The life of Leeuwin
A steady stream of warm water f lowing from the tropics shapes the coastal f lora and fauna of Western Australia.
Warm water flowing from the tropics shapes the coastal flora and fauna of WA.
THE OCEAN BENEATH Busselton Jetty, a 220km drive south of Perth, ripples with life. Vibrantly coloured nudibranchs, sponges, bryozoans, ascidians and other invertebrates cover every available centimetre of wood piling, creating a submerged forest-like habitat that extends up to 8m under water and teems with fish. Despite Busselton being well within temperate latitudes, many of the more than 300 different marine species found at the jetty are tropical. These include stunning soft corals that are mostly found much further north in deeper water. This astounding assemblage, and the reason so many tropical species survive here, is due to the Leeuwin Current.
The Leeuwin is one of four major currents inf luencing life and climate in Australia. It starts as a great f low of warm water that pushes south from Indonesia along the edge of Western Australia’s continental shelf, sending huge swirling eddies into coastal waters and out to sea.
This remarkable force of nature then continues all the way down the west coast, before rounding Cape Leeuwin and continuing into the Great Australian Bight, its inf luence extending as far as Tasmania.
About 60,000 visitors f lock to the jetty each year, largely for the spectacular array of life it supports and that can be viewed through the glass windows of the site’s underwater observatory, one of only f ive in the world. The current manager of this facility, Sophie Teede, can recall being overwhelmed as a little girl by the marine life at the site. “I grew up in the goldfields and have fond holiday memories of riding the little train along the jetty in a carriage with my two brothers,” she says. “To end up working here doing marine science is something I’m very passionate about.”
Visitors to the observatory descend a dramatic stairway that passes through four different viewing levels, eventually arriving at the sea f loor. At each level there are large acrylic windows through which visitors can see the jetty pilings colonised by a lush, living veneer of soft corals and invertebrates of every colour of the rainbow. It’s a potent reminder of the power of the Leeuwin Current.
IT WAS MORE THAN a century ago that a study of marine life off the Houtman Abrolhos Islands, west of Geraldton, suggested to English-born marine biologist William Saville-Kent that there was probably a warm southward-f lowing current off WA. He documented the warm waters and tropical marine species around the islands but his theory wasn’t conf irmed until the 1970s, when a CSIRO study using drifting buoys and satellite images finally proved the current’s existence. Two of the researchers involved in that study, George Cresswell and Terry Golding, named the current after a Dutch merchant ship that explored Australia’s south-west coast in 1622 called Leeuwin, the Dutch word for lioness.
Just as the famed East Australian Current (EAC) impacts reefs – both tropical and temperate – off eastern Australia, the warming Leeuwin Current affects the range of species and reefs off the west coast.
Thomas Wernberg, a marine ecologist with the University of WA’s Oceans Institute, has been studying kelp and other seaweeds, the keystone species, in reefs south of Kalbarri in the Leeuwin’s path.
He’s worked on WA’s kelp forests since moving from Denmark to Perth to complete a PhD on them 20 years ago. “I was always into seaweed,” says Thomas, who grew up beside the Baltic Sea. “My friends would spend their weekends birdwatching, but I would always spend mine seaweed watching.”
His research explores how coastal habitats respond to human-induced stressors such as climate change. He is particularly interested in two things: the way WA’s temperate reefs are changing under the inf luence of a Leeuwin Current that’s set to get warmer, and how tolerant the kelp is to sudden temperature spikes.
Marine heatwaves are extreme ocean temperature events that have been increasing on a global scale in both severity and duration during the past 100 years, with an increase in frequency since the 1980s. The consequences of this trend were made clear by mass coral bleaching on the Great Barrier Reef in 2016 and 2017 (AG 142) and the recent loss of giant kelp along Tasmania’s east coast (AG 139), events that were inf luenced by a warmer-thanusual EAC.
“We had an extreme and unprecedented event off the west coast in 2011 that resulted in severe impacts on our temperate reefs where we lost extensive forests of common kelp and other species of seaweeds,” Thomas says.
“We had an extreme and unprecedented event off the west coast in 2011...we lost extensive forests of common kelp”
We also saw unusual occurences of tropical species, such as whale sharks off Albany, and had a massive recruitment of tropical herbivorous rabbitf ish, which continue to graze back kelp at several locations.”
