The Southward Stampede Pt.2
| Jock Serong
| Jock Serong
Imagine you’ve launched your tinny in the estuary of the Derwent River, somewhere in the suburbs of Hobart. It’s mid-summer but a chilly morning, so you’re rugged up. On a whim you make a slow pass under the Tasman Bridge and cut an arc towards the reefs off Bellerive Bluff, trailing a new lure for ‘couta or salmon. The rod bends double. The reel screams. After a brief but spirited fight, you tip your net onto the floor of the boat to reveal a long, bullet-shaped pelagic fish, counter-shaded dark above and silver beneath. The golden gleam of the fins is unmistakable: it’s a yellowtail kingfish. But your GPS says you’re at latitude 42 degrees south. What on earth’s going on?
Ocean temperatures are changing, all over the globe. Species are moving with the changes, trying to adapt to a new world, wrought by humans. But a range of unexpected benefits – like catching kingfish in Tassie – could be distracting us from some devastating losses.
The yellowtail kingfish is a visible illustration of changes in ocean temperatures. Marine scientist Curtis Champion worked on kingfish redistribution in the Tasman Sea as his PhD topic at the University of Tasmania and CSIRO. He describes the movement of kingfish out of their range as a long-term phenomenon, punctuated by rapid southern movements in years when ocean temperatures are particularly warm. “There’s an underlying warming trend that’s driving many marine species poleward,” he says of the Tasman. “It’s the same as in southwest WA. These redistributions are occurring rapidly at the tail end of the East Australian Current and the Leeuwin Current.”
In southeast Australia, the rate of increase in ocean temperature is three to four times the global average. We know this because satellites have made it possible to survey the temperature of the entire globe’s oceans in segments of just 20km2. The mechanism responsible for increasing southeast Australian temperatures is the increase in westerly wind velocities over the Southern Ocean, which causes a natural anti-clockwise spiral called the South Pacific Ocean Gyre, to spin faster. The East Australian Current is the western boundary of this gyre, and as it spins faster it delivers warmer water from the Coral Sea down to Tasmania.
Martha Lavinia beach on King Island is currently the site of a proposed salmon farm, but warming ocean temps could see warmer-water species like kingfish farmed there. Photo: Stu Gibson.
The vast majority of marine species are ectotherms, meaning that their body temperature is largely dependent on the ambient temperature of the surrounding water. As the ambient temperature changes, mobile species will move to track their thermal preference. For kingfish, that’s about 21 degrees Celsius. Historically, this was the summer water temperature off the Victoria/NSW border: now it’s often the temperature off eastern Tasmania in summer. A sea temperature buoy that’s been collecting data off Maria Island for 60 years indicates the local waters have warmed by about 1.4 degrees over that period. And marine heatwave events – like Kalbarri’s in the west – are becoming more frequent and intense in the Tasman Sea.
An obvious question arises: if pelagic fish can move to chase the temperatures they prefer, do their predators and prey move as well? “This is an under-researched area,” says Champion. “Few models exist, and most are specific to a single species. Preliminary work suggests the kingfish off Tasmania are capable of adapting their diet to what’s available locally, like redbait, rather than the slimy mackerel and scad they would’ve eaten off NSW.”
There’s strong anecdotal evidence that similar movements are occurring with King George whiting and pink snapper, but those species according to Champion are more site-attached than the highly mobile large pelagics like kingfish. Studies are now looking at whether those more static species are rendered more vulnerable to temperature change through effects on reproductive viability, and resilience to competition, parasites and predation. If warmer water makes a fish sluggish, it’s more likely to be picked off by another fish.
Kingfish are a useful vehicle for the discussion about sea temperature changes because they’re such a popular angling species – Champion refers to “the sociology of kingfish” – but the likelihood is that other large pelagics like dolphin fish and spotted and Spanish mackerel might also now be on the move.
The comparison between these species engaging in range-extension, and fixed species like kelp is that the latter are facing the range-contraction to which Scott Bennett referred in Part 1 of this series. “This phenomenon is happening at the poleward tips of several continents,” says Curtis Champion. “This can be seen as a global fingerprint of climate change on biodiversity.” More than 20 ocean warming hotspots have been defined globally. Where they are located disproportionately corresponds with social disadvantage, and with locations where dependence on seafood for human nutrition is greatest, such as Southeast Asia and western Africa.
Another side-effect of these redistributions is what they might mean for indigenous traditions. As Champion puts it, there are cultures that have certain species embedded in their very identities, their idea of who they are and where they came from. What happens when, after thousands of years, that animal leaves? Or another appears in its place? The decoupling of traditional societies and their totemic animals is happening already on land, as witnessed by changes to the timing of reindeer migrations in northern Europe, which used to form the basis of traditional seasonal calendars.
Species redistribution in response to temperature changes is not only a scientific discussion – it’s also an ethical one. “One aspect of all this that will be important is the ethics of warming-driven species redistribution,” according to Scott Bennett. “How should we consider and respond to new species that are moving into our local ecosystems?”
Curtis Champion is wary of the juxtaposition of good news stories about climate-driven range shifts over bad news. As he says, if the celebration of the arrival of a new species is motivated by recreational fishing interests, we’re inclined to see it as a positive. But if terrestrial mammals colonise and dominate a new area because of other pressures, we would cull them. He says, “the question of whether to persecute, protect or ignore species that are turning up in novel environments due to climate change, is at the current forefront of this field of research.”
