Do Wild and Hatchery Trout Interbreed?

This would be one of those "mover" hatchery fish found no where close to where he/she was likely stocked. See below for more details.

I’m happy to report that after months of studying (and neglect of this blog), I successfully passed my comprehensive exams this week. Over the course of a 40-hour written and 3-hour oral exam, I delved into everything ranging from statistical models, population genetics, adaptive management, and the history of ecology. My brain is mush.
 
But, in the middle of my exam, I took one day to attend the Pennsylvania Chapter of the American Fisheries Society to present the results of our study on hatchery and wild brook trout interbreeding. I know I’ve been holding those results hostage for a few months, but now that I’ve started spreading the word and our manuscript is nearly published, I think it’s about time I let you all in our secret. Drum roll please…..
 
In Loyalsock Creek, hatchery brook trout don’t seem to breed much with wild brook trout.
 
Seriously? Yes. 
 
Are we sure? About as sure as we can be.
 
How can this be? We don’t know exactly, but we have guesses.
 
Let’s step back a little and digest this study. Throughout their native range, hatchery brook trout are commonly stocked in streams frequented by anglers to increase the probability of catching a keeper trout. Stocking programs are largely successful in accomplishing that goal, but yet often remain controversial because of the possibility that hatchery trout might breed with wild trout. The process of wild and hatchery interbreeding is more formally known as introgression.
 
Why do we care about introgression? In short, introgression can decrease wild population reproductive success and increase the likelihood that a wild population will be extirpated. The mechanisms behind this response are a bit complicated, and have been covered in another blog post. But, it largely centers on the fact that fish used in the hatchery system are so genetically different than wild fish (especially when the focus of the hatchery is in recreational stocking…more on that in other blogs). The last time a wild fish was brought into many hatcheries was 50-100 years ago. In that time, humans have bred hatchery fish to grow really fast, really large, and produce a lot of offspring. While these traits produce a great hatchery fish, it often makes hatchery fish very unsuccessful at life in the wild. So, when a wild and hatchery trout introgress, their offspring often have low survival and low reproduction. To put another way- think of offspring as the average of their parents. The average of two wild fish is usually going to be far superior than the average of a wild and hatchery mating.
 
It only takes a few generations of introgression for there to be a noticeable decline in population size, and for the risk of extirpation to rapidly increase. However, much of our understanding about introgression in brook trout is borrowed from studies of salmon, and in situations where there was a large-scale accidental release from a captive facility and/or where a stocking program was terminated 10-20 years ago. There is very little information about introgression in a ‘typical’ stocking reach- where stocking is still ongoing, and has been occurring for 10-20 years. So, prior to this study, we didn’t really have a great idea on how active stocking might influence introgression in wild brook trout streams.
 
And, we are just as surprised as you when our results showed very low rates of introgression. We surveyed 30 sites across the Loyalsock Creek watershed and analyzed genetics for 1742 fish. Only 97 of those fish (<6%) had evidence of introgression.  And, it’s worth noting, our methods were very sensitive, so if anything we overestimated the number of introgressed fish.
 
Now, this isn’t a call to go out and stock freely and openly. One troubling find was that we did find introgression at sites several miles away from the closest stocking location. Often times, stocking is justified because it doesn’t occur directly on top of a wild brook trout stream.  However, our results suggest that the impact of stocking can extend far beyond the spatial scale of direct stocking efforts. How stocking could influence wild trout populations needs to consider not only the status of the stocked stream, but also streams nearby.
 
Second, it’s important to keep in mind that we did not account for how stocking could influence wild trout reproductive success. Does competition for food and space with hatchery trout decrease wild trout reproduction? Do wild trout readily attempt to breed with hatchery trout, but offspring die before we are able to sample them? We’re not sure, but both of those scenarios are plausible non-genetic effects of stocking that our study cannot address, and that would still result in declines in wild population survival.
 
Why introgression seems to be substantially lower in our study relative to other studies of trout and salmon is a bit of a mystery. It’s possible that stocked brook trout die (either through harvest or natural mortality) quickly after stocking. We know there is some truth in this, as stream surveys rarely find stocked fish even a few months after stocking. It’s also plausible that there is an environmental gradient over which stocking is more or less possible. Because we did not find much introgression to begin with, we can’t really test hypothesis about how the environment may influence introgression. However, we did find some patterns where introgression did seem marginally higher at sites with higher densities of wild adult brook trout, and where pH was higher and temperature low.  In short, it seems (though very preliminary and without high certainty) that robust wild trout populations may be a little more resistant to introgression.
 
Overall, our study suggests that until a better understanding of the factors that influence hatchery and wild trout introgression are known, a cautious prescription of hatchery stocking may still be warranted.
 

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