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?

This is what I think about when I hear "healthy trout population." But looks, and counts, can be deceiving.

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.