Establishment of the long-spined sea urchin (Centrostephanus rodgersii) in Tasmania: first assessment of potential threats to fisheries

Project Summary PRINCIPAL INVESTIGATOR: Prof. Craig R. Johnson The pattern of distribution of the long-spined sea urchin Centrostephanus rodgersii over ca. 40 y in the Kent group, Bass St., suggests initial establishment in the mid 1960s with subsequent expansion of populations to its current status as the dominant invertebrate on shallow subtidal rocky reef. On the east coast of Tasmania, C. rodgersii is most abundant in the vicinity of its location of initial discovery in 1978, but it occurs throughout the east coast between Eddystone Pt in the north and Recherche Bay in the south. Barrens habitat, supporting high densities of sea urchins but largely devoid of macroalgae, occurs extensively in the Kent group and at several sites on the northern half of the Tasmanian east coast, but declines with increasing latitude and does not occur south of the Tasman Peninsula. At the southern extent of barrens habitat on the open coast, barrens are incipient and occur as small patches in macroalgal beds. Evidence suggests that the barrens habitat in the Kent group and on the open rocky coast of Tasmania is formed by grazing of C. rodgersii and not by Heliociaris erythrogramma, another sea urchin that occurs on these barrens. This is largely because there is a significant positive relationship between C. rodgersii density and extent of barrens but not between H. erythrogramma density and extent of barrens, and because H. erythrogramma is not know to form barrens on exposed coast. These collective patterns suggest that the incursion of C. rodgersii into Tasmanian waters was from the north, and that spread on the east coast of Tasmania propagated from an ‘epicentre’ in the vicinity of St Helens in the northeast. We suggest that the initial incursion was via larvae transported from NSW in the East Australian Current, which has increasingly influenced the east coast of Tasmania over at least the past 4-5 decades. The lack of any genetic differentiation among C. rodgersii populations in NSW, the Kent group and the east coast of Tasmania is consistent with this view. On the east coast of Tasmania, there is a clear negative relationship between the abundance of C. rodgersii and the density of commercially fished abalone (Haliotis rubra) and rock lobster (Jasus edwardsii). The density of abalone is significantly lower on barrens habitat than in adjacent macroalgal beds at the same depth and on the same substratum type. We conclude that abalone and rock lobster are unlikely to occur in commercial quantity on C. rodgersii barrens. Given these findings, the spatially patchy distribution of existing extensive barrens, and particularly if existing incipient barrens (consisting of small barrens patches scattered through seaweed beds) develop to become extensive barrens, then a stronger focus on spatial management of fisheries on the east coast of Tasmania may be warranted. In Tasmanian waters, large continuous tracts of C. rodgersii barrens do not develop in shallow water (2-10 m) as occurs in NSW, but largely occur within a depth range of ca. 10-20 m in the Kent group, and ca. 15-35 m on the east coast of Tasmania. Barrens habitat is more prevalent on boulder substratum than other types of consolidated reef, extending to cover >75% of the seafloor on this substratum at some sites, and averaging ca. 33% cover on boulder substratum across all sites in Tasmania where incipient barrens occur. Given these collective observations, and estimates that boulders comprise ca. 55% of consolidated reef in depths ≤18 m and 34% of consolidated reef to ca. 40 m depth, barrens habitat could potentially expand to account for ca. 50% of rocky reef on the east coast of Tasmania, as currently occurs in the Kent group and NSW. This scenario would have serious implications for abalone and rock lobster fisheries on this coast. However, the capacity to predict future patterns of barrens habitat requires better understanding of the mechanisms that initiate barrens formation and that determine the position and dynamics of boundaries between barrens and macroalgal-dominated habitat. Given these considerations, and evidence worldwide of the connection between fishing of sea urchin predators and formation of sea urchin barrens, we suggest that management intervention to limit the spread of C. rodgersii barrens in Tasmania is warranted.