Pros vs. Cons of Hatchery Stocking: Part 2

A nearly 18-inch stocked brook trout. What kind of havoc can these guys cause? Photo courtesy of PA Fish and Boat Commission.

Picking up where we left off last week, this week I’m going to flip the switch and talk about the potential negative consequences of stocking. I highly recommend that those who missed, or maybe just don’t remember, last week’s post start there before continuing. Some lingo and concepts may be a little fuzzy without the background information.
 
You didn’t press the back button, did you? (I don’t blame you, I wouldn’t either). So, here’s a quick refresher: wild trout populations have genes that are locally adapted to their native environments. This means that trout have genes that make them successful at life in their home stream, but their genes may not be great for surviving in another stream. However, not all fish in a stream can be identical clones of one another. There needs to be some level of genetic diversity in order for populations to survive disturbance and be able to adapt to future conditions. I call this the “eggs in many baskets” insurance policy. The genes that are best this year may not be the genes that are best next year, and so there needs to be high diversity so that at least some fish can survive and reproduce if conditions change in the future (if you’re a financial guru, this concept is very similar to having a diversified stock portfolio). 

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Easy, right? Well, here’s some of the ways that hatcheries can disrupt this balance. I’m focusing specifically on recreational stocking programs because conservation stocking programs have taken more precautions to avoid these potential pitfalls (though, they do sometimes still happen).

• Reduced genetic diversity: The goal of hatcheries is to grow as many big fish as possible. Once humans found out that growth was linked to genetics, we started weeding out the slow-growing fish, along with their genetics, leaving only a select few genes remaining in hatchery populations. This selection process (termed artificial selection) results in hatchery populations with desirable genetic make-ups, but very little genetic diversity. To think about this another way, imagine you have a bag of Skittles. If you scour through the bag and set aside every red Skittle, you’re left with a highly desirable handful of all-red candies. But, what if you want purple? Same idea with artificial selection in hatcheries.  The fish that are in our hatchery systems may have some highly desirable traits, but overall they have very little diversity and little ability to adapt to changing conditions. When these low-diversity hatchery populations are released into the wild, average population genetic diversity goes down.  
• Introduction of maladapted genes: Not all genes are created equal.  Not only do we care about overall genetic diversity, but we also care about how much a particular gene increases survival and reproduction. Some genes decrease survival though, for example, making fish more predisposed to disease, heat stress, or predation. Many “bad” genes are either not present or in very low abundance in wild populations because natural selection removed them a long time ago. But, in hatcheries, there are very few selection pressures (aside from growth rate), and these genes have a tendency to pop up more frequently. Once hatchery fish are released into the wild, they bring these “bad” genes with them like an unwelcomed guest to the dinner party
  
• ​Reduced survival and fitness of offspring: If hatchery fish never bred with wild fish, then the two aforementioned points would be of little concern. However, interbreeding between hatchery and wild fish is what causes those bad genes to enter into wild populations, and interbreeding is what causes the next generation of trout to have lower genetic diversity than offspring from two wild fish. I’ve made this analogy before, but think of offspring as an average of their two parents.  A fish that has two wild parents will receive mostly genes that have been naturally selected to increase survival in that stream.  Now think of a fish that has one wild and one hatchery parent. Half of that fish’s genes will be adapted for life in the stream, and the other half will be adapted for life in a concrete raceway. Because of this, fish that have one hatchery and one wild parent tend to have lower survival, lower reproduction, and, in general, less vitality, than fish that have two wild parents. This effect is known as outbreeding depression (breeding with fish OUT of the population depresses offspring survival), and the effects can be long-lasting. One generation of interbred fish with low survival will lead to fewer offspring for several more generations, and those future generations may still have lower genetic diversity and a higher rate of “bad” genes in their genetic composition.

Tiger trout are absolutely gorgeous, but I hate knowing that they steal from next year's native brook trout population.

• Reduced fitness of wild populations: In addition to the genetic effects above, wild fish may invest a lot of energy into reproducing with hatchery fish, only to have few, if any, of their offspring survive. This energy, if spent reproducing with another wild fish, could have resulted in thousands of fertilized eggs with higher probability of survival. Sometimes we may not find many interbred individual in a population, and the initial reaction may be that hatchery fish are not negatively affecting wild populations.  But, it’s hard to tell how much a wild population would be reproducing if it wasn’t for their wasted efforts in trying to breed with hatchery fish. And, this potential negative effect extends beyond just brook trout stocking, but also to hybridization between brook and brown trout.  Every tiger trout (a brook x brown hybrid) represents a failed native brook trout reproduction. 
• Decrease maximum size of wild trout:  This one might sound the most far-fetched, but hang with me. As I already mentioned, a trout’s maximum size has a genetic component.  Just like I am never going to be 6 feet tall, many trout will likely never reach 7 inches (the legal harvest size in most Pennsylvania streams). But, there are a few trout in every stream with the genetic potential to reach 7 inches and larger, and those fish may survive and reproduce to make more large fish for a very long time because most people don’t know they are there, and many wild trout anglers return catches of any size back to the stream.  Now, if we stock the stream, there is more angling pressure directed towards stocked fish, but the large wild trout are also more likely to get caught. And, because the wild trout are the size of hatchery trout, they are more likely get harvested because it’s assumed to be a hatchery fish. Eventually the select few fish that were carrying the “grow large” genes get removed from the population, and now no fish in the population has the genetic potential to get very large. . 
• Increased competition: Alright, let’s move away from the genetic effects of hatcheries.  Hatchery fish are generally larger and more aggressive (which is, in part, the result of artificial selection). When they are released into the stream, they can outcompete native fish for food and occupy the best habitats in a stream. If hatchery fish remain in the stream for long, they can ultimately hurt native population reproduction by minimizing the amount of nutrients that wild fish can put towards growth and reproduction.
• Predation: Bigger fish have a higher probability of being piscivorous, meaning fish-eating. Yes, wild fish eat juveniles, but the probability of this behavior occurring is much higher in stocked populations.
• Disease: Incidences of disease are more common in hatchery populations, and stocking of diseased fish occurs fairly often.  In trout, the most common diseases tend to be gill lice and whirling disease, but there are also potential internal and fungal infections that don’t get as much attention due to their less-subtle appearance.   

I want to emphasize two points.  First, while these are the most likely negative effects of stocking, there are others that I didn’t cover. We would be here all day.  Second, not every stocking program is going to suffer from these negative effects. But, they are all possible and frequently observed. In the last part of this series I am going to attempt to pull together results from a few studies to comment on how likely it is hatchery stocking will have positive or negative effects on a wild populations.  But, I’m away at the annual meeting of the American Fisheries Society next week, so you may just have to wait in suspense….