A key impact of that 2011 event was the disappearance of common kelp from about 100km of coast south from Kalbarri. It is yet to recover.
There were also severe impacts on other environmental, social and economic values of the region, noticeably negative f low-on effects to the economically important western rock lobster and abalone f isheries, which contribute $500 million and $8 million to the state annually.
“All of the predictions suggest there are going to be more of these events, and so it is just a matter of time before the next one comes along,” Thomas warns. “There has always been natural variation, but as the average temperature increases so does the maximum of the extremes, and the spikes just keep getting warmer and warmer.” An increase in mean water temperature translates into an increase in frequency and severity of marine heatwaves.
To better understand the impact on kelp, Thomas and his team are using large experimental devices – each about 4sq.m – called growth rings. These hang suspended 5m off the sea f loor, anchored to the bottom and pulled upwards in mid-water by a large f loat. “A big PVC frame sits in the middle, with lines spreading out laterally where we can attach 50–100 kelp plants representing an entire kelp population allowing, for the f irst time, population-level f ield experiments on marine heatwaves,” Thomas explains.
Kelp collected from cool water as far south as Hamelin Bay is translocated to warmer water near Kalbarri to test how it handles the stress. It’s then taken back and its recovery documented in a methodology that allows the team to replicate the effects of a marine heatwave through experimental manipulation.
“We have different collection and depositing points,” Thomas says. “We have growth rings off Perth and Kalbarri, and are planning to deploy one soon off Jurien Bay [north of Perth]. It’s perfect because there are different increments of temperatures between those locations and so the temperature gradient off the coast makes for a perfect natural underwater laboratory.
“We are also investigating what genetic changes occur when you grow kelp in a warm location for a couple
of generations,” Thomas says, explaining a component of the study with a longer-term research objective.
“We are basically investigating kelp’s adaptation to warmer waters and, down the track, we’ll use this to inform restoration plans and explore options to increase its resilience through, for example, selective breeding.”
A shortcoming for most temperate reefs globally is that not many have been the subject of a monitoring series longer than about 10 years, and Thomas hopes to address that. Having monitored the same locations since completing his PhD, his sites are now becoming longterm monitoring locations.
“So, as long as I can keep doing what I am doing, I will keep returning to those sites,” Thomas says. “By the time I am done with this work, hopefully in many years from now, there will be a long-term dataset to indicate decadal change.”
NEAR THE EDGE OF the continental shelf off Geraldton, the waters around the Houtman Abrolhos islands support the most southerly true reef-building corals in the Indian Ocean, partly as a result of the Leeuwin Current.
Due to their latitude, these waters also provide habitat for cooler water species including the Australian sea lion, once over hunted for its fur and now listed as vulnerable. It means there’s an eclectic mix of tropical and temperate species around the islands.
The area is a hot spot for the western rock lobster fishery, which is acknowledged as one of the best managed and most sustainable in the world. The f ishery occurs between Shark Bay and Cape Leeuwin, uses baited traps, involves about 280 boats and has recently moved to a quota-managed framework.
Lobster larvae numbers are affected by changes in environmental conditions such as the rate of f low of the Leeuwin Current, and frequency and intensity of lowpressure systems generating westerly winds.
To test productivity and help plan future management, the WA Department of Fisheries collects late-stage rock lobster larvae, called puerulus, at several sites where they settle off the west coast. The data contribute to setting annual caps for f ishing, which helps maintain breeding-stock levels.
El Niño conditions have been associated with belowaverage puerulus settlement. When the El Niño effect is strong, the result is usually a weaker southern f low of the Leeuwin Current and historically the settlement has also been poorer. The easterly winds across the Pacific Ocean vary over time, being stronger in La Niña years and weaker when under an El Niño inf luence.
The changing wind strength alters the amount of water piled up in the west of the Pacif ic and, therefore, the sea level near Indonesia – higher during La Niña years and lower for El Niño. The changing sea level, in turn, inf luences the strength of the Leeuwin Current. The strongest f low is during La Niña years, which results in higher puerulus settlement.
