Reversed parasite-mediated selection in sticklebacks from eutrophied habitats (2010) Heuschele & Candolin, Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-010-0937-9

The Crux

We’ve all heard the stories of turtles choking on plastic bags, or birds swallowing sporks. Algae grows out of control due to chemicals being added to the water, which changes the native habitat. For fish like sticklebacks, where males compete for the attention of females, the loss of visibility has the potential to be a problem for the females. Male sticklebacks defend a territory, develop a bright red spot, and fight other males. The long and short of it is that the bigger, stronger, and prettier fish mate more and the weaker and uglier fish don’t. One thing that weaker males tend to have more of than stronger males are parasites, which isn’t a surprise as a parasite’s entire existence revolves around surviving at the expense of whatever organism is unfortunate enough to have them.

Understanding how eutrophication affects female choice in relation to parasites, and thus the reproductive dynamics of this system, is quite important. This paper tries to map that out, using the Baltic Sea around Southern Finland. This experiment is well-suited to the problem, as there has already been evidence for algal growth changing the dynamics of stickleback reproduction.

Did You Know: Eutrophication

What is it? This is where a body of water, like a lake or pond, experiences an increase in nutrients in the water. These nutrients can come from chemicals being dumped in natural waters, sewage leaks, or even runoff from farmland. Cows defecate and urinate everywhere, including the water that they drink from. The result of this is a lot of extra nitrogen, which plants use to grow, and grow they do. In abundance. Ever stuck your hand in the water to touch or grab that silky, feathery green stuff and come out covered in gunk and slime? That’s a product of eutrophication. What happens because of it? Algal blooms can cover much of the surface of the water and limit how much oxygen gets to the rest of the pond. When this happens, organisms that need oxygen to survive, like fish, die. These events are suitably called “fish kills”. This also makes eutrophied areas stink. They can also cause toxic blooms, which can affect humans directly. What can we do? The most obvious solution is to limit pollution and runoff from farming, but another option is introducing organisms like shellfish to the eutrophied areas. Shellfish remove nitrogen from the water, and they feed by filtering the water and consuming what they find in it. This can improve overall water quality. There are also technological solutions like the PondBot, which can help clear affected areas.

How it Works

The authors built artificial habitats, ten which mimicked eutrophied areas and ten which mimicked naturally-vegetated areas, and then allowed the spined sticklebacks (Gasterosteus aculeatus) to populate the different habitats. After fish had started to advertise and defend their territories, the authors waited two days before collecting males to measure their condition. They waited because males only accept eggs laid in their territory for two days before they cease their defences and start taking care of the eggs. The authors then measured different aspects of male condition, habitat choice, and reproductive success, explained below:

Male condition – the authors used “Fulton’s condition factor”, which is the weight divided by the standard length. The higher the number, the higher the male’s quality. They also measured how many parasites a male had, with a lower number of parasites indicating a higher quality. Male habitat – Where the male was caught, either dense vegetation (artificial eutrophication habitat) or sparse vegetation (natural vegetation habitat). Reproductive success – The authors quantified how successful a male was by counting the number of eggs deposited in the male’s territory. The more eggs deposited, the more successful that male was.

What they found out

Most males chose to nest in the dense (eutrophied) habitats, though there was no difference in male size between the two habitat types. Interestingly, males nesting in dense habitats did have more parasites than those in the sparse habitats. This could be due to parastized males choosing the dense habitats, or it could be the result of parasitized males in the sparse habitats being more likely to be eaten by predators than their parasite-free counterparts. Another possibility is that because males cannot see one another as well in dense vegetation, the weaker and more heavily parasitized males were not “kicked out” by the stronger males.

Regardless of the reason for this increased proportion of parasitized males in the dense habitat, they had more eggs on their nests than their healthier counterparts. This is the exact opposite of what we would normally see in a non-eutrophied habitat, as female sticklebacks are choosy and prefer high-quality mates to sire their offspring. This is most likely due to parasitized males being more likely to show off than healthier males, at least in the short term, and the poor visibility made it hard for females to see just how bad of a choice they made.

Any Problems?

Nothing huge, the only thing to consider is that this study was a measure of how eutrophication affected the reproductive rate of healthy and parasitized males. While measuring how many eggs the males had collected in territory is a good way to approximate how successful they were, the authors didn’t actually measure the quality of the offspring after they hatched. It could be the case that the parasitized males had more eggs, but they were of a lower quality.

So What?

Eutrophication is a huge problem in natural systems, but its effects are not always as direct as killing everything in the water when the algae grows out of control. This study showed that the poor visibility caused by denser vegetation allowed poor-quality males to sire more offspring than healthy males. In the long term, this could result in weaker fish as the more heavily parasitized males out-reproduce the healthier males in polluted waters. This will not only harm the fish species, but it could also have an impact on both trophy and commercial fishing. Imagine that the only fish we can catch are smaller, sicker, and full of parasites due to pollution of the natural systems.