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PAUL NELSON FISHING: Who knows what the summer will bring

Paul Nelson

July usually has the highest average temperatures of the season, both on the land and in the lakes. Summer is officially here, so enjoy it while it lasts.

Most lakes are now warmer than 70 degrees, with the best time to measure the surface water temperatures early in the morning, before the sun has a chance to warm the water.

It is not how high the surface water temperatures can spike during a hot windless day, it is how much heat the water can hold overnight that is the more accurate measure of water temperature.

Temperatures more than 70 degrees feel warm to the fish, which are cold blooded. The 70s are within the normal temperature ranges for most species of fish in the northern lakes.

Temperatures more than 80 degrees start to present a problem for the cold water species in the lakes.

Warm water species like largemouth bass, sunfish and crappies do better in warm water temperatures than species like walleyes, northern pike, muskies and smallmouth bass.

Fish can escape the heat of summer a number of ways. One way is hiding under the shade of standing vegetation, to avoid the most direct sunlight during the day.

Another way fish can avoid the heat is to go as deep as they can to find cooler water, but some lakes will become oxygen depleted below the thermocline later during the summer.

The oxygen levels in deep water are usually higher in the beginning of July than the end of July, with the warmest part of the summer usually happening in late July or early August.

The deepest parts of the lake may become inaccessible to the fish at some point during the summer because of low oxygen levels.

Most lakes will be at their lowest oxygen levels in deep water during the same time when the water temperatures are at their peak. The longer the temperatures stay close to their peak, the lower the oxygen levels will become in parts of the lakes that are deeper than the thermocline.

It usually doesn’t get hot enough in northern Minnesota for the lakes to get into the 80-degree range for long periods of time, but it can happen.

During the hottest summers there may be a “summer kill” in the lakes caused by high water temperatures and low oxygen levels.

The most susceptible species to summer-kill are the cold water species like tullibees, whitefish and suckers, but all species in the lakes are affected to some level.

A summer kill is nature’s way of regulating populations of forage species when their numbers get too high and the largest predators like muskies and pike can not keep their numbers in check by predation.

Anglers can figure out what species of fish are most affected by the water temperatures by how far south their home range extends.

Fish that live in the south like largemouth bass, crappies and sunfish are less affected than species that only live in the north or in lakes with colder water that are generally considered “trout” lakes.

Individual fish of all species that are old, injured (usually from getting caught by anglers) or other fish that are not healthy for whatever reason may not be able to catch enough food to keep up with the demands placed on them by their increased metabolism in the warm water.

If the fish can’t keep enough food in their stomachs to fuel the calories they are burning with their increased metabolism, then they will eventually burn up all of their reserves and become a casualty of summer water temperatures.

The last time high water temperatures caused a summer kill in the local lakes was several years ago. There were so many dying tullibees and suckers on the surface over deep water, it changed the feeding patterns of many muskies.

The dying fish flopping around on the surface caused many muskies to leave the shallows and suspend over deep water, so they could target the dying tullibees and suckers on the surface.

Who knows what this summer will bring…

Paul A. Nelson runs the Bemidji Area Lakes Guide Service. Guided trips for 2018 can be booked at