The rivers and streams that feed the Great Lakes are heavily dammed, and many of these dams are old and deteriorating. For decades, the dams have separated two groups of salmonids (salmon and trout): the resident Brown Trout (Salmo trutta) and the steelhead (Oncorhynchus mykiss) and other salmon species that migrate from the lakes to spawn. As we begin to remove some of these broken old dams, we are mixing these two groups, and a Michigan Department of Natural Resources study suggests that might be bad news for the brown trout.
Read the original paper here. (Subscription required)
Researchers with Michigan’s DNR introduced steelhead to a section of Hunt Creek dominated by brown trout, where no steelhead had been previously found. Other studies have looked at the interaction between the two species, but have not taken this extra experimental step and observed how introducing one to a stream section affects the other. The steelhead adults introduced to Hunt Creek spawned and then headed downstream, unable to return because of dams that block upstream migration.
Over a six-year period, this introduction was repeated and populations of trout were studied, not only in the experimental stream section but also two controls: a reference area of Hunt Creek without steelhead, and a section of nearby Gilchrist Creek. Numbers were also compared with data collected before the introductions, to provide a before-and-after look at the effects of adding steelhead to the stream. The data was analyzed to determine whether the population density, survival, or growth of brown trout was affected by having the steelhead spawn in their waters, as they would probably do if the dams blocking them from the Great Lakes were removed.
The researchers found about half as many brown trout one year old or older in the experimental years as in the years before the introduction. There was no such difference between years at either of the control creeks, suggesting that the introduction was the cause of the change.
But the study also found that when given access to these cold-water streams usually blocked by dams, steelhead can be extremely successful in the Great Lakes area. Currently, steelhead populations are maintained largely through hatcheries, rather than natural runs. Stocking from hatcheries is expensive and inefficient, and allowing these steelhead runs to re-establish after dam removal could both reduce the cost and increase the quality of this popularly fished species.
Dam removal has a lot of benefits, not only in terms of opening up habitat for migratory species, but also improving physical and chemical attributes of rivers and creeks (temperature, salinity, sediments, etc). But as we ramp up removal of some of the oldest and most dilapidated dams in the country, we must acknowledge the costs as well, including the effect it will have on resident fish and other creatures that have lived, sometimes for a century or more, separated from marine and lake species.
Special thanks to Todd C. Wills of Michigan DNR, one of the co-authors, for providing us with a copy of the paper.