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Why lobsters are the best thing to hit Tassie's urchin-encrusted reefs

The kelp forest’s best defence against long-spined urchins is hiding deep within the crevices of Tasmania’s rocky reefs.

By day, the forests of giant and common kelp along Tasmania’s east coast are powerhouses of productivity, providing food and shelter for a wealth of fish, crustaceans, and molluscs, including two of Tasmania’s most valuable commercial species, rock lobster and abalone.

Yet by night, hordes of creatures bearing masses of long, black spikes emerge from their crevices, devouring all that stands before them. Patch by patch, long-spined urchins are razing the kelp forests, reducing them to desolate ‘urchin barrens’ on the sea floor.

Professor Craig Johnson, Head of the Ecology and Biodiversity Centre at the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS), describes an urchin barren as a scene of complete ecological devastation. He says it’s akin to taking a machine through and clearing out “every last stick of seaweed”.

“All you have remaining is bare rock,” he says.

Long-spined urchins are a relatively recent arrival to Tasmanian waters. Over the past 50 years, eddies of the warm East Australian Current have flowed further south with increasing frequency. The larvae of long-spined urchins came along for the ride.

Scientists predicted that long-spined urchins would have the potential to reduce up to half of Tasmania’s productive rocky reefs – home to the kelp forests – into barrens.

“There were real issues happening here,” says Professor Johnson. “It was a threat not just to biodiversity in the barren areas, but also to some of the key fisheries involved.”

The problem with urchin barrens

According to Professor Johnson, a kelp forest with an emerging urchin problem resembles a slice of Swiss cheese. “It’s mostly kelp, but there’s a hole in it here, there, and somewhere else.”

As urchin numbers increase, so too do the holes in the cover of kelp and other seaweeds, until there are more holes than kelp, and the ecosystem collapses into an urchin barren.

And even that doesn’t stop the urchins. They simply switch up their diet to live off the microalgae growing on rocks, or they feed on drift seaweed and filter nutrients directly out of the water.

“The problem is that these urchin barrens are extending,” says Professor Johnson.

“In some places they’re down to well over 60 metres below the surface. It’s not a problem that’s easily solved by divers, because no diver is going to work for very long at 25 metres, and certainly not below 30 metres. You’ve got extensive barrens and it’s just too deep for divers to get there.”

In response, the researchers turned to a species they knew had an appetite for long-spined urchins and could work at depth, at night, every night of the year: the rock lobster.

The key to healthy kelp forests

Professor Johnson and his colleagues tested the role of lobsters by moving hundreds of large lobsters to an area that had suffered significant damage – an extensive urchin barren. Here they could determine whether they could drive urchin numbers low enough to kickstart seaweed recovery.

They also tested whether increasing the number of large lobsters on an incipient barren – a region with localised barren patches – could prevent it from collapsing into an extensive barren.

With the agreement of the fishing industry, the Tasmanian Government closed several kilometres of coastline at research reserves on the north-east and south-east coasts, so the rock lobster experiment could proceed undisturbed.

The lobsters did their work, feasting on the urchins.

The team found that even though the rock lobsters ate tens of thousands of urchins on the extensive barrens, the seaweed did not recover, because urchin densities were still not reduced far enough. But on the incipient barrens, the rock lobsters were able to control urchin numbers enough to allow for the seaweed’s recovery.

Lobsters were the key to saving existing kelp beds, even if they could not help to restore extensive areas of urchin barrens.

“We were able to show really clearly that lobsters were absolutely the key predator,” says Professor Johnson. “By building up lobster populations in healthy kelp beds, you can prevent barrens from being formed.”

Boosting lobster numbers

Just as marine ecologists were revealing the critical role of lobsters, fisheries scientists at IMAS were reporting that rock lobster stocks, as measured in biomass, were at historically low levels off eastern Tasmania, in the region where urchins had become more established.

Director of Sustainable Marine Research Collaboration at the University of Tasmania, Professor Caleb Gardner, says rock lobster numbers had been reduced by recreational and commercial fishing down to about 10 percent of their natural level.

“Regardless of urchins, that is an unacceptably low level for any fished species, and action was needed.”

