The significance of sharp falls in the numbers of freshwater fish, shellfish and other aquatic animals in recent centuries has been frequently overlooked by natural resource managers worldwide, including Australia, according to a study of historical records by a Charles Sturt University (CSU) researcher and his US colleague.

 

Fish biologist and water management lecturer with CSU’s School of Environmental Sciences, Dr Paul Humphries, together with Mr Kirk Winemiller from Texas A&M University in USA, argue that as a result of this neglect of historical records, water catchment planning and river restoration is often built on estimates of original numbers of fish and freshwater mussels that are much lower than past population numbers.

 

“Therefore, planners underestimate the likely far-reaching effects these animals had on their watery ecosystems before Europeans arrived,” says Dr Humphries, who is also a researcher with the CSU Institute for Land, Water and Society.

 

Although precise historical numbers cannot be known, written accounts dating from the 1600s in USA and 1800s in Australia suggest that abundances were much greater than they are today.

 

“Travellers and diarists from these early days of European settlement reported rivers so full of fish that a spear thrown into the water only rarely missed one, salmon runs that spanned the whole width of a river, and fish so plentiful that they were used as pig feed,” Dr Humphries says.

 

Dr Humphries and Mr Winemiller point out that European settlers in Australia and North America could easily move inland from coastal communities to supplement seafood with food taken from freshwater lakes and streams. Within a few decades, they started building weirs and mills that hindered fish migration and put further pressure on populations. Stocks of fish and shellfish declined rapidly soon after. The effects of this early loss of wildlife on the river ecosystems, the authors contend, has not been adequately considered.

 

To add further weight to their argument, relatively unexploited freshwater systems seem to confirm the strong effects of fishing pressure in exploited freshwater ecosystems. “For example, fish are far more abundant in rivers that feed into the Lake Eyre Basin in central Australia compared to fish stocks in the Murray and Darling rivers which are far more heavily exploited,” Dr Humphries explains.

 

“To help return some balance to our exploited rivers, we need to consider more fully the role of top predators in river ecosystems, such as the Murray Cod in Australia.

 

“We also urge the creation of freshwater protected areas. Some of these protected areas could be used for restoration experiments in which the effects of reintroduced species could be explored.”

 

The research was published in the September 2009 issue of the prestigious international science journal, BioScience.