Background

The longspined sea urchin (or Centro) was not traditionally found in Tasmanian waters, with the first record of its occurrence in 1978. Rather, evidence suggests that this species has recently undergone a range extension to Tasmania from NSW due to extensions in the warm East Australia Current brought about by climate change (Johnson et al. 2005, Ridgway 2007, Ling 2008). Increased populations of longspined sea urchins are of concern because they can damage kelp forests through overgrazing (Ling et al. 2009a, Johnson et al. 2011, Marzloff et al. 2016). Once established on a reef, increases in urchin density and subsequent grazing pressure of this species leads to discrete patches of bare rock termed ‘incipient’ barrens (Johnson et al. 2005). If urchin density continues to increase in incipient barrens, the grazed patches grow and join together into larger patches, leading to the formation of ‘extensive’ urchin barrens (Flukes et al. 2012), a habitat largely devoid of macroalgae (Lawrence 1975, Chapman 1981, Andrew and Underwood 1989). This fundamental change in the ecosystem has a substantial impact on a broad range of species and reduces the utility for human activities including diving, and recreational and commercial fishing of a number of species.

Urchin Density Control

In Tasmania, there has been research into various strategies for reducing urchin densities to prevent or reverse barren formation, one of which is to increase numbers of their predators. Worldwide, there are numerous examples of where overfishing the apex predator has led to a loss of kelp forests through the creation of urchin barrens (Steneck et al. 2002) and also the reverse effect where reduced harvest rates on urchin predators have resulted in the reestablishment of kelp forests, such as rebuilding sea otter populations in Alaska (Estes and Palmisano 1974) and rock lobster populations in South Africa (Mayfield and Branch 2000).

Take-all Harvest

A take-all harvest removes juvenile and well as mature long-spined sea urchins from an area with the aim of reducing urchin grazing pressure for the sustainability of the kelp beds and the health of the reef.

Predation

Southern Rock Lobster Jasus edwardsii is the key predator for the native sea urchin Heliocidaris erythrogramma in Tasmania (Pederson and Johnson 2006). Southern Rock Lobster has also been shown to predate the Longspined sea urchin and is the only known predator of large emergent urchins in Tasmania (Ling et al. 2009a). As a result of prolonged intense fishing pressure, the Southern Rock Lobster biomass off eastern Tasmanian had dropped to extremely low levels prompting the development of the East Coast Rock Lobster Stock Rebuilding Strategy in 2013. This has maintained catches at below half the recent peak in the mid 2000s and has led to stock rebuilding that will continue into the future. The interactions between seaweed, Longspined sea urchins and Southern rock lobster in Tasmania have been examined in detail by a simulation model of Tasmania reef communities, called TRITON (Marzloff et al. 2013, Johnson et al. 2014). One of the main findings of the model was that the initial prevention of urchin barren formation through increased predator numbers would be more effective than reversal through the same strategy, and that reduced catch of lobster on incipient barrens could mitigate the formation of extensive barrens in these areas within a 20 year time frame (Johnson et al. 2014). That modelling was conducted at a time when the current high catches taken by the urchin fishery were not anticipated.

Culling

Other strategies have been explored in Tasmania to reduce urchin densities in an effort to prevent or reverse urchin barren formation. Culling is an alternative removal method to harvesting in diveable depths. When harvesting, divers remove urchins generally in the mid size range of ~85mm test diameter (Johnson 2016), leaving smaller urchins, and are limited to finding urchins with roe quality that is acceptable to the processor. This means urchin harvesting takes place outside of spawning season, with most harvesting taking place between approximately December and June. In comparison, divers that are culling can kill urchins of most size ranges (that are visible) at any time of year by smashing them with a spike or similar instrument. When culling, divers are not limited to choosing urchins of high roe quality or transporting urchins to the boat and truck. On extensive barrens in Victoria, the rate of culling is reported to be close to 3x faster than harvesting (Personal communication John Minehan). However, culling is labour-intensive and can be costly compared to subsidised harvesting depending on subsidy rate, whether divers are paid for culling, and the density of urchins (Tracey et al. 2015, Cresswell et al. 2019). Culling by commercial divers has been funded in small areas on the Tasman Peninsula and there is also volunteer culling by abalone and recreational divers.

Quicklime

In other parts of the world, problems of high urchin densities and associated extensive urchin barrens have been dealt with by use of quicklime (Leighton et al. 1966). Quicklime, which is made by heating limestone and is used in cement, has been used to control starfish in oyster bed and sea urchins in commercially harvested kelp beds (Bernstein and Welsford 1982). It releases heat when combined with water and kills echinoderms by causing epidermal lesions that permit bacteria to enter (Bernstein and Welsford 1982). Kill rates in excess of 96% can be achieved with an apparatus that mixes quicklime with sea water at the surface and then pumps the slurry through a hose to the bottom. In some cases, greater precision is achieved by a diver who directs the flow onto sea urchins (Bernstein and Welsford 1982). In Tasmania, with other methods available for removing sea urchins with diving depths, quicklime offers potential for removing urchins from depths deeper than 25m.

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