I hope you have read parts 1, 2, and 3 of this series - those posts have all shared a similar theme, the importance of connectivity. Fish dispersal (movement) allows fishes interact with their landscape to fit their needs during different times of the year and conditions. The first post was about the idea of "the neighborhood" and how connectivity is important. The next post was the ecological theory - sources and sinks, complementary and supplementary habitats - behind why connectivity matters. The last post was applying ideas from the first two posts and how ecology informs management in northern Wisconsin (which is applicable elsewhere). This post flies in the face of parts of those first three posts but mostly because humans have changed the riverscape and moved around non-native fishes so much that sometimes we need to "break the rules" to restore native fish populations.
Using Barriers to Restore Native Fishes
While a bit counterintuitive, barriers can be effective to restore native fish species because they are able to prevent non-native species from accessing portions of the stream network. Here in the Midwest, it is mostly Brown Trout (Salmo trutta) that invade the habitats of Brook Trout (Salvelinus fontinalis). In southern Appalachia, like in the Great Smoky Mountains, Rainbow Trout (Oncorhynchus mykiss) out-compete native Brook Trout. And in the Western United States, browns, rainbows, and brookies are all responsible for the declines of native salmonids of the genus Oncorhynchus. Barriers and removal of non-native species have been used in all of these areas to restore native species.
My first experiences with this management strategy were the removal of Brown Trout above the dam on Seas Branch (West Fork Kickapoo watershed) which began in 1997 and on Hermosa Creek in Colorado's Hermosa Creek Wilderness where native Colorado River Cutthroat Trout (O. clarkii pleuriticus) were above a barrier and below the barrier we caught brook, brown, and rainbow trout in addition to the native cutthroats. On the way to Hermosa Creek, Ty Churchwell, our host from Trout Unlimited, tried to reduce our expectations. After all, a lot of westerners do not loving fishing the creek we were about to go to, it is "techie" and small. But they do it to catch the streams particular strain of Cutthroat Trout, a subspecies that occupies a small percentage of its native range. As we passed Purgatory Ski Resort and turned the corner to see the headwater valley of Hermosa Creek, Henry, Rich, and I all thought, "Bohemian Valley".
Henry and I went down the hill and Rich and Ty walked upstream until they too walked down the hill. Before they were even able to start their way down the hill, Hank and I were into fish. "F- yeah, you Sconnie guys can fish!" we heard Ty yell from up on the road. I don't remember how many Colorado River Cutthroats we caught but it was a very good morning of fishing. They did not get big in this little stream but every pool had fish and they were willing to rise up to hit a dry fly, a trait in fishes I greatly appreciate. And they were stunning little fish.
After the morning on the upper Hermosa, we drove downstream to below the barrier that kept the non-native trout species from accessing the upper watershed we had fished in the morning. Below the barrier, we caught browns, brookies, rainbows, and less commonly, Colorado River Cutthroat Trout. Additionally, we caught a few "cutbows", a hybrid between the native cutthroat and the non-native rainbows. Hybridization is another threat to the Colorado River Cutthroat Trout as well as other cutthroat subspecies and other Oncorhynchus species such as Gila Trout (O. gilae) and Apache Trout (O. apache). Barriers work to prevent non-native species from out-competing and from hybridizing with native species, two significant issues limiting the distribution of native Oncorhynchus species.
In Wisconsin, we have restored Brook Trout populations using similar barriers with somewhat mixed results. Where they have worked, they have been hugely successful but we have a few failures under our belts too. Yes, it is not as if Brook Trout are endangered as some of the western Oncorhynchus species are but they are generally in decline in much of their range, largely due to changes to their environment and the effects of non-native species. The restoration of Brook Trout in their native range is growing in importance as climate change limits the amount of habitat suitable for Brook Trout in their native range. Here in Wisconsin, we are managing for native Brook Trout and the negative effects that non-native Brown Trout have on them but elsewhere in the East, it is also Rainbow Trout that are outcompeting Brook Trout. Great Smoky Mountain National Park has used a number of different strategies to manage for Brook Trout in the park.
