Pāua abundance trends and population monitoring in areas affected by the November 2016 Kaikōura earthquake

The November 2016 Kaikōura earthquake caused coastal uplift that resulted in massive pāua (Haliotis iris) mortality and loss of critical pāua habitats. This has seen the ongoing closure of the pāua fishery along approximately 100 km of the coastline that supports significant customary, recreational, and commercial pāua fisheries (the ‘closed area’). Commercially, the closed area spans portions of the PAU 7 and PAU 3 quota management areas (QMAs), which at the time of the closure had total allowable commercial catches of 93.6 t and 91.615 t, respectively. The closed area accounts for approximately 60 t of pre-earthquake commercial catch, approximately 15 t and 45 t from PAU 7 and PAU 3, respectively.

This project is a two-year continuation of initial work that was undertaken to estimate baseline pāua densities and length-frequency profiles at selected sites within the closed area (McCowan & Neubauer 2018). Initial estimates were made with novel methodologies using Global Positioning System turtle units and underwater electronic callipers. The objective of this project was to monitor changes in pāua abundance and length frequency (to assess recruitment), to ultimately inform fisheries management decisions at the scale of the closed area.

Of the initial 35 survey sites, 21 were successfully re-surveyed during the first survey period of this project (2018–2019), and 34 out of 35 were surveyed during the second survey period (2019–2020).

Initially an assessment was made of the appropriateness of using the number of measurements per unit effort (MPUE) as a proxy for pāua density to overcome issues with missing data from GPS dive units (originally used to delimit area to estimate density) and to enable the use of significantly larger data sets of measurements and counts of pāua at each site. The measurements per unit effort, as well as biomass per unit of survey effort (BPUE; number of measurements multiplied by the length-frequency distribution of measured pāua), correlated well (R2=0.86) with density. Therefore, MPUE and BPUE were used as indices of changes in pāua density.

An overall increase in pāua abundance was observed at a QMA-wide level in both QMAs over the three survey periods. Increased abundance was generally more pronounced in PAU 7 than in PAU 3. In PAU 3, abundance trended downwards in the second survey period, which was likely due to the consistently poor survey conditions during the period, as well as a potential bias towards sampling sites with lower rates of increase. There was high variability in abundance trends across sites. This variability was in part related to variability in the amount of uplift at each site, because sites with a larger increase in abundance were those with less uplift. Variability in abundance trends across sites could also be linked to habitat related factors and pre-earthquake abundance. Comparison of length-frequency profiles across the three survey periods showed reasonably stable profiles in larger size classes (125–160 mm), with an increase in the number of individuals in the 80–100 mm size range in both QMAs which is likely to be indicative of post-earthquake recruitment. Recruitment signals were variable between sites due to differences in available recruitment habitat and variability in uplift.

At the time of publication, one likely pathway for the reopening and management of the closed area is via the recently approved PAU3 Fisheries Plan (PāuaMAC3 2021), which outlines criteria for when reopening of the fishery should be considered. Outcomes from this project indicate that these criteria have likely been met, and that reopening could be considered with future monitoring.