Crowded eels

In my last post, I mentioned some of the big issues facing eel populations around the world: habitat fragmentation, overexploitation, climate change. We’re constantly gaining information on how these problems impact eels, and we’re also getting a better sense of how to mitigate some of the challenges. Once again, I’m largely looking at those eel species that use both fresh and salt water habitats.

In 2004 the Octoraro Dam in Maryland was removed- photo courtesy of USFWS

In 2004 the Octoraro Dam in Maryland was removed- photo courtesy of USFWS

Starting with habitat fragmentation, we know that eel populations respond to improved habitat and we also know that, above a certain density, too many eels are not helpful. When a protected area of coastal marsh in France was compared with a fished area, the protected area had more than three times the number of silver (mature) eels (Cucherousset et al. 2007), so, if we create suitable habitat, the eels will use it. We could remove dams on rivers that are no longer being used and work on cleaning up pollution. But that can only get us so far- a second study in France found that glass (young) eels returning to brackish and fresh waters from the open ocean exceeded the carrying capacity of the study area, meaning that they hit a point where they were competing with each other and demonstrating a skewed sex-ratio (Acou et al. 2011). So, if we have suitable habitat, we can’t keep adding to the eel population indefinitely. Why is this information helpful? It means that we could restore areas currently unused by eels and then move ‘excess’ eels from locations that have reached their carrying capacity. And since all of the eels from different locations head to the open ocean to meet up for spawning, we’re not actually removing the translocated eels from the original breeding population.

Moving on to overexploitation, this one presents some larger challenges. I’ve learned quite a bit about how they have succeeded in getting eels to breed in captivity, and the information doesn’t make me feel particularly good. It turns out that eels need encouragement in captivity to be female- when population densities are high, as they are in aquaculture, eels (which essentially start life without being one sex or the other) mature into males (Okamura et al. 2014). If you want them to become female, you have to feed them hormones. You then need to inject both sexes with different hormones to make them become sexually mature and produce eggs and sperm. And successfully raising larvae in captivity has, at times, used the eggs of an endangered shark for food. You can understand why the process is expensive, time-consuming, and not exactly positive for fish conservation in general.

As for climate change, we know it’s having and will continue to have an impact– a study out of Norway suggested that warmer waters in the spawning areas mean less food for larvae (Durif et al. 2010). We already have cyclical warming and cooling of ocean waters (for example, El Nino), but warmer waters as a result of climate change would compound the issue in certain years. And changes in ocean currents could push larvae into new areas. So we need to recognize that there are changes coming and try to make sure suitable habitat is available in a wide variety of areas so that, when eels go looking for the habitat they need, they can find it.

This feels kind of doom-and-gloom as I read through this post, but I feel like there must still be ways that we can, in fact, support eel populations. So I’m going to go looking for our options in my final post of the month.

Works cited:

Acou, A., E. Rivot, J. A. Van Gils, A. Legault, F. Ysnel, and E. Feunteun. 2011. Habitat carrying capacity is reached for the European eel in a small coastal catchment: evidence and implications for managing eel stocks. Freshwater Biology 56:952–968.

Cucherousset, J., J.-M. Paillisson, A. Carpentier, V. Thoby, J.-P. Damien, and M.-C. Eybert. 2007. Freshwater protected areas: An effective measure to reconcile conservation and exploitation of the threatened European eels (Anguilla anguilla)? Ecology of Freshwater Fish 16:528–538.

Durif, C. M. F., J. Gjøsæter, and L. A. Vøllestad. 2010. Influence of oceanic factors on Anguilla anguilla (L.) over the twentieth century in coastal habitats of the Skagerrak, southern Norway. Proceedings of the Royal Society of London B: Biological Sciences rspb20101547.

Okamura, A., N. Horie, N. Mikawa, Y. Yamada, and K. Tsukamoto. 2014. Recent advances in artificial production of glass eels for conservation of anguillid eel populations. Ecology of Freshwater Fish 23:95–110.