Fish are “ectotherms.” There body temperature is the same as the water in which they swim. They are unable to control their body temperature as the water they live in gets warmer or colder. Thus, fish either must move in search of favorable conditions or “tough it out” until temperatures change. Generally, the “tough it out” option is most successful for fish under cold conditions. Most species of fish in Lake Champlain can withstand the coldest conditions. The alewife, being an invasive, warm water species, is an exception. Die-offs of alewives in Lake Champlain occur when alewives experience a sharp decline in water temperature. On the other hand, extreme warm water temperatures are lethal to most fish species in the lake. Fortunately, extreme temperatures generally occur in very shallow waters in Lake Champlain. Most fish simply move to deeper water. The problem with warm water is that as temperature increases, water is not able to contain as much oxygen causing fish to “suffocate.” Seasonal changes in the lake water temperature occur as the thermocline develops in the summer and subsequently disappears as waters cool and mix in the fall. Fishes like lake trout and Atlantic salmon that orient themselves relative to the cooler water below the thermocline find favorable cool temperatures all the way to the surface when the thermocline breaks down. By the fall breeding season, lake trout can be found on shallow spawning grounds often very near shore. Lake trout and salmon anglers take advantage of this behavior to take fish with smaller boats and gear when they are found closer to the surface.
On the other hand, warm water fishes such as northern pike, yellow perch, brown bullhead and many others species seek shallow, near shore waters in the spring as they seek spawning areas. Shallow water warms up and provides suitable spawning and incubation temperatures sooner than deeper water. Anglers also take advantage of great near shore fishing opportunities in the spring.
The mouths of rivers and streams as well as underwater groundwater sources may at times provide refugia of warmer or cooler water. Large concentrations of minnows, smelt and other species may be found at these sites at some times of year when temperature differences exist between lake and river water.
Water temperature is one of the most important factors determining the distribution of fishes within Lake Champlain. To find where the fish are, pay attention to water temperature. Source – mychamplain.net