ANOTHER LOCATION WHERE the inf luence of the Leeuwin can be seen is around Rottnest Island. The Leeuwin f lows gently around the eastern (mainland) side of Rottnest but frequently bathes the western and south-western sides, inf luencing the f lora and fauna there. Sea temperatures in those areas in winter are several degrees higher than along the mainland coast.
As a result of the topography of the coast here, the shelter provided by Rottnest and the slower current, extensive seagrass beds grow inshore from Rottnest creating a marine ecosystem that provides shelter and food for a diverse range of species.
The area is home to some of the healthiest seagrass meadows in the Perth metropolitan area, but when such sites are disturbed they can take decades to recover.
The area is also home to more than 200 bottlenose dolphins that survive among the sheltered waters of the
bays and islands adjacent to the coastal city of Rockingham, between Warnbro Sound and Cockburn Sound.
Dolphin-swim tour guide Kameika Anderson explores these waters daily, spending time swimming with resident bottlenose dolphins.
“I grew up living in the country and during school holidays my family would travel to Shark Bay every year, so I had an early connection with dolphins,” Kameika says. “We moved to Rockingham when I was eight, and we would take the boat out here and the dolphins were everywhere and they would cruise right up next to us – you can just tell straightaway how friendly they are in the water.”
Rockingham Wild Encounters has been operating a swim-with-wild-dolphin cruise since 1989. “It’s up to the dolphins to initiate the interaction,” Kameika says. “They’re just curious and want to have a play: they are not enticed and there is no food or noise to attract them. Every encounter is on their terms.”
Sometimes the Encounters team sees unusual behaviours, such as a rarely documented hunting technique of dolphins following giant smooth stingrays in the hope of catching a fish, octopus or crab disturbed by the large rays.
“We do have a lot of seagrass meadows in the area, which is important for the health of the marine environment,” Kameika says. “The dolphins just follow the stingrays and wait to grab an easy meal.
“Most of the time when we see that behaviour it is the females with their calves and so it is the mums showing their young calves this particular feeding technique, which is even more amazing to witness. Because they have a long rostrum they can get in there before the stingray and steal their catch, so it is a pretty amazing learned behaviour.”
ASPECTACULAR RUN of Australian salmon occurs in the south-west as the Leeuwin Current begins its autumn and winter f lows. There’s been commercial salmon f ishing off the west coast since 1940, making it one of WA’s oldest commercial f isheries.
The fish begin their swim westward along the southern coast and then turn north up the coast to spawn, and the resulting larvae are distributed back south into the Bight by the Leeuwin Current.
Population f luctuations among the west coast salmon have been associated with variations in strength of the
Leeuwin Current and north-f lowing Capes Current. When the Leeuwin is f lowing strongly and warmer waters penetrate onto the continental shelf, it’s thought the f ish may migrate offshore into deeper, cooler waters rather than continuing around and up the coast.
“My grandfather started f ishing off Cape Naturaliste and always had a link to f ishing to help subsidise his farming work,” says commercial f isher Shane Miles. “Unfortunately, Gramps died in his mid-50s, much too early in life, and that’s when my dad took over the fishing business.” Shane learnt from his father and has been involved in the commercial f ishing industry for the past 35 years.
During salmon season, every morning is an early start for Shane and his crew, who meet in the dark before taking their four-wheel-drives down onto the beach.
“We are on the beach before sun-up, before we can even see through the water,” Shane says. “As soon as the sun comes up the fish start to move and so we are there in anticipation and hopefully we f ind something in the first hour to catch. Otherwise, we hang around and wait, try again to look for some more, or try another beach further along.”
These days salmon is mostly sold as bait for the commercial western rock lobster industry because there has not been a big market for human consumption, although that is changing. “Our small processing site allows us to take out the middleman,” Shane says. “We can catch it, process it and sell it to the end user, and can value-add the product. But for the last f ive years we are f inding we are providing a lot more salmon to the edible market.”
Shane seems to be content with the current status of the salmon stock.
“I’ve never seen so many fish than in the past 15 years,” he says. “But it is cyclical: we have seen the f ish in patterns...that my father saw in the ’60s and ’70s and we are actually seeing them revert back to those sorts of patterns, which is good news.”
