Ask and you shall receive! My recent posts have focused on the potential influence hatchery fish can have on wild populations, particularly if they starting reproducing with one another. With stocking a common, but often contentious, practice, I knew many people would be interested in our pending data analysis and results. But, I never thought it would get as much interest as it has. Even better, I’m getting some really great questions from many of you trying to wrap your head around the pros and cons of hatchery stocking.
Unfortunately, the manuscript I am in the process of writing will do very little to help clarify those questions. Heterozygosity, allelic richness, Hardy-Weinberg equilibrium, FST, genetic distance, bottlenecks …do these words mean much to you? No? That’s okay- they all refer to genetic measures that scientists look at to determine how an event (like stocking) is influencing a population. We know that certain events cause some of those numbers to go down, and other events will cause those numbers to go up. Describing how those genetic statistics have changed (or not) as a result of stocking is largely what I’ll be talking about in my manuscript. Not very reader friendly.
But, those statistics are the nitty gritty. Just like you don’t need to know how an engine works to drive a car, you don’t really need to know the exact genetic details to understand the pros and cons of stocking. But, understanding some basic genetics concepts and a little lingo will go a long way in helping tease apart why the potential effects of stocking on native populations aren’t so cut and dry. It’s also helpful to have this background knowledge when deciding whether you feel the risks of stocking are worth the rewards. So, for the next three weeks I’m going to flesh out the possible pros (this week) and cons (next week), and then round out the mini-series by summarizing the consensus among biologists as to how much stocking is affecting fish populations worldwide.
Where to start. Let’s first talk a little about why we have to think about genetics when assessing the effects of stocking. The brook trout of today are the product of millions of years of natural selection- genes that produce healthier fish and the most offspring are more likely to get passed on to the next generation, whereas genes that are associated with lower survival and reproduction eventually get removed from the population. Making things more complicated, the best genes for one population are unlikely to be the best genes for another population. This is called local adaptation- millions of years of natural selection have left fish from a given stream with genes that give them the best chance of survival in that stream (highlighted for extra emphasis). Local adaptation makes fish successful at living in one environment, but potentially not very successful if they are transferred into another environment.
Following that line of thought, it seems like fish should stay in their home stream to fine-tune local adaptation and increase survival. But, it’s not that simple (for starters, fish don’t choose to adapt, but that’s a story for another day). Streams are highly variable environments, and local conditions change faster than fish can adapt. So, stream fish populations need to put their eggs in more than one basket- they need some fish that have genes that are successful under certain conditions and other fish that are successful in other conditions. In other words, populations need a lot of genetic diversity. With increased genetic diversity there is increased survival and resiliency to changing conditions, but also increased potential for populations to be able to adapt to future stressors (like climate change).
A lot of things influence genetic diversity, but some of the biggest contributors are population size and population connectivity. Big populations that have a lot of fish moving into and out of them tend to have high genetic diversity. Historically, this is probably how many brook trout populations existed. But, times have changed. Population sizes have declined following natural disaster, disease, habitat loss, etc. And, many populations are now isolated by waterfalls, road crossings, and thermally unsuitable habitat. When populations get too low and too isolated, genetic diversity quickly erodes as fish start inbreeding and other genes get randomly removed from the population (a process known as genetic drift). For all of these reasons, one of the biggest priorities fisheries conservation managers have is to restore genetic diversity by increasing population size and connectivity.
Did you follow all that? No? That’s okay. Big picture- fish become genetically specialized to the local environment (local adaptation), which increases their survival. But, the population can’t become too specialized because it needs high genetic diversity in order to be withstand disturbance and have the potential to adapt to future conditions.
Okay, so where does stocking come into play?
First off, there are two basic forms of fish stocking. It’s important to keep them separated as we discuss the pros and cons of stocking because they are very different from one another and have their own benefits and drawbacks. The less common form is conservation stocking, where the goal is to increase population sizes and genetic diversity of critically threatened or endangered fish or reintroduce a species to its native habitat after restoration. In conservation stocking, the fish populations are generally not harvested, and stocking is used to prevent future population declines and extinction.
Conservation stocking is very tricky business. The genetics of every individual used for reproduction are carefully considered so that the stocked population is locally adapted to the wild environment, thus giving fish the best chance of surviving and reproducing. This also helps avoid outbreeding depression - when a native, locally adapted fish spawns with stocked fish that is not locally adapted and the offspring have lower survival and reproduction. Outbreeding depression has the potential to cause rapid population declines as each generation continues to have lower and lower survival and reproduction. To avoid potential effects of outbreeding depression, fish used in conservation stocking are often brought in directly from the wild and only kept in captivity for a few generations, thus minimizing genetic differences between wild and stocked populations.
Now, contrast that with recreational stocking. Recreational stocking, especially for trout, is by far the most common. The goal here is to stock as many large fish as possible in order to increase angler satisfaction. Genetics are considered, but mostly as a means to grow bigger fish, faster. In other words, fish used for recreational stocking have been artificially selected. As opposed to natural selection (where the environment picks the best genes), in artificial selection humans are the ones deciding which genes are best. It takes many years for humans to artificially select the genes that will make a population grow large and fast. But, once that goal is achieved, wild fish are no longer brought into the hatchery because wild fish will not grow as large and as fast as their artificially selected counterparts. In fact, it’s been over 100 years since the last wild fish has been introduced into many hatcheries used for recreational stocking. Because of this, fish used in recreationally stocking are genetically incompatible with the wild environment, and have lower survival and reproduction once released.
So, what are the pros of stocking?
So, yes, stocking can have positive influences on native populations. But, I’ve already hinted a few times that most stocking and hatchery practices are unable to realize some of these potential benefits. But, tune in next week for a more detailed discussion about the possible negative consequences of stocking.