The Perth metropolitan area is influenced in almost every aspect by the Swan and Canning Rivers: the layout of the city, transport on and along the waterway, for recreational activities and for the wildlife of the Swan Coastal Plain.

As people live alongside these waters, the population has impacted the health and destiny of the rivers, and management and conservation of the waterway has become more complicated. Proof of the positive and negative impacts that have come with a growing city are evident in the wildlife of the river ecosystem, such as blooms of Alexandrium, an algae present in the Swan River since 2019, that poses health risks to people and wildlife alike.

The most recent Alexandrium bloom was in February 2023, creating the risk of Paralytic Shellfish Toxins that can cause sickness and even death in humans if consumed, and to which there is no antidote. But the effects of these toxins are not only limited to people.

“The toxins produced by Alexandrium impact other mammals as well, and indeed also birds and possibly fish,” Department of Biodiversity, Conservation and Attractions acting senior environmental officer and planktologist Jeff Cosgrove says.

This could pose a threat to Perth’s osprey population, which have already been suffering from the ongoing increase in global temperatures brought on by climate change. These birds of prey can be seen in trees and on light posts surveilling the waterways for fish, the sole prey item in their diet. For a body of water to host a population of ospreys is a key marker of the biodiversity and health of the waterway.

BirdLife WA volunteer osprey tracker, Markus Singor has been tracking ospreys in the Swan/Canning river system for 13 years. He said that recent breeding seasons have seen contrasting success rates: “In 2021 we had those weeks of 40-degree heat … that basically wipes out all the young ospreys around Perth because they just got fried up there. Some were just jumping out of their nests to escape the heat.”

The 2020 breeding season was a successful year for Perth’s ospreys due to a mild summer, to which Mr Singor attributed the birds’ success. In the years following however, the increased heatwaves experienced over the summer has begun to heavily impact the nestlings.

The exact population count of ospreys along the Swan River is unknown, however in 2020, 17 nests were occupied by breeding pairs, putting the estimated population at around 50 individuals.

In the 2021 season, 12 juvenile ospreys hatched in six nest sites along the Swan River, but only six fledgelings survived the heatwave that arrived in late December. In the 2022 season, 15 nestlings successfully fledged at nine nesting sites with a loss of three nestlings. Successful nestlings disperse across Westen Australia, with some juveniles banded in Perth being recovered as far away as Lancelin and Albany.

Alongside the effects of climate change hazardous algal blooms in the river system can have far reaching impacts on wildlife that rely on the Swan River.

Dr Cosgrove said that the smaller animals that consume Alexandrium then carry this up the food chain, and when larger animals consume a high enough amount of the infected prey items, this can cause sickness or death, just as is possible with humans or other mammals that come into contact with the algae.

The DBCA tracks all aspects of river health, including algae concentration levels, and recognises that the growth of the metropolitan area, like climate change, is impacting the resilience of the river system, and changing the ecosystems within.

DBCA acting principal scientist Peter Novak says that river health is overall looking fairly positive: “Some of the indicators are tracking pretty well. We still have a really diverse fish community, the fishermen index, which has been done for the last 10 years has shown a consistent pattern in the fish. It hasn’t improved markedly, but it hasn’t gotten worse, and I think that’s consistent with a lot of other indicators as well.”

Through initiatives like the fisherman index, a community reporting system used to gauge the populations of different species of fish, frequent water testing in specific areas of the catchment to find levels of plankton, algae, and chemicals present in the water, scientists are tracking the overall health of the waterway. This then informs what protocols must be put in place to lower the risk of further deteriorating river health and limiting algal blooms.

Dr Cosgrove says, “we’ve identified, for example, the Ellenbrook system as being a major contributor of nutrients to the system, so the Ellenbrook artificial wetlands have been put in at the end of the drainage system to try and strip out the nutrients going in there.”

Built in 2014 the Ellenbrook artificial wetlands is WA’s largest man-made wetlands, a $4 million project that converted part of the Ellen Brook tributary into a catchment area that would filter phosphorous and nitrogen, the two biggest pollutants found in the Swan River, from the tributary water.

“We’ve been doing all of this, but it becomes a little bit hard to disentangle how effective some of these things when you overlay climate change, loss of rainfall, and streamflow into that equation.”

Dr Cosgrove said that in the future, a less healthy rainfall pattern will have an impact on the river system, and that by using other methods to upkeep the health of the waterway, a resilience can be built up to help deal with the changes in rain patterns.

