Health of rivers the best in a decade

The variety of fish in the Swan and Canning rivers increased significantly last year, prompting scientists to announce that the waterway is in the best ecological health for a decade.

The Murdoch University report on fish communities revealed a "notable increase" in species living close to riverbanks, up from 29 to 35 last year.

It also found 21 species in the upper reaches of the Swan, up seven from 2012. Although the rivers continue to suffer water quality problems, including high levels of nutrients and low oxygen, the report on the system's ecological health showed a gradual but sustained improvement over the past 10 years. "A higher diversity of fish species is one indication of an estuary in relatively good condition," author Chris Hallett, from the Centre for Fish and Fisheries Research, said.

Overall, the estuary improved slightly and was again rated in good to fair condition. The near-shore waters scored highest and the Canning estuary the lowest, because of an algal bloom that killed about 70 fish.

The results came amid other promising signs, including for the river's much-loved dolphins and western school prawns.

Long-time prawners said last summer was the best season in decades, coinciding with the conclusion of a three-year restocking project to rejuvenate the historically popular pastime.

Although the two can't yet be definitively linked, the Swan River Trust said the program had exceeded expectations and it would seek funding to continue it.

Researchers, including from the Challenger Institute and Murdoch, put almost two million prawns into the river this season, doubling early targets and taking the total to more than 2.6 million.

Will Smithwick, who watched numbers fall dramatically over 30 years fishing with friend Kevin Reid, said the prawns were already "more prolific than in ages and ages".

The pair, who helped gather brood stock for the trial, hoped it would give future generations a chance to enjoy the tradition.

Mr Smithwick said he had also noticed other signs of new life in the rivers over the past few years, including seahorses and the return of big cobbler.

There was also encouraging news for the river's dolphins, with the birth of two calves since January.

Although there is a high mortality rate - a third calf recently died - Murdoch researcher Delphine Chabanne said the births were "a positive sign" and a further indication that the population had recovered from six deaths in 2009.

Dr Hallett said the positive fish results reflected high, stable salinity in the river because of a lack of rain, higher oxygen levels and the absence of major algal blooms.

"There is quite a bit of assorted evidence that the mid-2000s were a pretty stressful time for the estuary and that the situation has improved in some ways since then," he said.

"Since then, the system has not experienced such severe or widespread algal blooms, possibly related to our drying climate."

Dr Hallett said there was growing evidence the health of the rivers was closely tied to the amount and timing of rainfall. Algal blooms were associated with summer and autumn storms.

Heavy falls could cause stratification, where fresh water sits on top of denser salt water, preventing oxygen from circulating and fuelling blooms and fish kills.

Low rainfall also meant the Swan-Canning estuary had salt water further up the rivers and for longer periods that Dr Hallett said was likely to continue.

The rivers were continuing to deteriorate in a range of areas, including rising salinity and temperatures, low oxygen, high nutrients, turbidity and fish deaths, according to an Auditor-General's report last year.

It concluded that the waterway was not adequately protected and while improving its health was achievable, it required a more concerted effort from State Government agencies and the community.

Swan River Trust principal scientist Kerry Trayler said the fish index results were good news but that did not mean everything was "hunky dory".

The upper reaches of both rivers were under most pressure, with long-term declines in dissolved oxygen that were mitigated with five oxygenation plants at vulnerable sites.

Nutrients, particularly nitrogen and phosphorus, also continued to enter the waterway in much larger than acceptable amounts.

Reducing those inflows remained the Trust's focus, Dr Trayler said.

As well as reducing fertiliser use in the catchment and slowing flows from urban drains, including by installing nutrient- stripping wetlands, Dr Trayler said it was vital to focus people on the river.

"In Perth, when you talk about prawning, people get very excited. It goes to the heart of community values and a lot of people have that historical connection," she said.

"We know from programs like Dolphin Watch, that when people are engaged with something, they are more inclined to want to protect it.

"One of the critical things to reducing nutrients is to get people to make the connection with the fertiliser they put on their lawn or the detergent they use to wash their car.

"You can't just blame it on the farmers. It's about getting that message across - that if you value the prawns or the dolphins, think about those things."

If you value the prawns or the dolphins, think about those things. " Swan River Trust principal scientist Kerry Trayler