Adding trees to a lake can do more than provide shelter for fish it can help support the entire aquatic food web. Researchers in northern Wisconsin United States, found that introducing 140 trees into Sanford Lake approximately doubled the lake’s capacity to support fish production highlighting the importance of natural shoreline habitats for healthy freshwater ecosystems.
The long term research project was conducted at Sanford Lake, a 92 acre lake in northern Wisconsin. Scientists placed 140 whole trees, mainly conifers along with some deciduous species into the water to investigate whether the added structures would simply attract fish already present or increase the amount of fish the lake could support.
According to Greg Sass who leads the Wisconsin Department of Natural Resources research team, the results indicated that the lake’s fish production capacity had approximately doubled following the addition of the trees.
The findings highlight how fallen wood, leaves and other natural materials can contribute to the productivity of freshwater ecosystems.
How trees support the aquatic food web
Trees that fall into shallow water create underwater habitats that provide shelter for small fish and protection from larger predators. Their trunks and branches also offer surfaces where algae can grow and aquatic invertebrates can establish themselves.
As leaves and wood decompose, they release organic matter that supports microorganisms and aquatic insects. These smaller organisms become food for fish allowing energy from the surrounding landscape to move through the aquatic food web.
Researchers found that the added trees developed into small underwater communities, supporting algae invertebrates and fish. Over time these interconnected processes helped increase the lake’s fish production.
The project builds on earlier research suggesting that a substantial share of the carbon found in largemouth bass comes from the surrounding landscape, including material from tree leaves and decaying wood.
Fish production is not the same as fish numbers
Researchers emphasised that doubling the lake’s fish production capacity does not mean every fish species doubled in abundance or that the lake immediately contained twice as many fish.
Instead, the findings suggest that the ecosystem could support a greater overall amount of fish production because the additional habitat helped increase the resources available through the food web.
Some larger fish also showed changes following the intervention. Researchers observed more largemouth bass and hybrids of bluegill and pumpkinseed sunfish. The effects on other species, including smallmouth bass, muskellunge and walleye, remain under investigation.
The benefits are not expected to appear immediately. Organic matter must break down support smaller organisms and move through the food web before its effects become evident in larger fish. Long lived sport fish may require years of monitoring to detect meaningful population level changes.
Researchers expand the experiment
Following the initial addition of 140 trees, researchers introduced another 240 trees to investigate whether increasing the amount of underwater habitat would produce further gains in fish production.
The team is examining how much wood is needed to improve habitat, where it should be placed and whether a point exists beyond which additional trees provide little extra benefit.
The project is designed to run for approximately 25 to 30 years, allowing researchers to observe changes over a period that better reflects the life cycles of larger fish. Muskellunge in northern Wisconsin can live for decades while walleye may survive for more than 20 years.
Long-term monitoring will help determine whether the initial results persist and how different fish species respond as the habitat develops.
Concerns about water quality
Adding trees to lakes can raise concerns about whether decomposing wood will release excessive nutrients, reduce water clarity or contribute to harmful algal growth.
However, a 2024 analysis from the Sanford Lake project found no negative effects on water quality or clarity, according to Sass.
Researchers noted that forested lakes in northern Wisconsin differ from large reservoirs surrounded by intensive agriculture. In some agricultural watersheds, phosphorus carried into water bodies can fuel algal blooms and deteriorate water quality. The results from Sanford Lake therefore should not automatically be applied to every lake or reservoir.
Protecting natural shorelines is essential
The research comes amid changes to lake shorelines, where natural forests are increasingly replaced by larger houses, lawns, docks and other developments. Fallen trees are often removed to improve the appearance of waterfront properties or reduce perceived boating hazards.
Such practices can reduce the amount of natural wood entering lakes and remove habitat that supports aquatic food webs.
Researchers argue that fish habitat is not merely a place where fish gather; it is also part of the system that sustains them. Fish stocking and harvest regulations can help manage populations but they cannot fully compensate for the loss of essential habitat.
The project is primarily funded through the federal Sport Fish Restoration programme, which uses revenue from fishing-related taxes and fees to support fisheries conservation, research and management.
Its findings offer a practical lesson for lake managers and shoreline residents: retaining suitable natural habitat, including fallen trees where safe and appropriate, may help sustain productive fish populations. Further research will clarify how much woody habitat is most effective and whether similar benefits can be achieved in lakes with different ecological conditions.