A Tale of Two trout

Pennsylvania brook trout have life easy compared to their southern cousins.

This one goes out to all my readers down south.
 
Northeast brook trout populations are what I would call “typical.” Fish move around some, but not a lot. Populations are not too genetically diverse, but there’s enough there for evolution to work with. No one stream contains a ton of fish, but we aren’t typically concerned that a population could be extirpated next year.
 
Drive a few hundred miles south and we’re telling a different story. There, brook trout populations are living life on the edge- literally and metaphorically. Southern-edge populations often live in streams that are hot, lack abundant food sources, and are threatened by barriers and an abundance of nonnative species. Compared with populations in the northeast, southern populations are also must older because they were never frozen out by the glaciers. But, with age comes genetic wear and tear- the older a population gets the more likely it is to have lost some genetic diversity due to random chance and catastrophic events that cause large population declines (floods, disease, etc.). Put all this together- the lack of connectivity, the low population sizes, and the limited genetic diversity- and southern-edge brook trout seem destined for population collapse.
 
When populations get isolated, and when genetic diversity starts to drop, biologists often start questioning whether we should intervene. It wouldn’t be hard- we can simply do what brook trout used to be able to do themselves and move individuals between populations. We call this physical movement of individuals among populations translocating. With a little luck and a lot of research and planning, the translocated fish will spawn with the resident fish, and their offspring will have increased genetic diversity that contributes to the population for many generations after. This is exactly what we want because, as genetic diversity increases, we often see an increase in number and size of individuals in a population. Perhaps more importantly, we also see that genetically diverse populations are better able to survive disturbance events. Translocations must be a no brainer, right?
 
But, here’s the catch. The populations down south have been isolated for so long that many of them have evolved their own identity, and potentially might be on their own evolutionary trajectory. Translocations are only successful if the fish moving into a population have genetics that are similar to the resident population. Otherwise, the offspring may be genetically more diverse, but those genes may make fish poorly adapted for life in that environment. This is tough, because it can take many generations to realize that the translocated fish are having a negative effect, and by then it could be impossible to turn back the clock.
 
The genetic risks associated with translocations have been known for a long time. But, the south brings up another, more philosophical, dilemma. If we start translocating fish across multiple watersheds, we potentially erase all of those genetically unique populations. Do we really want to do that?
 
Seriously.  That is my question to you. What is more important? A genetically distinct population that could collapse within the next 50 years?  Or, a population that loses some of its uniqueness, but perhaps has more long-term stability?
 
Here’s the fun part- scientist haven’t decided the right answer. On the one hand, you have to balance the risks of translocation with the potential for population collapse due to low genetic diversity and isolation.  But, who’s to say that the isolated, distinct population wouldn’t survive just fine on their own?  Populations above waterfalls have existed for hundreds of years and they are doing just fine. On the other hand, what is the value of these genetically distinct populations?  Are they locally adapted to those streams, and therefor possess unique genes that are worth conserving?  Or, are they just one in the same with the neighboring populations?
 
We’ve definitely got some important decisions to make, and the right answer will surely vary across watersheds. But, perhaps the decision doesn’t need to be so black and white, either. For example, we probably don’t want to move fish with strong hatchery influence to watersheds that are comprised of completely wild genetics. This is particularly true given that fish stocked in the southeast are often descendent of the northeast (turns out…southeastern brook trout are hard to reproduce in captivity). But, what if we look at the watershed as a whole, identify the populations that have completely wild genetics, and only translocate to/from wild-only populations that are somewhat similar?  Maybe this is a good compromise that would lead to moderate increases in genetic diversity while still maintaining some unique genes in the population.
 
Now is the time to be having these discussions. I recently sat down with the Trout Unlimited Southeastern Volunteer Coordinator and we chatted about how some states have very restrictive translocation policies making it difficult to do reintroductions or translocations. Good for those states, because they are probably also limiting the spread of hatchery genes into wild populations. There are still so many uncertainties that I think conservative approaches are probably the best choice right now for most streams.  After all, most brook trout populations will be fine for the next few years while we take the time to do our due diligence and research needed to make the right decision.
 
But, pretty soon we do need to start having these discussions.  Will you be ready to contribute? 

This post was inspired by recent research published by Kasey Pregler and colleagues. I would encourage all to read the original manuscript found here. 

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