Pros vs. Cons of Hatchery Stocking: Part 3

I’m back!  And, boy was my absence untimely. While I enjoyed soaking up the rays attending the annual meeting of the American Fisheries Society in Florida, I unfortunately missed the Pennsylvania Wild Trout Summit. The PA Fish and Boat Commission was quick to post presentations online, so I’ve been able to catch a few talks (including the one below by my advisor, Ty). But, I’ve also been reading some feedback from a few attendees and my takeaway is that the best talk wasn’t by a platform presenter- it was among members in the audience. One of the reasons I love studying trout is the passionate anglers and citizen scientists that are invested and devoted to wild trout conservation and restoration. There is no other angler base that is as informative and fun to interact with as you all, and I was sad to miss the opportunity.

My other observation is that there was some disappointment in what wasn’t discussed. Most notably, it seems a lot of people in attendance wanted to discuss the state’s trout stocking plans.  I’m not surprised. Stocking is controversial and there will probably never be a stocking plan that makes everyone happy. But, I’m also encouraged. The public is trying to voice their opinions on this really complex problem, and, from what I’ve seen, seem to largely understand the delicate balance between the science of native fish conservation and the social dynamics of recreational fishing.  It’s not an easy line to walk.
 
I’m also encouraged because it means there is interest in our current research beyond the scientific community. Our manuscript on native and hatchery fish interbreeding is nearing completion, and the results are getting closer to being released. Until then, I’ve been spending most of my days pouring over manuscripts published over the last 20+ years from other studies of hatchery-wild interbreeding and trying to summarize their findings. From this, I’ve already summarized the pros and cons to hatchery stocking, but I’ve left you in limbo the last two weeks. Overall, do hatcheries have more of a positive or negative effect on wild trout populations?
 
Before I answer that question, there are two caveats. First, I’m only discussing recreational stocking- or stocking done to temporarily increase population sizes to allow for increased angling opportunities. The potential pros and cons to conservation stocking are a bit different. Second, I am only focusing on the hard science. I’m not going to attempt to compare the social benefits of stocking with the impacts to native fish diversity. But, you should. Everyone should weigh the pros and cons and make their own informed decisions about stocking. It’s not my place to make the decision for you, but it is my job to present the science so that you can be informed. We know that stocking increases recreational opportunities and can be an economically profitable business, both of which valuable. Taking that into consideration, I have drawn a line in my mind where I think stocking is worthwhile and where it’s not. You need to find that line without someone telling you where they think you should put it.
 
So, after 20+ years of study, what do we know about the effect of hatchery stocking on wild trout populations? 

• Hatchery stocking doesn’t always result in interbreeding. But, you have to understand that our data sources are also not the greatest. Most studies of hatchery-wild interbreeding focus on either accidental escapees, which are common in salmon aquaculture facilities, or historic stocking practices that were terminated long before the start of the study. Far less attention is given towards quantifying current rates of interbreeding in actively stocked populations. For a variety of reasons, those other studies, which do offer valuable insights, aren’t ideal comparisons to current stocking practices and may underestimate the amount of interbreeding. I’ve also had trouble finding studies on brook trout, and most that I have found are on lake populations in Canada. But, across dozens of studies, the overall conclusion is that stocking doesn’t always result in hatchery-wild interbreeding. In some stocked populations interbreeding is nearly undetectable (but recall the possibility for underestimation); however, other populations now appear to be 100% interbred. But, overall, most studies do detect at least moderate levels of interbreeding suggesting that stocking does usually change wild population genetics.   
• We don’t really understand why rates of interbreeding are so variable. But, it probably depends on two factors - the vitality (or lack thereof) of stocked fish and their offspring and the environment they are stocked in. Hatchery fish survival is very low in wild environments, and so fish may not live long enough to spawn. Likewise, juveniles with at least one hatchery parent often do not survive to adulthood (which is the life stage we usually sample when we conduct these studies). So, interbreeding may still be occurring, but high juvenile mortality may keep us from detecting it. Environmentally, studies have shown that interbreeding can be more common when wild population sizes are low, habitat is limiting, and/or water quality is better. Together, these results suggest that interbreeding is likely more common when competition is lower, encounter rates between wild and hatchery fish is higher, and when water quality doesn’t stress hatchery fish. However, it should be noted that there are not many studies that explore the influence of habitat on interbreeding, and there are still a lot of unexplored hypotheses for how habitat could influence rates of interbreeding.
• Stocking almost never has a positive effect on native populations. I say “almost never” to cover my back, but the consensus is really more like “never.” Yes, stocking can increase genetic diversity and local population size. But, that assumes stocked fish are highly diverse to begin with, and usually they are not.  Either way, more is not always better. Stocked fish are not adapted to local conditions, and so any increase to genetic diversity usually just dilutes the frequency of genes that increase survival to local conditions. A highly diverse population that isn’t locally adapted still isn’t a very stable population. Not to mention, stocked fish can compete with wild populations, introduce diseases, and influence nutrient cycling in aquatic and terrestrial ecosystems.
• But, negative effects of stocking are often hard to definitively find. We know that stocking often changes wild population genetics. But, understanding how these changes influence survival, reproduction, and adaptive capacity is not so clear. It’s hard to tease apart effects of stocking from environmental variation and natural gene flow, and it can take many generations before there is an observable effect of stocking (i.e., long after scientists have stopped collecting data). Much of what we know about the negative effects of stocking are theoretical, and while we think they probably do occur in wild populations, they can be very hard to document without a lot of data (which means a lot of money).
• When there is a negative effect of stocking, the consequences can be severe and long-lasting. If interbreeding doesn’t occur, then wild population genetics go largely unchanged and the biggest effect hatcheries have on wild population is to lower reproduction through competition. That’s bad, but wild populations can rebound provided that the stressor is removed (i.e., stocking stops).  Once interbreeding starts occurring, it becomes a negative feedback loop. The genes that hatchery fish pass on to their offspring stay in the population for many generations, perhaps indefinitely. It’s kind of like the spread of a contagious virus.  It starts with one person, but quickly escalates to a large proportion of the population. Similarly, a handful of hatchery or interbred fish that reproduce can spread their genes throughout the population in only a few generations.  It doesn’t take much, or long, for a wild population genetics to be significantly influenced by interbreeding. And, once interbreeding occurs, it’s nearly impossible to remove those genes from a population without total population extirpation. Making matters worse, the genes that are being passed along are generally associated with lower survival, lower reproduction, and less adaptive potential.  So, as the population becomes increasingly more interbred, the health and vitality of the population declines.

 
So, where does that leave us?  With a lot of uncertainty. Hatcheries can have negative effects on wild populations. But, not always.  And, hatchery interbreeding can be high in stocked populations. But, not always. And, we know that there are long-term negative consequences of interbreeding.  But, yet again, not always. We just don’t know.
 
Perhaps a more important question- where does that leave you in your thoughts on stocking?