Paua biomass estimates and population monitoring in areas affected by the November 2016 Kaikoura earthquake

The November 2016 Kaikoura earthquake caused extensive coastal uplift resulting in massive paua (Haliotis iris) mortality and loss of critical paua habitats. An immediate fisheries management response was the closure of the p aua fishery across the uplifted area. The closed area spans portions of two paua quota management areas, the southern aspect of PAU 7 (Marlborough) and the northern aspect of PAU 3 (Kaikoura). The closed area historically accounted for approximately 60 t of annual commercial catch, and supports significant customary and recreational paua fisheries. The objective of this project was to estimate paua abundance and monitor paua populations in the earthquake-affected area to inform management decisions relating to the re-opening of the paua fishery. To estimate abundance, we developed novel methodologies using GPS dive loggers and underwater electronic callipers. We also established fixed monitoring points within surveyed areas to monitor discrete paua populations through time.

To allocate sampling effort, we devised a stratification scheme based on GPS dive logger data from the fishery. This allowed us to place sampling effort in areas that are relevant for the fishery. Our new survey method combined estimation of survey area from GPS units worn by pāua divers (turtle loggers) with estimates from a Bayesian model for the survey that integrates estimates of pāua detection probability. The integrated model allowed us to estimate pāua density with reasonable confidence. Uncertainty was relatively high (i.e., CVs were elevated) due to high within-stratum variance in pāua densities, as well as due to a thorough treatment of parameter uncertainty. Highest densities were found in the closed area of PAU 3, especially in areas of high and medium fishery use. Pāua densities were relatively uniform in PAU 7, which had lower densities over-all than areas in PAU 3.

We found that scaling density estimates to total biomass or abundance was difficult due to the lack of robust estimates of habitat area for pāua. In the absence of a defensible solution, we opted to calculate density only, and suggest that this quantity could be used to monitor change in the local pāua population. However, how useful the current survey and potential subsequent follow-up surveys will be to inform management will depend on the over-all science and management framework for the re-opening of the fishery.