In studies of logging practices prior to passage of the act in 1971, changes to fish habitat were documented from the use of stream channels as transportation corridors for logs and from other changes to riparian areas adjacent to streams. One of the landmark studies of such practices occurred in the Alsea River watershed in the late 1960s.
Starting in 2006, a team of researchers from OSU, the U.S. Geological Survey, Colorado State University and the forest products industry returned to the Alsea basin to study the impacts of modern logging practices conducted in compliance with the act. They assessed the numbers of cutthroat trout, aged one year old and older, annually through 2014 in the watersheds of Needle Branch and Flynn creeks. The researchers also documented forest cover, stream habitat conditions and stream temperature and discharge.
Logging occurred in Needle Branch in 2009, but Flynn Creek was left unharvested, just as it had been in the 1960s. The results were published March 1 in a professional journal, Forest Ecology and Management.
“In the 1960s, the stream channel in Needle Branch got hammered, and the cutthroat took it in the shorts,” said Doug Bateman, the lead author of the paper, now a retired researcher in the College of Forestry.
In the latest study, the biomass of cutthroat trout in Needle Branch increased after the tree harvest relative to the trout in Flynn. In the headwaters area of Needle Branch, nearly all of the trees were cut with the exception of the required buffer strip along the fish-bearing portion of the stream. By monitoring the movements of fish up and downstream, the researchers were able to determine that increases in Needle Branch were related to local changes rather than to influxes of fish from other areas.
“It’s rare to be able to say that,” said Bateman. “In other studies, it is often unknown whether changes in population size are associated with fish movement, but here we show that the fish responded to conditions in the harvested portion of the channel. We weren’t set up to study the causes of the increase, so we can’t really say for sure. It’s possible that increases in sunlight or increased export of invertebrates from upstream areas contributed to the increased fish biomass, but it could also be related to any number of other factors, such as stream temperatures, changes in predators or disease.”
The researchers also recorded changes in numbers of juvenile coho salmon, which were found generally downstream from harvested areas where increases in cutthroat trout were most apparent. No changes were observed in numbers of salmon over the course of the study, possibly due to the fact that these fish were located downstream from the logged area.
The salmon numbers are also complicated, said Bateman, by the species’ migratory behavior, which exposes the fish to a variety of factors such as sport and commercial harvest and to conditions in the ocean and estuaries that most cutthroat trout do not experience.
“We can confidently say that, in this watershed, cutthroat trout were not negatively affected by logging activities over the course of the study,” said Bateman. “We’re cautious about generalizing these results to other watersheds, since conditions can vary so much. Still, these fish are probably well adapted to changes in the streams, and forests provide some of the best remaining habitat for them. When you move downstream into areas adjacent to farm fields and urban areas, the changes to rivers and streams can pose significant challenges. It’s important to look at the watershed as a whole.”
Funding for the study came from public and private organizations, including: the College of Forestry and Watershed Research Cooperative at Oregon State University; the U.S. Geological Survey; Oregon Watershed Enhancement Board; USDA Forest Service; the federal Bureau of Land Management; the National Council for Air and Stream Improvement; the Oregon Forest and Industries Council; Plum Creek Timber; Hancock Forest Management.