Why are so many freshwater fish threatened with extinction?

Our local plants and animals help define who we are as a culture and make our cities and towns unique. For instance, one of us was recently working in Fiji. Locals were extremely proud to talk about how they were protecting the Lekutu goby (a stream fish, Redigobius lekutu), which only occurs in a few streams on the island of Vanua Levu.

Fiji ten dollar note
The local Gobys are so important in Fijian Culture that one endemic species (Redigobius leveri) is depicted on their $10 dollar note.
Photo Courtesy of: Dr Chris Brown









So it is alarming that we are losing freshwater fish species faster than we figure out why they are going extinct. Over 57 North American freshwater fish have gone extinct since 1889.  A further 37% of freshwater fish that have been assessed by the International Union for the Conservation of Nature are also classified as being under threat. Whereas, in the oceans only about three fish species have gone extinct over the last century (though the exact number is controversial).

In our own home town of Brisbane, we are sad to have lost one species already (Maccullochella sp., the Brisbane River cod which was likely endemic to this catchment) and the iconic Australian lungfish (Neoceratodus forsteri, endemic to south-eastern Queensland) is now severely threatened.

Why are so many freshwater fish threatened with extinction?

Undoubtedly humans have a role. Many of our activities put fishes under pressure. In fact, there are so many pressures, figuring out what causes extinctions is a challenging task.

We introduce fish species to places they are not meant to be. For instance, tilapia and carp have been deliberately released and/or escaped from ornamental ponds and can eat native fish and compete with them for food and space.

We also block of rivers for dams, preventing fish from migrating to their spawning grounds. By regulating rivers so we can use the water for hydropower and agriculture and to protect our infrastructure we change the natural cycle of river flows. Fish typically depend on these natural cycles to cue their migrations up and down rivers to their feeding or spawning grounds.  Sometimes the rivers just dry up, not many fish can survive in a dry river.

Farming, industry and households also pollute the rivers. Pollution can make native fish more susceptible to disease and lead to fish kills due to poor water

Many freshwater fish are popular food. The extinct Brisbane River cod was popular for fishing – it could grow more than  1 metre in length and was reportedly good eating.

Finally, if fish populations get really small, they can become inbred, which contributes to their demise.

Knowing why these fish species go extinct is important. If we know the causes, we can better manage our rivers and lakes to stop further species going extinct.

If you were a detective trying to crack the case of a fish extinction you would have a tough job.

Brisbane is a city with three universities and a long history of scientific research and even here, the exact cause of the Brisbane River cod extinction is unknown. Over-fishing, habitat loss and pollution all likely contributed to its demise (Pusey et al 2004).  Similarly, Australian lungfish are threatened by habitat alteration and loss (e.g. due to dams and weirs), introduced fish species and other factors that have caused declines in reproduction and recruitment success (Arthington 2009, Hughes et al 2015).

The problem is that with so many potential causes it is difficult to attribute extinction to any one cause.

In fact, multiple human activities can interact with each other to make extinction more likely. For instance, fishing usually targets the largest cod that also happen to be those that produce the most eggs.

Conservation scientists, like ourselves, are increasingly trying to understand how the interplay among different human pressures causes extinction. Understanding this interplay can help us inform managers about how they can best save threatened fish species.

Informed management like this can help us slow down the alarming pace of freshwater fish extinctions.

Blog Courtesy Of:
Dr Chris Brown and Associate Professor Mark Kennard  
Griffith University and Water Future Australia

Please note that the ideas, thoughts, information or views expressed within this blog are those of the authors. and therefore do not necessarily reflect the opinion of Water Future.