These projects are not without their issues however and not all have been successful. For one, these projects require a lot of effort to remove the non-native species. In Wisconsin, we have used electrofishing which is labor-intensive and does not capture all the fishes to be removed. In other places, rotenone has been used to remove fishes but that too is not without some issues as rotenone negatively impacts macroinvertebrates. Rotenone is an indiscriminate fish killer - native species like sculpin are also killed. The aim is not necessarily to removal all the non-native fishes but remove enough so that they have a difficult time successfully reproducing. A use of the Allee effect which tells us that below a threshold population, despite being well below carrying capacity, populations will fail to grow due to their scarcity and difficulty in finding mates. Barriers are not without their issues either. In one project where a barrier was constructed - a small artificial waterfall - floods breached it and Brown Trout recolonized the stream. Today Brook Trout are every bit as rare as they were before the project was initiated. More successful projects have been above flood control structures with large and impassable outlets.
Maple Dale Creek is a work in progress. It is in about the fourth year of the project and Brown Trout are still being removed but they are getting much less common and the Brook Trout have increased exponentially. It will be interesting to watch this project unfold. For those on Facebook, there was a recent presentation a the Trout Unlimited Driftless Area Restoration Effort's Driftless Science Symposium well worth the watch (link).
Arguments Against Barriers
Barriers are not without their issues. First and foremost are the issues associated with habitat fragmentation and the loss of connectivity. When populations are confined above barriers, they potentially suffer from both density-dependent and density-independent population regulation issues. Isolated populations are susceptible to natural disasters (density-independent events) and to Allee effects. There are also genetic issues associated with fragmentation and isolation. Fortunately, our highly productive spring creeks, populations are probably large enough to make the density-dependent issues less significant but that is not necessarily the case in less fertile waters. For example, Hilderbrand and Kershner (2000) asked, "How much stream is enough?" and they found that many isolated populations are not likely to be sustainable over time. And Wofford et al. (2005) found that isolated populations of Cutthroat Trout had lost genetic diversity due to genetic drift, leaving them more susceptible to extirpation (local extinction). Similarly, Cook et al. (2010) found that small populations above barriers, while they may be successful for decades, genetic issues are likely to eventually cause their extirpation. And that is enough scientific writing for one post...
Additionally, barriers are not fool proof, as we saw on Tenny Springs, an Elk Creek tributary. This is the project I alluded to earlier where Brown Trout were able to move above the man-made barrier after their removal. It certainly is not the first project "where nature found a way." Additionally, we are but a reintroduction away from having issues on any native fish restoration project. We are fighting non-native introductions all over the World, whether they were purposefully done by resource agencies or of the bait bucket biologist variety.
The Wrap-Up
Barriers can be an effective way to restore fish populations but they are not without issue. All the issues in the previous three posts about the role of dispersal and the importance of connectivity are evident when using barriers to restore native fishes. Increasing the length of stream above the barrier seems to be the best solution for making these project as sustainable as possible. Expect to see more of these projects, maybe coupled with assisted gene flow to mitigate the effects of genetic drift in small, isolated populations. Some of the Brook Trout restorations in tributaries to warm water streams like the lower Kickapoo and the Mississippi River are all based on similar ideas and have similar issues associated with them.
In my own NE MN and NW WI experience, yes and no to barriers.
I grew up on the North Shore amid small, infertile freestone streams. There were no native fish above the barrier falls after Lake Superior receded to its current level. Legend has it that early settlers moved pails of brook trout from below the barrier falls to their upland streams, planting them as a food source. I read that the logging companies did the same to feed the loggers in the camps. If the fish weren’t native above the falls, they were at least feral, sharing genetics with the brookies from below the falls. One of the neighborhood streams was planted with hatchery brook trout. I preferred…