Algae bloom, low oxygen levels affect fishing on Bemidji area lakes
The weather in the Bemidji area this past week was closer to what is considered normal for the end of July, which still means temperatures were into the 80s.
Surface water temperatures in most lakes are holding in the upper 70s, which is providing the perfect conditions for an algae bloom in many lakes.
The lakes are really starting to look dirty with the heavy algae bloom and all the broken floating weeds in the water. Lake home and resort owners fight a constant battle this time of year trying to keep their beaches clean of dead fish and weeds that keep blowing into their shore.
Algae blooms can filter out much of the sunlight going into the lakes and can reduce visibility in the water to only a couple of feet or less.
Anglers and other boaters are starting to see dead tulibees, whitefish and suckers floating on the surface of the lakes. Once a summer-kill begins it usually gets progressively worse until a cold snap or significant rainfall drops water temperatures in the lakes several degrees over a short period of time.
Oxygen levels below the thermocline in many lakes are already too low to support fish respiration. Cold water species usually prefer to stay below the thermocline where water temperatures are cooler but once the oxygen levels drop too low they are forced to go above the thermocline to find more suitable levels.
The high water temperatures in the lakes raise metabolism rates in all species of fish so everything has to eat more or burn fat reserves and eventually risk death.
High water temperatures put more stress on some species of fish and less on others. Species like largemouth bass, crappies and sunfish are well suited to survive warm water temperatures with their home ranges extending far into the south.
Species like northern pike, muskies, smallmouth bass and perch prefer cooler water but they are tolerant of warmer water. Their home range is further north and only extends into the south in some cool water reservoirs and larger river systems.
Cold water species like tulibees, trout, whitefish, suckers and eelpout are much better suited for northern waters or the Great Lakes where they have access to high oxygen levels below the thermocline all summer, regardless of the surface water temperatures.
When cold water species live in marginal waters like many of the lakes in the Bemidji area they usually have to survive part of the summer above the thermocline but they usually only have to do it for a few weeks so most of the fish survive.
Eventually there is a summer that is unusually hot for a longer period of time which can cause a summer kill in the lakes.
The bad side for the lakes is there are many dead fish washing up on shore, including some walleyes and muskies that have been unsuccessfully released by anglers or were sick or weak for some other reason.
On the positive side, adult suckers and tulibees only have a few natural predators like muskies, northern pike, and a few of the largest walleyes and bass. They can become over populated because of the lack of enough large predators to keep their populations in check.
Sucker and tulibee populations can use up a lot of biomass in the lakes so a significant die-off allows other species to take their place in the biomass.
Walleye anglers are still finding walleyes in shallow water on top of flats, bars, humps and points in 7 to 12 feet of water. There may also be walleyes feeding on the edges of structure in 12 to 18 feet of water. There may also be a few walleyes hugging the top edge of the thermocline in some lakes.
Anglers often have to almost hit the fish in the head for them to see their baits in the algae colored water. Any presentations that add flash, color and vibration may help anglers get more bites because the fish are able to hear or feel the baits coming before they can actually see the baits in the water.
PAUL A. NELSON runs the Bemidji Area Lakes Guide Service. He can be contacted at firstname.lastname@example.org