Pāua abundance trends and population monitoring in areas affected by the November 2016 Kaikōura earthquake, February 2023 update.

The November 2016 Kaikōura earthquake caused coastal uplift resulting in high pāua (Haliotis iris) mortality and extensive loss of critical pāua habitats. The uplift affected approximately 120 km of coastline that supports significant customary, recreational, and commercial pāua fisheries. There has been a closure of the pāua fishery from the Conway River in the south to Marfells Beach in the north (the ‘closed area’) since November 2016 to allow for recovery of affected pāua populations. The fishery was re-opened for the first time for a three-month period on the 1st of December 2021.

The objective of this project was to monitor annually (since 2017) the abundance and length frequency of adult pāua populations in the closed area to estimate biomass trends to inform management actions at the scale of the closed fishery until mid-2023. This was achieved by continuing to monitor baseline estimates of pāua abundance and length-frequency profiles at selected sites within the closed area since 2017. These surveys have employed a modified timed-swim methodology to estimate site and area-wide (QMA) trends in pāua abundance and length frequency. Results from these surveys have shown an overall increase in adult biomass, and widespread juvenile recruitment, which supported the decision to re-open the fishery.

Data analysis followed the methods of McCowan & Neubauer (2022), namely using generalised linear mixed models. Additional diagnostics to those presented for previous surveys were inspected to ensure that the model adequately captured variation in survey data across various strata (e.g., survey strata, quota management areas, survey periods) and covariates. These efforts suggested that multi-modality in the response within survey sites is a key driver of high variability (CVs) in survey estimates, a feature that arises from patchiness of pāua distributions and is largely irreducible with limited survey effort. During surveys undertaken during October-November 2021, all previously established survey sites were re-surveyed immediately prior to the re-opening of the fishery. After the fishing season, follow up surveys were attempted, but only 6 of 34 sites (in PAU 3A) were able to be surveyed due to poor conditions.

Analyses show that abundance of pāua (represented by estimated biomass per unit time effort) across all sites and at a fishery-wide scale has continued to increase during the most recent survey immediately prior to the fishing season. However, a decrease in abundance was estimated in PAU 3A after the fishing season. This is potentially due to the influence of poor survey conditions and bias towards low abundance sites with low site coverage.

General trends of increasing biomass are driven by the emergence of post-earthquake recruitment of small (<100 mm) pāua that are now visible during dive surveys, which is more consistently apparent in PAU 3A. In PAU 7, increasing biomass is more strongly driven by the increasing abundance of larger (>140 mm) pāua.

There is high variability in abundance and recruitment trends across surveyed sites which could be attributed to site-specific variability in uplift and habitat related factors. Overall, current trends continued to support the criteria for fishery re-opening until immediately prior to re-opening. There is uncertainty about the decrease in abundance detected after the fishing season in PAU 3A which will likely be resolved by data from surveys conducted in 2022–23.