Fish kills expected statewide: MI

Article Posted: March 20, 2003

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Fish kills expected statewide

State Fisheries officials today reminded residents to expect fish kills in many lakes throughout the state, noting that spring fish kills are most often natural and not related to environmental contamination.

Large-scale fish die-offs usually are one of three seasonal types, sparked by natural causes that cannot be controlled. The spring kill predicted by state fisheries biologists is related to the recent weather shift from a long, cold winter to above-normal, spring-like temperatures.

“Most Michigan lakes have been locked under ice and snow for months, which can lower dissolved oxygen levels in the water and stress the fish,” said DNR Fisheries Biologist Todd Grischke. “The abrupt warming trend of the past few weeks has the potential to raise lake temperatures too quickly, further stressing the fish and potentially causing some die-offs in a variety of water bodies.”

Winter fish kills occur in shallow lakes with excessive vegetation. Dying weeds rob dissolved oxygen from the water, and ice cover prevents lakes from drawing fresh oxygen into the water. Fish also suffocate in particularly hot summer months, when plants consume large amounts of oxygen at night as part of the photosynthesis process and create large, oxygen-depleted areas in lakes. These summer kills claim fish in the hours just before dawn, when dissolved oxygen levels in shallow, weedy areas are at their lowest.

“Michigan experiences a full range of weather conditions in a normal year,” Grischke said. “Fish kills are simply nature’s way of thinning fish populations and, ultimately, ensuring healthy fish populations.”

While it does occur, rarely are fish kills attributed to pollution or improper use of herbicides or other chemicals. Fish kills are unsightly, but most are not cause for alarm. However, if a chemical odor is noted in the area of a large die-off, or a strange water color or surface film is observed, residents should contact DNR Fisheries Division immediately at 517-373-1280.

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