Each species of aquatic organism has its own best water temperature. If the water temperature shifts too far from the optimum, the organism suffers. Fish can regulate their environment somewhat by swimming into water where temperatures are close to their requirements. Fish usually are attracted to warm water during the fall, winter and spring and to cool water in the summer. Notice how fish swim down to cooler parts of the lake to escape the heat of the noonday sun.
Fish can sense very slight temperature differences.
Fish migration often is linked to water temperature. In early spring, rising water temperatures may cue fish to migrate to a new location or to begin their spawning runs. The autumn drop in temperature spurs baby marine fish and shrimp to move from their nursery grounds in the estuaries out into the ocean, or into rivers, as the case may be. All sorts of changes take place in aquatic organisms when water temperatures change.