Longspined Sea Urchin CPUE

Commercial Catch Per Unit Effort (CPUE)

Data was filtered to exclude dives with the top and bottom 5% of effort (hours per dive). Annual catch rate was calculated by averaging all individual dive catch rates for the year. We then standardised the catch rate by the effects of individual diver ID, latitudinal region and time of year (month).

There is no discernible trend in CPUE for the entire east coast of Tasmania over the course of the fishery, and the catch rate has remained stable for the last 3 years of the fishery. There is also no discernible trend in CPUE for Region 2, St Helens which is the with the greatest abundance of urchins and the longest history of fishing on the east coast.

Catch-per-unit-effort (CPUE) or catch rate is calculated per dive day by the total wet weight of harvest (in kg, measured at the boat ramp) divided by the dive time (hr). CPUE can give an indication of relative biomass over time. However, with a highly spatially aggregated/patchy species such as sea urchins, CPUE throughout the range of the fishery may remain stable as the stock is being depleted. This is especially the case for developing fisheries that target spatially structured stocks, because fishers may continue to move to new areas and/or depths while maintaining a high CPUE but sequentially depleting reefs over time and space. Longspined sea urchins are patchy because of low movement rates and homing to crevices (Flukes et al. 2012), so density is highly dependent on substrate. In addition, the fishery data is currently recorded at a very coarse spatial scale, with commercial fishing blocks at the scale of 11.1 km by 11.1 km. An area of this size may contain many different reefs. Due to the coarse spatial scale, divers may be undertaking a rotational or sequential harvest of different reefs within the one block without affecting CPUE data. The results here should be considered in the context of these limitations.

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