If you fish for sport, you want a trout or bass that puts up a fight. But if you raise fish for the frying pan, you’re better off with fish that are more laid back.

“Stress reduces fish survival and growth,” says Jeffrey Malison, director of the Aquaculture Program at the University of Wisconsin-Madison. Malison and his colleagues have been studying how to minimize stress so fish are healthier and grow more rapidly. The scientists also hope to produce domesticated strains of trout and perch for fish farming.

Most agricultural animals were domesticated hundreds of years ago, according to Malison. But the trout and perch that fish farmers raise are nearly identical to wild stocks.

Last spring Malison published a study showing that minimizing aquaculture practices that disturb perch can increase their growth by as much as 30 percent. In a new study, he and colleagues in the College of Agricultural and Life Sciences report that they may have a method to identify rainbow trout that respond less strongly to stress.

“Our findings suggest that it may be possible to genetically select faster growing fish based on the concentration of a hormone in their blood several hours after they”ve encountered a stress,” Malison says. “Such fish might show improvements in many areas besides growth, such as disease resistance and domesticated behavior.”

The results could pay big dividends for Wisconsin”s aquaculture industry. Wisconsin is the nation”s fifth largest producer of rainbow trout. According to a producer survey, the state”s fish farmers sold $3.7 million worth of fish for food in 1997. And the industry is expanding more rapidly than most agricultural sectors.

When faced with threats, fish produce cortisol, a hormone that helps them mobilize the energy they need to flee or fight. But this natural response comes at a cost. Elevated concentrations of cortisol cause fish to grow more slowly and suppresses the animal”s immune system, leaving it vulnerable to infections.

On fish farms, fish are subjected to netting, handling, pond cleaning and other stresses that can elevate the level of cortisol in their blood.

Scientists have tried to relate stress and growth by looking at the peak blood cortisol levels, which occur one hour after fish are stressed. But peak levels didn”t successfully predict which fish would grow faster than others.

In the latest issue of journal Aquaculture, Malison, Lynne Weil and Terence Barry report that cortisol level measured three hours after fish were stressed was significantly correlated with the growth of rainbow trout. Moreover, fish that had consistently low cortisol concentrations three hours after stress grew significantly faster that those with consistently high cortisol levels measured at that time.

The Wisconsin study is the first to find a significant correlation between fish growth and a specific measure of stress response. The results suggest that how rapidly cortisol levels decrease after stress is more important in predicting how fish will grow than how much cortisol concentrations rise immediately after stress.

The UW-Madison researchers followed the growth of 400 1-year-old rainbow trout over 10 months. The researchers stressed each fish by netting it and holding it out of water for a minute. They administered the stress and blood test to individual trout three times at three-month intervals during the study.

The research was supported by grants from the University of Wisconsin Sea Grant Program and the USDA North Central Regional Aquaculture Center, and support from the UW-Madison College of Agricultural and Life Sciences, the College”s School of Natural Resources, and the Wisconsin Department of Natural Resources.