Lower rainfall results in less flushing events for the system, creating concentrations of nutrients in the rivers and allowing better conditions for algae and bacteria to grow.

Friends of Attadale Foreshore Inc. chair Mike Nichols suspects that the algae levels could also be influenced by the damming of the upper reaches of the waterway: “When I was a kid, the Swan River was brown for six or seven months of the year, but because we are building more dams and our rainfall is falling off, the brown water is not coming down the river.”

The Alfred Cove wetland has been developed and maintained by the Friends of Attadale Foreshore, a community group that has focused on rehabilitating the Alfred Cove area into a conservation site. This wetland has been recognised by Ramsar, an organisation that develops and protects wetlands under the international Convention on Wetlands, ratified in 1971.

Mr Nichols says, “We have 30 to 40 species in the Alfred Cove area, recently we had roughly 100 cormorants, we have dozens of swans, pelicans, so you’ll see a lot of bird life on the sandbanks that are out there. We have birds that come in from Alaska in September, it’s about a 12,000km flight.”

Mr Nichols has helped to create more successful nesting options for ospreys around Perth through his conservation efforts in Alfred Cove. The conservation efforts have introduced new nesting infrastructure for ospreys, utilising disused crayfishing pots as a structure that better supports nest building than the flat boards that have been used in the past.

“We found that once we got winds over about 80 kilometres per hour it blew the nests off the platforms that we had. I travelled right up through north of the Abrolhos Islands quite a few times and found that the old cane cray pot was well often used by nesting birds,” he says.

He said that in his youth there were five ospreys in Alfred Cove, but the population dropped as low as two. In the last breeding season, there were four ospreys present in the wetlands. The breeding pair had made use of the modified cray pot, a success that has been shared within the conservation community.

In spite of these successes climate change and human impacts continue to create challenges for the DBCA to understand and manage. Algae thrives in low rainfall conditions, and blooms are fed by phosphates found in garden fertilizers when the chemicals enter the river system.

In an effort to reduce the amount of phosphates entering the catchment, DBCA recommends growing native gardens: “People in the Swan Canning catchment can really help out by having gardens that have plants that are native to the Swan coastal sand plain that can survive and thrive in low nutrient soft soils, and then when they use fertilizer, if they use slow-release fertilizers.

“A lot of people think that by using organic fertilizers, such as manure, they’re helping out, but it’s fast release fertilizer. Because we’ve got such sandy soils, those nutrients just pass through the soil really quickly before the plants have a chance to uptake the nutrients, and it just ends up in the groundwater which flows into the Swan Canning.”

Other pollutants, such as fishing and crabbing equipment has been a significant issue for the health of the animals that rely on the Swan River.

Mr Singor said that fishing line can pose a threat to ospreys: “if someone catches a fish and just cuts the fishing line, they leave a hook in the mouth of the fish. If an osprey catches the fish and starts eating it, they can get the fishhook stuck in their beak, and they can’t open their beak and they’ll starve or die because of it.”

The fishing waste issue became apparent in 2013, when a dolphin was seen struggling after becoming entangled in fishing line. The River Guardians is a DBCA program that engages with the community on matter regarding the Swan River, through education programs, awareness campaigns and Dolphin Watch. It started the Reel It In campaign in an effort to tackle fishing waste.

The campaign launched in 2013, and has put 74 bins on popular jetties, bridges, foreshores, and fishing platforms along the Swan River. This campaign has been successful, with 190km of fishing line, 22,000 hooks and sinkers, and 15,000 bait bags being disposed of in these bins since its inception. The campaign has spread through Western Australia, with Mandurah and Walpole being two examples of towns that have installed the Reel It In Bins.

The management and conservation of the Swan River by the DBCA and community initiatives points to a significant effort by the people of Perth to upkeep the health of the waterway.

Some initiatives such as the Reel It In campaign have clearly calculable effects on the health of the river. Other initiatives, such as attempts to slow phosphates and nitrates from entering the waterway through artificial filtering systems, and awareness campaigns regarding fertilisers seem to be limited in the effectiveness when Alexandrium and other algae blooms arise.

However, significant ecological markers provide conservationists with a way to gauge how effective a program or initiative is. Dolphins and ospreys are two such markers, and tracking their respective populations can provide clear information to how effective the Swan River management efforts are.