In Defense of Nonnatives

X marks the spot, in this case where you can find stocked trout in Loyalsock Creek.

We stock.  We stock nonnative brown and rainbow trout in native brook trout streams, but we also stock brook trout in native brook trout streams.  You may feel uneasy about nonnative stocking in native streams- there is clear evidence that brown and rainbow trout cause declines to native brook trout populations. But, what about stocking brook trout in brook trout streams?  That seems pretty harmless, right?
 
Or is it?
 
You may have already heard about the negative genetic consequences that can occur if native brook trout spawn with stocked brook trout (otherwise known as introgression).  In short, stocked fish are not genetically compatible with local stream conditions. If a native trout spawns with a hatchery fish, the offspring generally have lower survival, reduced growth, and go on to make fewer, less successful offspring. It’s a negative feedback and ultimately results in overall reduced population health that can take many generations to repair.  
 
But, hatchery and wild brook trout don’t always reproduce with one another, and even some evidence that there are very adults that are the product of hatchery and wild interbreeding (whether this is a choice or a failure for eggs to survive remains unclear).  So, genetic consequences of stocking are sometimes quite minimal.  In that case, the effects of stocking may depend more on how hatchery and wild brook trout interact with one another on a daily basis, otherwise known as conspecific (meaning same species) interactions. 

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Following my post last week on the influence of stocking on ecosystem nutrients, I was sent a paper that discussed whether native or nonnative trout stocking is a bigger disruption to native ecosystems. The authors reviewed hundreds of research papers that documented the effects of stocking on everything from individuals, to populations, and on up to entire ecosystems. They found that native fish stocking can actually be WORSE on wild, native populations than nonnative stocking.
 
How can it be so? How can more of a good thing (native fish) be a bad thing? It all makes sense in light of ecological theory which states that the more similar two individuals are, the more they will compete for resources. So, two toddlers are going to compete over the same toy more fiercely than a child and adult might.
 
There is no doubt that brown and brook trout compete with one another- so much so that it causes declines in one species (usually brook trout). But, individual brook and brown trout compete less with one another than two brook trout compete with each other. Or, to put it another way, intraspecific (same species) competition is always higher than interspecific (between species) competition. 

Stocked or not? It can be hard to tell because even stocked trout are gorgeous, but lack of color, rounded pectoral fins, and abnormal size are generally great signs a fish was stocked. Photo from kneedeepclub.org.

And, that’s where we have a problem. Competition between hatchery and wild trout of the same species can cause a shift in individual-level properties. Things like stress, physiology, growth, reproduction, movement, behavior…basically everything…are influenced by competition. And, because hatchery fish are often artificially selected to have higher growth, the competitive edge is given to them. Once hatchery fish outcompete wild fish, only the non-adaptive hatchery genes are left to sustain the population, which could speed up population collapse.  
 
Of course, I’m not going to let nonnative stocked fish off the hook entirely. The authors showed that nonnative stocked fish have a significant negative effect on aquatic communities (which entails all living organisms in the stream, including frogs and other terrestrial species that only occasionally visit the water) and the ecosystem (which includes not only interactions with other organisms, but also nutrient processing and energy flow). Unfortunately, there aren’t many studies on how native stocked fish influence community and ecosystem-level processes, so we can’t be sure whether native or nonnative stocked fish have a greater impact at these levels of organization.
 
So, once again, I warn you. Stocking has significant positive influences on angling opportunities, and is probably responsible for getting people off couches and into streams.  But, the long-term consequences of stocking may be both negative and long-lasting, and in ways we still don’t entirely appreciate or understand.
 
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 so you can contribute to the discussion. 

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