Population Size: When Big is Bad
Take a second and think about a healthy brook trout population. Your mind may wander to a stream rambling through a backwoods valley on a crisp autumn day, where there’s a deceptively calm mirror glaze reflecting the forest canopy. It’s deceptive, of course, because you know underneath the water’s surface trout are swirling around and preparing to spawn. Now, habitat aside, what qualities really constitute a “healthy” population, and what do biologists use as a measure of health?
I’m willing to bet most you said something about population size. And, why wouldn’t you? It’s one of the most common metrics biologists use to gauge population status, and everything you’ve probably ever been told about trout suggests more fish is better. You’re probably feeling pretty confident that large populations are healthy populations.
Science is a cruel beast. As a biologist, I’ve come to learn that the only thing you can be confident in is that nothing is ever ALWAYS true. Even things that seem so completely obvious and undeniable….like the correlation between population size and health. Yes, there are many benefits to large population sizes such as higher genetic diversity, higher adaptive potential, and more resiliency to disturbance. Many conservation objectives aim to increase population sizes in order to preserve exactly these qualities.
But, bigger is not always better. There’s a limit to how big populations can get before high abundance actually becomes a negative thing. How can this be? An obvious answer is food availability- more fish in a stream means less food for each individual. Have you ever gone fishing in a pond and caught what seems like hundreds of small bluegill? You could be catching the same individual, or just fishing at the wrong time of year. But, more likely you were fishing a stunted population- a common phenomenon in sunfish were there are too many fish and not enough resources for any of those fish to grow very large.
Food is not the only resource fish compete for, and may not even be the most important limiting resource. As population sizes increase, competition for habitat, specifically spawning habitat, also increases. As competition for spawning habitat increases, the average reproductive success of individuals starts declining. So much so that some fish may not get to reproduce during their entire lifetime. In other words, if there is a lot of competition for spawning habitat, population size may be large, but the number of breeders can actually remain quite small. In brook trout, it’s not uncommon for a population of 1,000 adults to only have 100 breeders, or only 10% of the population contributing towards reproduction.
In this case, population size is a poor indicator of overall population health. If I surveyed 1,000 adult trout in a stream, I would be amazed at how healthy that population is. But, if only 10% of those fish are contributing towards reproduction, then that population may be rapidly losing genetic diversity. You have to remember that to increase adaptive potential and maintain genetic diversity and population stability, the goal is to increase the number of fish that are reproducing- not necessarily the number of fish in the stream. So, a population that has fewer individuals but has, say, 40% of the individuals contributing towards reproduction may actually be much healthier than a large population where only 10% of adults are breeding.
More problematic is that we often have no idea the relationship between population size and the number of breeders. It’s pretty easy for us to go out and sample fish and determine the population size. To determine the number of breeders, we have to sample juveniles, and then run genetic analyses to determine how related the juvenile fish are. If the fish are mostly siblings and cousins, then we know the number of breeders must be fairly small and there are only a few families in the population.
What does this mean for those of you that may be actively trying to plan and execute various projects to improve the health of your local trout stream. Yes, it’s difficult to determine the number of breeders in your stream. But, that doesn’t mean you should just throw your hands up and hope for the best. The number of breeders is often directly related to habitat availability. So, to increase the number of breeders, we should often focus restoration efforts on increasing habitat, not stocking to increase abundance. Stocking fish in streams with limiting spawning habitat could actually decrease the number of breeders, thus decrease the overall adaptive potential of the population.
Of course, this isn’t to say that small populations are healthy populations. The ratio of breeding adults to total population size isn’t the only metric of population health. It’s just one of the many things to consider as you think about protecting the health of your local waterway.
Are We Making Fish Stupid?
I’ll start by acknowledging that this is a bit of a bold idea, but bear with me.
Researchers recently determined that egg incubation temperature decreases the social learning ability of adult lizards. They conducted an experiment where they incubated some eggs at about 80°F and others at 86°F. Once those lizards reached adulthood, they tested their ability to socially learning a new behavior. Specifically, lizards were allowed to watch videos of other lizards opening a sliding door (a behavior that lizards don’t usually know how to do). After watching the videos, the researchers tested whether lizards hard learned to open the sliding door for themselves. Lizards that were incubated at warmer temperatures were significantly less capable of socially learning the behavior, and thus were less successful at opening the door compared to lizards incubated at the cooler temperatures.
