Adding wood to streams benefits brook trout
Trout fishing is a popular pastime around the world. Most trout species are known to put up a good fight on the line and are valued for their table fare. Brook trout (Salvelinus fontinalis; Figure 1) are a small species of char native to eastern North America. They require cool, well-oxygenated water to survive, and are largely restricted to small headwater streams. They were a valuable food source for Native Americans and European settlers and are prized amongst recreational anglers today.
By the turn of the 20th Century, brook trout populations had been decimated by overharvesting, pollution, the introduction of nonnative trout species, and logging methods that altered the stream habitat. In many Appalachian rivers and streams, these early logging practices included indiscriminately clear-cutting riparian forests and “log driving”. Log driving was a quick and inexpensive way to transport wood to sawmills and pulp mills. The trees were felled and floated downstream with the help of the spring floods. “Splash dams” were constructed at strategic points along the river to enhance the water flow. The rivermen would close the spillway to raise the water level in the impoundment behind the dam. While it was closed, crews quickly cleared large pieces of wood and boulders (usually with dynamite) from the river to prevent the logs from getting snagged. When they were ready, the spillway was opened allowing the water and logs to rush downstream. Though log driving became obsolete with the construction of logging roads and railways, widespread clear-cutting continued for many years afterward.
These historic logging practices had an adverse effect on stream habitat. The absence of mature riparian trees that provided shade to streams caused water temperatures to increase. There was also a lack of large woody material entering the streams to provide structure for fish habitat. Excessive sediment deposits from eroded stream banks covered gravel spawning beds used by brook trout.
In order to reverse the damage from previous generations, stream restoration projects have become widespread in recent decades. These projects focus on restoring riparian and stream habitat, stabilizing eroded streambanks and enhancing the natural functions of the stream. The placement of large woody material such as logs, branches and root wads have become increasingly common in many trout streams. In addition to providing refuge for trout, large wood slows the stream flow allowing gravel to settle on the streambed. As the flow is altered, deep pools are scoured out of the streambed immediately downstream of the wood. The wood also traps organic matter and improves the habitat for benthic macroinvertebrates, an important food source for trout (Figure 2).
While adding woody material has positive effects on the stream ecosystem, it is unclear if trout populations actually increase due to these efforts or if the trout simply congregate around the improved habitat. Dr. Jud Kratzer, a Fisheries Biologist with the Vermont Fish and Wildlife Department (VFWD), led a study to examine how the addition of large wood influences brook trout populations. Since 2012, the VFWD and Trout Unlimited, a nonprofit conservation organization dedicated to protecting and restoring coldwater fisheries and watersheds in North America, have partnered to improve stream habitat in northeastern Vermont (Figure 3).
The study took place in the East Branch Nulhegan River and its tributaries, a watershed that experienced multiple rounds of riparian clearcutting and log driving in the past (Figure 4). Brook trout populations were monitored every July from 2012 to 2017. Backpack electrofishers were used to temporarily stun the fish so they could be captured and measured. In 2013, woody material was placed at several “treatment” sites in the watershed by felling trees and positioning them in the stream using a griphoist. The “control” sites were nearby locations that did not have wood added to the stream.
At the treatment sites, brook trout abundance and biomass (the total weight of the trout population) tripled after large woody material was added. In the year following the addition of wood, trout biomass decreased at the control sites before steadily increasing over the next several years. This suggests that the trout initially concentrated around the improved habitat before the population increased and spread out throughout the streams. The woody habitat also benefited juvenile trout by providing additional refuge and food when they were most vulnerable.
The strategic placement of large wood had a positive effect on brook trout abundance and biomass. It should be noted that not every trout stream restoration project will require the addition of wood. Large wood is not an important habitat feature in every stream channel (e.g. steep, boulder-dominated streams). In some streams, there may be some other factor (such as water temperature) that is limiting trout abundance, so adding wood will not make a difference for trout. A thorough evaluation should be conducted to examine the habitat, natural wood loadings and limiting factors for each stream before wood is added. In streams where the lack of woody habitat is a limiting factor for brook trout, the addition of wood has potential to be an effective restoration tool as the surrounding riparian forests mature.
Kratzer, J.F.. 2018. Response of brook trout biomass to strategic wood additions in the East Branch Nulhegan River Watershed, Vermont. North American Journal of Fisheries Management 38: 1415–1422.