Size matters: extinction risk in small-bodied freshwater fishes

Sabanejewia romanica (Photo by Gerhard Ott)

Sabanejewia romanica, a near-threatened small-bodied freshwater fish from the Danube River Basin, camouflaged in its typical habitat, sand (Gerhard Ott)

By Casey Shaw, Honours Student, Charles Sturt University

Generally, for marine fishes, it is larger species that are at most risk of extinction. This is because large species tend to produce fewer eggs and have longer generation times than small species, which results in low population turnover and recovery times if populations are disturbed. This is also the case for marine mammals and birds. But for freshwater fishes, the opposite is true: small-bodied freshwater fishes are at the same or greater risk of extinction as their larger-bodied counterparts.

It has been thought that high reproductive rates and early maturation of small-bodied vertebrates buffer these species from extinction risk, as they can recover more quickly from reductions in population. But this does not seem to be the case for small-bodied freshwater fishes. And this group is among the most imperiled of all faunas worldwide, with 37% at risk of extinction! Looking at ‘extinction prone’ traits may shed light on why it is small-bodied species that seem to be the most at risk

I recently undertook my Honours, investigating the reasons why small-bodied species of freshwater fish are more vulnerable to extinction than large-bodied freshwater fish. The aims of my Honours study were to: 1) determine whether functional traits (traits like numbers of eggs, size at maturity, reproductive behaviour) can be used to predict the threat status of small-bodied freshwater fishes; and 2) to evaluate whether the functional traits of threatened small-bodied fishes are unusual compared to other vertebrate groups.

Twenty-one functional traits associated with reproduction, age and growth, habitat and the environment in which fish live, together with IUCN (International Union for Conservation of Nature) threat status, were collated for all fishes smaller than 12 cm from four river basins: the Murray-Darling, Australia; the Danube, Europe; the Mississippi, United States of America; and the Rio Grande, Mexico. A generalised linear mixed model was used to determine whether functional traits could be used to predict the threat status of a total of 204 small-bodied species from these basins. Seventy percent of species were used to construct a predictive model, and 30% used to test it. The best model identified the presence of parental care (one or both adults looking after the eggs and/or young in a nest) and preference for fine particle size substrate as the best predictors of threatened status. Other models included small maximum lengths and the production of live young (some fishes actually give birth to live young, and have internal fertilisation, rather than expel eggs and have external fertilisation) as important traits.

Providing parental care may put small-bodied freshwater fishes at greater risk, because of the vulnerability of the parents to predation and other threats. And during the caring process, if the parent dies, so do the young. So there is a double loss. Producing live young is thought to increase vulnerability of small-bodied freshwater fishes for the same reasons. I speculate that the finding that fine substrates increase vulnerability of small-bodied freshwater fishes to extinction indicates more of a general effect of habitat degradation. Small species are often associated with particular habitats, for refuge or spawning. But it was interesting that habitat did not show more of an effect on extinction risk, as I originally thought it might. Finally, the smaller the small-bodied freshwater fish, the more likely they were found to be at risk of extinction, which probably reflects that small fish are more at risk than their larger counterparts from a whole range of factors associated with human-induced change. While this results does not tell us specifically what it is about ‘smallness’ that is inherently risky, it does confirm the unusual nature of small-bodied freshwater fishes relative to other vertebrates.


The live-bearing mosquitofish, Gambusia holbrooki, an alien species occurring in the Murray-Darling Basin (Mitch Meller)

I also found that functional traits can be used to predict the threat status of small-bodied freshwater fishes in these basins, with 76% accuracy (correctly predicting the status of 86% threatened and 73% non-threatened species).

The collection of data for my study highlighted a real lack of functional trait information for small-bodied freshwater fish species. To aid conservation in the future, I would encourage researchers to focus on the collection of consistent, comprehensive functional trait information for small-bodied freshwater fish species. Information from both highly degraded and non-degraded, species-rich ecosystems is needed. Unfortunately, often we know relatively little about the traits of species in a river basin until degrading processes have been underway for a while.

Useful references

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