Lake drawdowns are a major point of contention for sportsmen. Some love the “fish in a barrel” situation they lead to – lots of fish in not a lot of water. Others cry that the fish community could be decimated by them. So how do drawdowns actually affect fish? A 2018 study of the Rainy-Namakan complex in Minnesota and Ontario, completed by USGS, the National Park Service, and the Minnesota DNR, can tell us.
The most obvious issue with spring drawdowns is the effect they could have on spawning. The new, lower water levels may leave historical spawning sites – quite literally – high and dry. Because most fish species have at least a partial tendency to return to their natal site to reproduce, the results of this upheaval could be significant, especially if there are no suitable alternative sites.
If there is a successful spawn, the eggs of many specialized species have to remain more or less where they were laid for optimal survivorship. For generalist species like yellow perch, which spray their eggs in nets that attach to anything available or just drift through the water, the issue is not so dire. Their offspring can blow almost anywhere the current takes them without being jeopardized.
Larvae aren’t automatically safe once they hatch either. As a body of water is drawn down, turbidity increases as benthic nutrients and solids are suspended. This influx of nutrients causes a spike in phytoplankton reproduction, which in turn exacerbates the turbidity even further. High turbidity levels have a negative effect on larval fish; larval fish in turbid water eat less and present with lowered health.
As the larvae grow into fry and even juveniles, they need to stay close to shallow macrophytes (underwater vegetation) to hide from predators. When water levels change drastically, this habitat could be lost. Overall, the result is a population structure with a very weak base of young fish and a dominating cohort of larger, more mature fish. While this could simply mean less competition and bigger fish, it could just as likely result in a population crisis if numbers drop too far.
Luckily a 2016 study in Nebraska showed that some conditions within a reservoir are not noticeably affected by spring drawdowns. The warming rate of a reservoir and its mean water temperature per month are unchanged by these larger fluctuations. And while the dissolved oxygen of a reservoir is lowered with a drawdown, the oxygen levels stay well within the required ranges for native fish species.
Furthermore, best management practices are being used more and more to compromise between humans and nature. Most water level changes in inland reservoirs follow strict rules, like keeping the water level change during peak spawning and nesting weeks to less than 0.66 feet (0.2 meters). Many reservoir systems, like the Rainy-Namakan complex, are returning to a more natural hydrologic regime. Less water is removed in total, and the process is much more gradual.
For anglers, the fishing immediately following drawdowns can be incredible. Fish are forced into less and less water, which makes the odds of finding them that much higher. However, finding the fish does not always mean catching the fish. To truly take advantage of the opportunity, sportsmen need to adapt to turbid conditions.
As turbidity increases, both prey and predatory fish move less. They spend less time cruising and more time holding tight to cover, especially to macrophytes. Even the diet of predatory fish changes. Bass, walleye, and pike in turbid conditions will eat fewer crawfish and aquatic bugs, and more shad and bluegill.
When targeting fish in these conditions, choose your presentation by action rather than color. Bladed, disruptive baits will get the attention of fish that can’t see very far in front of them. Focus your casts in and along the edges of weed beds, and keep the lure suspended rather than hitting the bottom.
Originally published in Outdoor News.
Clark, Mark E. et al. 1998. Simulating smallmouth bass reproductive success in reservoirs subject to water level fluctuations. Environmental Biology of Fishes 51: 161-174.
Cott, Peter A. et al. 2008. A review of water level fluctuations on aquatic biota with an emphasis on fishes in ice-covered lakes. Journal of the American Water Resources Association 44(2): 343-359.
DeBoer, Jason A. et al. 2016. The influence of a severe reservoir drawdown on springtime zooplankton and larval fish assemblages in Red Willow Reservoir, Nebraska. Journal of Freshwater Ecology 31(1): 131-146
Larson, J.H., Maki, R.P., Vondra, B.A., and Peterson, K.E. 2018. Before-after, control-impact analysis of evidence for the impacts of water level on Walleye, Northern Pike, and Yellow Perch in lakes of the Rainy-Namakan Complex, PLoS ONE 13(6): e0198612.