So what? Why does a trout biologist care about lizard egg incubation, social learning, and sliding doors? Okay, maybe I don’t care about sliding doors. But, social learning, or simply learning by way of watching or imitation, is something that humans take for granted. We watch a friend solve a puzzle a certain way and we instantly know how to solve the puzzle the same way. Or, if we see a group people heading for a different register at the store, we’re inclined to follow thinking they found a faster way to check-out. Humans use social learning all the time, both consciously (like solving a puzzle) and unconsciously (like finding a new cash register). But, what about fish?
My first few research projects as an undergraduate all focused on understanding how trout acquire important information about their environment. The underlying assumption is that every individual learns information the hard way via trial and error where each fish has to learn everything on it’s own. While some information is acquired this way, it’s not a very good learning strategy for most things. It can take a long time to develop a new behavior or learn about a threat, and a fish only gets one chance to learn about a new predator before it’s eaten. So, one alternate strategy is to pay close attention to the behavior of other fish, and pick up new information via social learning.
Social learning speeds the learning process, and is particularly helpful in situations where information changes quickly and individuals need to be ready to adapt to their surroundings. For example, and completely hypothetical, streams are subject to rapid changes in flow conditions, which can also change where the best location is for a trout to sit. A fish can try to roam around and actively determine where to go as flow changes. But, the individual is unlikely to gather all the information before flow changes again, plus moving around exposes them to a lot of predation. Alternatively, a fish can use social learning to watch to see what all the other fish are doing around them, and use that information to update their map of the stream without really moving.
As it turns out, trout are incredible social learners. One of my first research projects focused on determining how brook trout gather information about changing food resources in a stream. As many anglers know, prey availability changes frequently in small streams, and trout have to constantly make decisions about whether something floating past them is food, a stick, or potentially something lethal. So, they typically develop a search image for a few insects, and let everything else float past (this is why it’s important to “match the hatch” and pick flies that resemble bugs currently in the stream). When food resources for which a fish has developed a search image to run out, trout have to learn a new search image and target a type of insect. It turns out, it can take over two weeks for a fish to develop a new search image on its own. That’s two weeks a fish could go with very limited food as it tries to learn what to eat. But, if a fish is watching another fish eat a new type of insect, it will develop a search image almost instantly. Here, social learning leads to more calories that can be used for growth, reproduction, and even survival.
Another project I did showed that brook trout also use social learning to avoid interacting with other fish they know will outcompete them. Brook trout readily fight with one another for spots in the stream that have the best access to food, concealment from predators, and where flow is not too fast or slow. Naturally, the most aggressive fish (which is usually the biggest) has access to the best spot, number two has the second best spot, and so on down the chain. There’s a benefit to occupying spots of highest quality, but it’s dangerous for a fish to pick a battle with another fish that it is going to lose to.
So, how does a trout decide who to fight? As before, it could be a trial and error process wherein a fish fights with most of the other fish around it to determine its rank. But, fighting is energetically costly and can be lethal, so trout try to avoid interactions when possible. To do that, trout watch other fish compete, and from those observations learn which individuals are more and less dominate. It’s like watching a series of playground fights to identify the bully you never want to mess with. By watching other fish compete, a fish can socially learn the competitive ability of many individuals in a pool without ever having to interact with them directly.
So, brook trout use social learning to find new food resources (which increases energy that can be used for growth and reproduction) and to limit competitive interactions with rivals (which decreases energetic output and the chances of injury). If increased stream temperature during incubation decreases social learning as it did with the lizards, what effect will that have on trout? It’s hard to say. We know that trout use social learning, and we can speculate on the energetic benefits to using social learning. But, we don’t know exactly what happens if trout suddenly lose the ability to learn socially. Could we see reduced growth, reproduction, competitive ability, and extirpation? Yes. But, it would be hard to say that those outcomes happened because of a loss in learning rather than the effects of some other stressor such as stream temperature rise or competition with nonnative species. It’s difficult, perhaps impossible, to isolate all of those stressors from one another.
But, what this highlights is that climate change is more pervasive that we probably think about on a daily basis. Yes, stream temperature rise can make certain streams too hot for trout to occupy. But, there are negative consequence of stream temperature rise that occur before population extirpation and that may affect more subtle aspects of fisheries ecology and behavior.