At low-levels of exploitation, commercial harvesting of long-spined sea urchins was found to prevent increase in urchin density. Adjacent unfished locations experienced an increase in both urchin density and grazed area over the 2014 – 2016 study period. Research sampling of populations remaining on reefs after fishing also found that mean urchin Test diameter and urchin age was smaller and younger respectively than on nearby unfished reefs, further supporting there is a measurable effect of urchin fishing on urchin populations even at low exploitation rates. These results demonstrate a clear potential for urchin fishing to reduce destructive grazing by urchins, or at least prevent further expansion of existing grazed areas even under a regime of low
fishing pressure.
The Tasmanian commercial fishery of long-spined urchin either directly overlaps or is immediately adjacent to commercially fished abalone reefs. As the presence of barrens has a clear negative association with abalone abundance, the potential for urchin fishing to lower urchin densities in key reef habitats highlights the importance of an ongoing long-spined urchin fishery for protecting key abalone fishing grounds (recreational and commercial). The ability to capture fine-scale spatial data on both urchin and abalone fisheries was critical to evaluating the potential for urchin fishing to benefit the abalone fishery. A long-term strategy for fine-scale data collection using passive GPS and Depth data loggers will be fundamental to ongoing assessment of the benefits of urchin fishing to the abalone fishery, but also to the broader users of shallow coastal reef systems in Eastern Australia.There is also the potential for positive-feedback loops from urchin fishing, with reduction in densities increasing roe production and output, and therefore ensure that the industry can remain economically sustainable.
Cartographic Exposure Index software developed during this project showed some capacity to identify coastlines at risk to destructive grazing, but further data collection to underpin the predictive model is required. Production of a high-resolution bathymetric map of key coastal reefs is considered to be a high priority for ongoing spatial mapping and analysis of the expansion of the urchin fishery and its consequent effects on urchin density.
There is a clear benefit to the urchin fishers if they switch from compressed air to using Nitrox gas for diving safety and access to greater reef area. Current beach price and likely decline in catch rates as the fishery expands however, will mean that investment in Nitrox based breathing systems may be economically marginal. From a health perspective, a switch to Nitrox while retaining the current bottom time would have clear safety benefits.
This study demonstrated positive benefits for abalone habitat at very-low urchin exploitation levels, with the extent and spatial magnitude of benefits expected to rapidly increase with increasing urchin exploitation. It is unlikely that commercial harvesting will lead to eradication of the long-spined sea urchin, but there is clear potential for commercial fishing to be a primary contributor to mitigating the destructive grazing of this species.
