Wood is Good

Wood Is Good for Wild Native Trout

Wood is Good

by Steve Hurley and Travis Drudi
Massachusetts Wildlife Magazine Jan, 2022
Republished by Permission

While everyone knows that wood in the form of trees provides critical wildlife habitat, its importance in aquatic systems is less well known. The land and vegetation surrounding a lake, pond, river, or stream has a major impact on a waterbody and its aquatic life. Trees alongside a pond or river can help shade a waterbody and moderate water temperatures from the heating impacts of direct sunlight. But it’s large wood in the form of fallen tree trunks and branches that is of particular importance to fish and, therefore, anglers. Organic matter trapped by woody material in streams and ponds provides nutrients and a food source for algae, plants, aquatic invertebrates, and a host of microscopic organisms at the base of the aquatic food chain. For more than 300 years, through unsustainable timber removal and clearing for pastures, stream straightening, and shoreline clearing, our streams, rivers, lakes, and ponds have mostly gone without this critical component of fish and wildlife habitat.

When a tree falls into a waterbody it offers instant physical habitat and cover for many species, including trout and largemouth bass. Many aquatic invertebrates rely on woody structure for different stages of their lifecycle as a source of nutrients and shelter. For example, aquatic invertebrates like caddisflies attach to instream wood and other hard surfaces in their aquatic juvenile stage before they transform to their terrestrial adult life stage. Fresh- water mussels, many of which are listed under the Massachusetts Endangered Species Act (see Flexing Freshwater Mussels, Massachusetts Wildlife, No. 3, 2020), reside under and around wood in rivers, streams, and ponds. Mammals like black bear, bobcat, fisher, weasel, and raccoon cross rivers and streams via fallen trees that span bank to bank. Anglers like to seek out downed trees, which hold forage fish species that use them as cover to hide from predators like birds and larger fish. Similarly, these trees attract larger predators like largemouth bass that also use the cover to ambush unsuspecting prey.

Downed trees are particularly import- ant in the formation of stream ecosystems because they can narrow the channel and create a diversity of habitats. They increase the flow in certain areas to create riffles and slow the flow in others to create pools and zones of reduced current velocity where fish can reduce their energy output while they lie in wait for food to drift by. Wood also works to improve water quality and stabilize stream banks. The dissolved oxygen in water that aquatic life depends on is in­creased by the movement of water over and around woody material. Downed trees also help to decrease stream bank erosion by slowing strong rushes of water during heavy rain events.

Large wood is particularly important in wild brook trout streams, where it pro­vides brook trout with important cover against predators like kingfishers, great blue herons, and river otters. Trout are often found close to wood, boulders, and other large objects in a behavior called thigmotaxis, which means that they feel safer when closer to objects. The more complex the wood is in terms of branches the more fish are attracted because it offers many more hiding places. A study in northern Vermont found that water temperature and large woody material were two of the most important limiting factors for brook trout streams. The number of brook trout approximately tripled at sites following the addition of large woody material.

Adding Large Wood to Red Brook
Large wood is being added to improve wild brook trout habitat in former cranberry bogs in southeastern Massachusetts. This dead tree was added to Red Brook in Wareham and Plymouth by the A.D. Makepeace Company under the supervision of Interfluve as part of a cooperative restoration project between MassWildlife, the Massachusetts Division of Ecological Restoration, The Trustees, and Trout Unlimited.

To some recreational users of rivers and streams, large wood is an undesirable ob­stacle that blocks passage of canoes and kayaks and should be removed. Because of the importance of wood in streams, the old term “large woody debris” used in early studies of its importance (often in the Pacific salmon streams of the north­western United States) has been replaced with the term large woody habitat. That is, what used to be referred to as debris is now recognized as a critical part of aquatic ecosystems. It’s a rare bass angler who will pass up the opportunity to flip a lure next to a downed tree. So, even if downed trees can snag the occasional lure or make canoe or kayak passage a bit more difficult, anglers and paddlers should recognize the importance of wood in maintaining abundance in the finned ecosystem beneath the surface. Now that forests again dominate the undeveloped landscape in Massachusetts, and with the addition of riparian buffers and cli­mate-smart approaches to forestry, we have become much better stewards of the forest. It’s time we recognize the impor­tance and encourage the restoration of large woody habitat for fish and wildlife.

About the Authors

Steve Hurley has been MassWildlife’s Southeast District Fisheries Manager since 1990 and has dedicated his career to the restoration of searun brook trout habitat. Travis Drudi is a Wildlife Technician II for the Northeast District. All photographs by Steve Hurley.

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