Fisheries scientists led by Professor Gardner used computer simulations to compare different management scenarios for recovering rock lobster stocks, and found that the simplest scenario – reducing and capping the lobster catch – was also the most effective and economical.

In response, the Tasmanian Government introduced a catch limit in 2013 to cut the rock lobster harvest by about two-thirds in the long-spined urchin-infested parts of eastern Tasmania. This now involves cuts to commercial quota and the recreational bag limits and season.

Under this rebuilding strategy, lobster stocks are expected to reach 20 percent of their natural (or unfished) level by 2023. The stock of large lobsters – the most effective urchin predators – will increase by about 400 percent at this point from levels in 2013. The lobster biomass is assessed each year, and so far, the recovery is on track.

Hope on the horizon

In 2015, a second strategy based on IMAS research kicked off, which involved moving lobsters from areas where they were abundant off south-western Tasmania to the urchin-affected areas on the east coast. Between 30,000 and 50,000 lobsters are now moved each year, which is accelerating the stock recovery.

Research at IMAS has also been used for direct removal of urchins by divers, which is helping in shallower depths. In 2016, the government launched a subsidy of urchin harvesting, which now supports a developing urchin roe industry of over 100 tonnes.

Professor Gardner says that, while most of the effort to date has been towards preventing barrens forming, separate strategies are needed for the recovery of barrens once they’ve formed.

This will be a challenge, because only a small number of urchins are required to maintain barrens once they form. IMAS is exploring strategies such as culling urchins in barrens, either by divers or with AUVs (autonomous underwater vehicles), as well as alternate uses of urchins harvested from barrens, such as special fertiliser applications.

Professor Johnson says the involvement of the fishing industry in the research was vital to its success and extension to fishery management.

“We had real help from both the rock lobster fishing industry and the abalone industry,” he says. “Without that help, it just wouldn’t have worked.”

Over the course of the study, it was the rock lobster industry that provided several tonnes of large lobsters to be dropped into experimental sites to boost the population. Abalone divers also assisted, testing whether localised removal of long-spined urchins was an effective control.

Professors Johnson and Gardner both agree that the dialogue and collaboration with commercial and recreational fishers will continue. Professor Gardner’s team at IMAS has resurveyed the urchin barrens that Professor Johnson first surveyed in 2000, with a report due for publication in 2018.

According to Professor Johnson, there is cause for optimism, looking into the future.

“We know that lobster populations have been picking up, which is great,” he says.

“But whether they’re picking up fast enough to really put out the fire in areas where there were incipient barrens is an issue that we’ll need to continue to monitor.”

5 key facts:

  • Long-spined urchins had the potential to turn up to 50% of Tasmania’s productive rocky reefs into urchin barrens.
  • Rock lobster stocks in the parts of eastern Tasmania affected by urchins were at 10% of their natural levels in 2011–2012.
  • In 2013, the Tasmanian Government introduced a catch limit on lobsters, which reduced the catch by two-thirds.
  • Stock recovery is a long-term operation, but an interim target has been set to restore stocks to at least 20% of natural levels by 2023, with greater gains beyond that.
  • Additional actions to combat urchins include translocation of 30,000–50,000 lobsters into the area each year since 2015, and subsidised harvesting of urchin for their roe since 2016.

About the researchers

Professor Craig Johnson

Professor Craig Johnson is Head of the Ecology & Biodiversity Centre at the Institute for Marine & Antarctic Studies (IMAS), and Associate Director of IMAS. He is a marine community ecologist interested in identifying the drivers of marine community dynamics, and how to interpret change in marine communities. He works mostly with shallow reef systems, but current work also includes the Southern Ocean. His work is evenly divided between field programs and computer modelling, and across animals and seaweeds. He leads a productive group of postdoctoral fellows and graduate students.

Professor Caleb Gardner

Professor Caleb Gardner is a Fisheries Scientist and Acting Director of Sustainable Marine Research. His research is mainly on high value invertebrate fisheries such as southern rock lobster and abalone. He has qualifications in both economics and biology, which interact in his research on commercial fisheries. His research on wild fisheries species generally have the objective of ensuring sustainable production and community benefit from Australia's fisheries. This has included the increased use of bioeconomic models in coastal fisheries for setting catches and assessing other regulations.

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