A river contains two species of troutâ€""brown trout and rainbow trout. Brown trout survive in temperatures between 40 and 73 degrees Fahrenheit. Rainbow trout survive in temperatures between 60 and 78 degrees Fahrenheit. The population of rainbow trout is much larger than the population of brown trout. A cold spell causes the river water temperature to fall to 58 degrees. The rainbow trout begin to die and the brown trout populations are unaffected. This is because brown trout have:.