Money is, of course, a factor. Eastern Tasmania has now gained a very valuable new fishery. “That opens a conversation about change and an opportunity for education,” says Champion. “These are the same communities who are seeing the long-spined sea urchin coming down the coast and devastating kelp forests, resulting in barren seascapes. Winter water temperatures along Tassie’s east coast are now warm enough that the urchins can complete their whole reproductive cycle in those southern waters.”
Australia has a clear biosecurity stance on ‘invasive’ species that come from overseas, according to Bennett. “Namely, prevention or eradication. But we cannot necessarily adopt the same position for species that arrive due to climate change. These species are modifying their range ‘naturally’ based on the suitability of conditions – so to try and stop them at their range ‘border’, or eradicate them, is arguably unethical and probably futile. At the same time, letting species come and establish unabated could modify habitats and threaten established species, with huge biodiversity and economic consequences. This is a wicked challenge that has not yet received much public attention.”
In measuring future trends in temperature change, there are several different scenarios, known as the Representative Concentration Pathway (RCP) scenarios. The most commonly used models are RCP2.6, (which has the lowest greenhouse gas projections), RCP4.5, RCP6 and RCP8.5 (which is the worst-case scenario). “Currently - and worryingly – we’re tracking RCP8.5 scenarios,” says Bennett, “which see rapid rates of ocean warming throughout this century. Increasingly, scientists go straight to RCP8.5 as an indication of what awaits us.”
Last year, the IPCC released its “Special Report on the Ocean and Cryosphere in a Changing Climate”, and it makes very grim reading. “Since 1993, the rate of ocean warming has more than doubled (likely). Marine heatwaves have very likely doubled in frequency since 1982 and are increasing in intensity (very high confidence).” Consider also the report’s talk of a “shrinking cryosphere” – that’s the cold bits. What will happen to the swell-exposed coastlines of the Southern Ocean when the weather systems to the south, that used to be born over the ice sheets of Antarctica, are now born over empty ocean or worse, bare rock?
But let’s turn our gaze northward for a moment to the Great Barrier Reef, which has suffered three major bleaching events in the past five years: 2016, 2017 and 2020.
The 2016 event, at the time considered the most severe on record, was most pronounced to the north of Cairns. In fact, taking a point about halfway between Cairns and Townsville, (say around Cardwell), north of there the bleaching was considered “most severe” and south of there it was negligible. In the 2017 event, the bleaching was concentrated just off Cairns and was less severe to the north and south. During this most recent event – which has now eclipsed the severity of 2016, however, almost all of it has occurred south of Cairns, down past Mackay to the reef’s southern extremity off Rockhampton. All of which boils down to this: no part of the reef is unaffected by bleaching.
Trevally, common along the Great Southern Reef. Photo: SA Rips.
This year, February had the highest monthly sea surface temperatures recorded on the Great Barrier Reef since the Bureau of Meteorology’s records began in 1900. The resultant bleaching event is the first one that’s ever occurred outside the context of an El Nino year.
Bleaching isn’t necessarily fatal for coral, and it affects some species more than others. A pale, or lightly-bleached coral typically regains its colour within a few weeks or months and survives. But when bleaching is severe, many corals die. In 2016, half of the shallow water corals died on the northern region of the Great Barrier Reef.
There’s a distinctly political flavour to the battle of ideas over how to combat ocean warming. As our government’s response has slowly turned towards a recognition of the scale of the problems, the concomitant action has been heavily tilted towards symptoms, and away from tackling the root causes. That is, ‘It’s too late. We’re stuck with this: let’s make the best of it.’
“So in one camp there is a growing effort to look for ways to restore habitats like coral reefs,” says Scott Bennett. “In the other camp are those who argue that doing restoration at the scale necessary for it to be ecologically meaningful is infeasibly expensive, with an extremely low success rate, and therefore a distraction from what we really need to be doing – cutting emissions. The media likes ‘positive action stories’ so the restoration efforts get a fair bit of air-time.” At the time of writing, Bennett’s words prove prophetic: there’s a sudden surge of media interest in a new technology called ‘marine cloud brightening’.
A pelican sentinel looks south. Photo: SA Rips.
But the other camp scoffs at restoration efforts. Professor Terry Hughes is a leader among them. As he tweeted recently, “Every time the Great Barrier Reef bleaches due to anthropogenic heating, the Australian government announces more funding to ‘control’ starfish – (a) it doesn’t work, and (b) culling some starfish doesn’t reduce greenhouse gas emissions.”
Scott Bennett favours a balance between the two views. “Like most things, there’s a bit of truth on both sides. Restoration (particularly on coral reefs) gets blown up, and scale is a major issue. The search for super-corals and super kelps is interesting, but unlikely to be our saviour, in my opinion. At the same time, many restoration efforts in other ecosystems, like Operation Crayweed in Sydney, or the Oyster Reefs in South Australia are having good success.”
And quietly building, there’s another approach, one that suddenly found attention after the Black Summer bushfires: a return to indigenous knowledge. The University of Hawaii, in a recent report on sea-level rise, made the uplifting recommendation that we should combat such problems by “Go(ing) back to the ways of ancient Hawaiian tradition, living along the streams and utilising the land more efficiently and independently.” The ancient ways are available to us, here in Australia, if only we’ll open our eyes and our hearts to them. Coupled with the modern expertise of scientists, they offer us a path to reversing some of the damage, by altering our lifestyle and our consumer demands to lower emissions and other wastes. In a year in which everything seems to be up for reconsideration, are we ready to reconsider our relationship with the ocean?
Banner image: Warming ocean trends off Australia’s east coast has seen optimal ocean conditions for the yellowtail kingfish moving south by about 95km per decade since 1996. Photo: Alamy.