may grow, reveals new research. The gastrointestinal system of fish is much more sensitive to temperature changes than previously believed, the researchers report.

"Our work is largely about trying to identify the physiological bottlenecks, in other words which parts of the body will fail first — whether the heart or the gut is the most sensitive part of the system," study researcher Albin Gräns, of the University of Gothenburg, in Sweden, said in a statement. They found that the gut was actually the most temperature-sensitive organ in many fishes.

The researchers looked at how gut function in various fish species is affected by both rapid and slow changes in water temperature, to better understand what will happen to different species when the climate changes.

"When the temperature of the water rises, the fish's body temperature climbs, activity in the gut increases, and more energy is needed to stay healthy," Gräns said. "Since changes in body temperature affect virtually all of a fish's organs, it's surprising that we know so little about how temperature changes impact on their physiology,"

Gräns studied sculpin, sturgeon and rainbow trout in saltwater and freshwater environments in western Sweden, California and Greenland. Because fish don't produce their own body heat, their body temperature is the same as that of their surroundings. When the temperature of the water changes, so does the temperature of the fish.

Some species may find it harder to absorb nutrients as water temperatures rise, while others could profit from the new climate, Gräns said: "If the water temperature in the Arctic rises further, some sedentary species, such as various types of sculpin, will probably struggle to maintain blood flow in the gut during the summer months, which will affect their health."

Other fish, such as those currently living at the lower extremes (the coldest enviroments they can survive) of their possible spatial distribution, could instead benefit from a slightly higher temperature. The effects of a rise in water temperature will therefore vary between species, and many of the changes are difficult to predict.

Gräns defended his thesis on this work on March 30. His recent sculpin studies will be published at a later date.