If the long-spined sea urchin behaves in Tasmania as in NSW, then large tracts of reef habitat will be destroyed and concomitant reductions in abalone and rock lobster fisheries are likely. Given fixed quotas, this will have the effect of increasing effort in productive habitat that has not been affected by urchins. It is imperative to assess whether the urchin is likely to behave in Tasmania as in NSW, and therefore whether it poses a significant threat to reef-based industries in Tasmania.
The pilot project proposed here will provide this initial assessment in establishing baseline information on the distribution of C. rodergsii 'barrens' (i.e. the current extent of the problem) and the urchin itself (i.e. the potential extent of the problem) on the east coast, and will determine whether abalone and rock lobster populations are reduced on relatively recent barrens.
If the threat posed by the urchin is significant, then it will be important to address possible responses to the problem while the extent of barrens may still be relatively circumscribed. The baseline data obtained in this project will also assist in assessing whether development of an urchin industry might be a suitable response to manage the problem.
The work outlined here is a necessary first step prior to a more expansive research program to identify (1) mechanisms triggering destruction of algal habitat, (2) the importance of interactions between the urchins and rock lobsters, abalone and algae in underpinning macro-dynamics of rocky reef systems and the population dynamics of commercial species, and (3) management responses to mitigate the impact of the urchin.
Final report
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.