After rounding Cape Leeuwin, the current f lows along the outer continental shelf to take an eastward passage. From here, the Leeuwin tends to move offshore again because of the distinct northwards aspect of the coastline.
Frequent reversals to westward f low occur during the summer months, and there can be an eastward current exceeding 3 knots in the core of the Leeuwin just off the continental shelf.
Population fluctuations among the west coast salmon have been associated with variations in strength of the Leeuwin...
SOUTHERN BLUEFIN TUNA (SBT) spawn off the coast of southern Java, and larvae and young tuna travel in the Leeuwin Current down Australia’s west coast, around Cape Leeuwin and then into the Great Australian Bight. By the time they reach the cape, they can be aged between a few months and a year.
They then live and grow along the southern coasts of Australia till they are three years old. The f ishery has been actively managed since the late 80s, but scientif ic data suggest the stock has not recovered signif icantly from what was then already an overfished state, although there are some recent promising signs of recovery.
Breeding is from September to April in warm waters south of Java, and juvenile fish then migrate south down Australia’s west coast.
During the summer months (December–April) they tend to congregate near the surface in coastal waters off the south coast of Australia and spend winters in deeper, temperate oceanic waters.
After they reach f ive years of age, they are seldom found in near-shore surface waters and embark on a migration that extends east and west into the Pacif ic Ocean. Adult SBT return to tropical waters south of Java annually to breed. Because SBT all breed in that same area and all look alike no matter where they’re found, they’re all managed as one breeding stock.
John Totterdell, a researcher and f ilmmaker with the Marine Information and Research Group and Cetacean Research WA, has been surveyng SBT recruitment off WA’s south coast since the early 1990s and more recently studying several cetacean species from the Kimberley south to the Recherche Archipelago.
The main transect line used for John’s SBT surveys extends from the coast out to the edge of the continental shelf and more than 16km beyond, into deep blue underwater canyons. John says it was the area around the second-last sampling station where his team tested water temperature, dissolved oxygen and salinity that became identif ied as the centre point for the high density of marine activity known as the Bremer Canyon. This includes one of the largest known aggregations of killer whales, or orcas, in the Southern Hemisphere (AG 136).
“The regularity with which we were seeing killer whales and sperm whales out there was unusual,” John says. “What was also really interesting was the scale of it. Killer whales had been sighted off the west and south
coasts on a number of occasions, but fairly randomly and at different locations. We started to get out there on either side of our tuna surveys and commenced a specif ic research focus on killer whales.
“There are now a couple of charter boats forming a new whale-watching tourism industry, and we get out there every year to continue the research.”
In 2012 John invited US-based whale expert Bob Pittman over, and he brought with him a wealth of knowledge and experience of these apex ocean predators. “He had already come to Ningaloo with me to work with the killer whales that were targeting humpback calves, and when I told him about the Bremer Canyon he was there the next season,” John says.
It’s not understood exactly how the Leeuwin Current inf luences this extraordinary assemblage of marine mammals. But when mixed with the nutrient-rich upwellings associated with deep-sea canyons, it creates a true oceanic biodiversity hot spot.
One factor under investigation in the canyon is diet. Determining the killer whales’ effect on the local offshore ecosystem is important. Deploying satellite tags gives distribution and dive prof iles, photo-identif ication aids population estimates, and biopsy analysis allows for genetic comparison and determining kinship within the group. Already, almost 200 individuals have been identif ied, but the team thinks there are probably signif icantly more. “We have good photos of about 150, another 50 that we have only seen once, and probably many more that we haven’t photographed yet,” John says.
Perhaps surprisingly, none of the individuals from the Bremer area have been recorded elsewhere. Although killer whales have been observed off Rottnest and much further north along the Ningaloo coast, none of the south coast whales have been sighted away from that offshore area. “A lot of animals f ind it [Bremer] a good place to forage,” John says.
By the time the Leeuwin Current leaves the Bremer Canyon and reaches across the Bight, it begins to dissipate and lose momentum, making a last push towards the west coast of Tasmania, before f inally petering out like a faint warm breath in a cool Southern Ocean.
Determining the killer whales’ effect on the local offshore ecosystem is important.