Fisheries staff from the Sebago Lake Region recently investigated a fish kill at Lake Auburn and collaborated with a variety of concerned stakeholders, including the Auburn Water and Sewer District. Lake Auburn not only provides the municipal water supply for the twin cities of Lewiston and Auburn, but also sustains one of the region’s most productive and well utilized fisheries for quality-size lake trout and salmon. Needless to say, regional fisheries staff remain deeply concerned by the unexpected and drastic reductions in deep water oxygen levels that precipitated the lake-wide kill of lake trout, a species with low tolerance of declining lake water quality.

The prospect of a fish kill involving lake trout in such a deep, clean “oligotrophic” lake seems rather improbable, particularly considering the watershed stewardship role assumed by the District and the City, which focuses on controlling nutrient inputs to this public water supply. That said, there are certainly other examples in Maine where such dramatic changes in water quality have historically occurred, including waters like China and Cobbosseecontee Lakes, which historically also supported fisheries for lake trout. Prolific planktonic algae blooms and associated changes in lake water quality on both these lakes now preclude management for native coldwater fisheries like lake trout.

Water quality data collected by the District revealed rather dramatic declines in water quality on Lake Auburn over the last two years. Last year the loss of deep water oxygen occurred in late fall when cool inshore water temperatures and oxygen levels provided lake trout suitable refuge in shallower areas of the lake. However, this year oxygen levels plummeted earlier in the growing season due in part to an early ice out, a very wet spring (conducive to nutrient loading from surface runoff) including a unusually large precipitation event in late June, and a long, hot growing season. These conditions allowed algae populations to flourish and became very abundant in August. As the algae died and settled to the bottom, oxygen levels were depleted by decomposing microorganisms. Since oxygen levels are restored to deeper areas of the lake only twice a year – during spring ice out and fall turnover just before ice in – the next opportunity for restoring oxygen to the deeper areas of the lake will be in mid-November, when the lake “turns over” as a result of cooling denser surface water sinking to the bottom. While the warm upper regions of the water column remain well oxygenated from plant photosynthesis and wind, this zone does not mix with the deeper water once a temperature density gradient is established along the “thermocline”. Since lake trout require cold water (50° F) found in deep water, and deep areas are devoid of oxygen, the lake trout were initially observed surfacing in 70+ degree water and gasping for oxygen, only to succumb to temperature shock. Salmon and smelt have not yet been impacted to the same extent and may survive this year, but it is unlikely many lake trout will pull through. Fall sampling for salmon and lake trout after fall turnover is planned to more fully evaluate the magnitude of the fishery impacts.

At this time it is unknown if the recent water quality changes observed in 2011 and 2012 on Lake Auburn will persist in the future. If they do, these changes will significantly impact our ability to manage for desirable native lake trout and salmon fisheries.

Logo courtesy Maine Department of Inland Fisheries and Wildlife

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