I’m hopeful that if I asked readers of this blog to make a list of conservation priorities for brook trout, increasing connectivity would make everyone’s top five. It seems I circle back around to connectivity in most posts, with discussions of how movement of individuals among streams increases population resiliency, adaptive potential, and overall population health. Last week I even posted about how we should prioritize culvert replacement to increase population connectivity.
So, I’m here now to say…..maybe we should build some dams.
No, I haven’t changed my mind on the benefits of population connectivity. And, no, I haven’t lost my mind (at least not in this regard). Movement barriers may be a saving grace for some brook trout populations.
How? Well, if brook trout can’t move, then neither can our favorite foes, the nonnative trout. Neither can most other species that may be moving into small headwaters to find cooler waters during summer, such as creek chubs, pan fish, and bass. It’s essentially like clicking pause on the species composition upstream of a barrier. Kind of cool, uh?
The idea isn’t a terribly new concept. Out west, they’ve been installing barriers for a while to prevent nonnative brook trout from accessing native cutthroat trout populations, as brook trout cause rapid declines in cutthroat trout populations. When bait bucket biologists don’t interfere, installing barriers can be an extremely successful management practice that prevents nonnative fish invasions, but also stops the spread of invasive macroinvertebrates, diseases, and hatchery fish.
But, connectivity is still key to fish population health. So, it comes down to determining which is the lesser of two evils- nonnative species invasion or population isolation. As you can probably imagine, there is no single solution for every stream. But, before we can even start discussing whether purposeful isolation is a viable management strategy, we need to answer two main questions. First, does isolation actually achieve the intended results; namely a stream composed of only brook trout and other native fish? If it doesn’t, then we are just wasting our time and money by installing barriers, and potentially doing a lot of harm by restricting movement. Second, is isolation just delaying the inevitable and eventually cause populations to collapse from inbreeding and environmental disturbance. If so, again, we may just be wasting our time and money.
Unfortunately, we don’t have a great feel for the long-term repercussions of purposeful isolation. All ecological theories would predict that an isolated population should eventually become extirpated through the effects of inbreeding, random loss of important genes in the population, and the inability for recolonization following a disturbance event that wipes out an entire population (which, as we’ve learned in rainy Pennsylvania the last few years, is a common phenomena in small trout streams). Nonetheless, for reasons really talented scientists don’t entirely understand, brook trout seem incredibly resilient to isolation. We are all well aware of thriving brook trout populations above waterfalls that seem to be completely fine despite hundreds of years of isolation. So, even if purposeful isolation only buys us a couple hundred years, I think most people would agree it’s worth the investment.
But, it is fairly easy to address the first question, which is exactly what researchers from Allegheny College recently did in a new publication. After assessing the species composition of 78 brook trout streams in Pennsylvania, they determined that brook trout-only streams were significantly more likely to occur above barriers, and that over 90% of streams with brown trout had no barrier present. This isn’t terribly shocking (again, barriers block fish). But, fish get into odd places all the time. This is especially true for species that are as beloved as trout, and for which there is no end in the number of people willing to invest their own time and money in moving them around watersheds to ensure their own angling opportunities. Sadly, it happens all the time.
So, with evidence that barriers do seem to be successful at blocking nonnative fish invasions, the weight might be shifting in favor or installing barriers. But, just keep repeating to yourself: ‘connectivity over isolation, connectivity over isolation…..’. Always prioritize connectivity where possible. Maybe not all streams are equally as vulnerable to invasion, and so maybe don’t need a barrier.
The research crew from Allegheny College also looked to determine which streams may be particularly vulnerable to trout invasion. Their findings suggested that brown trout have the highest invasion potential in streams that obviously don’t have barriers, but also streams that are larger, with lower slopes and a few degrees warmer. So, in short, brown trout are most likely to invade streams that are a little lower in the watershed and, thus, those sites might be the most reasonable locations to consider barrier installation.
Though not discussed in the research study, it’s also possible that barriers could be particularly beneficial if trying to remove nonnative species from a stream reach. Once a species invades and establishes, it is difficult, if not impossible, to remove them from a system because there will likely be a constant influx of individuals from elsewhere in the watershed. But, if a barrier is installed, and then there is a couple years of manual removal, then it might restore a stream back to native-only.
But, remember….connectivity over isolation, connectivity over isolation. We still don’t have a great handle for the long-term consequences of artificial isolation. Until then, we can think of this as another useful tool in the management toolbox. But, think of it like a highly specialized, expensive tool that we should only use for very special occasions.
*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent. I encourage you to read the manuscript, found here, so you can contribute to the discussion.