The long-spined sea urchin, Centrostephanus rodgersii has expanded its distribution southwards from southern New South Wales, through Eastern Victoria, the Bass Strait Islands and down the east coast of Tasmania. In some areas of Tasmania abundance of C. rodgersii has increased substantially, even to sufficient densities to form destructive grazing aggregations, removing the overstory macroalgae that are essential component of a healthy reef ecosystem. The degradation of these coastal reef systems has serious implication not only to biodiversity conservation and ecosystem value, but also to high value fisheries, with the potential to significantly compromise sustainable fisheries management practices.
This project aims to test the effectiveness of systematic culling in discrete areas as a strategy to reduce the density of C. rodgersii to minimise the potential for destructive over grazing, and secondly provide an estimate of the cost of culling areas of reef on the east coast of Tasmania.
Methodology: To test the effectiveness of culling in discrete area the project applied an experimental design utilising multiple treatment (n = 8) and control plots (n = 4), each 1,500 m2. These plots were assessed using a combination of randomised belt transects and census counts. Approximately two weeks post-culling the density of C. rodgersii within the eight treatment and four control plots was assessed using randomised belt transects, followed immediately by a second systematic cull within four of the treatment plots. Approximately 12 months after the initial culling exercise the density of C. rodgersii in all 12 plots was re-assessed using randomised belt transects, and the eight treatment plots further assessed by a systematic census count.
Two bio-economic models were developed to determine the cost of culling areas across the east coast of Tasmania, as well as sub‐areas (case study areas). The models were developed based on information on the cost of employing commercial divers to cull areas of reef down to 20 m, beyond which dive times were considered a limitation to implement a cost-effective culling program.
Results: Systematic culling of Centrostephanus rodgersii in spatially discrete plots in Wineglass Bay on the east coast of Tasmania was highly successful with average urchin density reduced from 1.51 to 0.13 urchin.m-2 within treatment plots when assessed approximately one year post‐culling.
Systematic culling was also effective at significantly reducing the patchiness of urchin distribution, where high abundance patches are, in theory, more likely to destructively over graze a reef area.
Four treatment plots were culled twice and the reduction in urchin abundance compared with the other four plots that were only culled once. The result was that there was no significant difference in the reduction of C. rodgersii between the single and multiple cull treatments.
The cost of manually controlling an invasive species in the marine environment is inherently expensive due to the costs associated with mobilising logistics to a target area, and secondly the limitations of diver time in the water. In this report we present models that can be used to generate cost estimates to cull a given area based on urchin density and dive depth, with the maximum depth chosen having a great effect on the overall cost. A local scale model estimates the maximum cost to cull Wineglass Bay to a depth of 20 m at $1,617,802, based on a constant density estimate of 1.5 urchins.m‐2. The cost to cull reef areas within Fortescue Bay to a maximum of 20 m using the same model at $877,019 based on a constant density of 0.29 urchins.m-2.
This report shows that systematic culling can significantly reduce the density of C. rodgersii in discrete areas. The implications of these findings are that culling can be considered a viable method in the management strategy evaluation of controlling the deleterious effects of C. rodgersii. The costing models provide tools to estimate the direct cost of implementing a culling strategy at a range of spatial scales across the east coast and can be manipulated to provide a bio-geographically accurate estimate of cost depending on the area (and size of area) selected.