Abstract
Drivers of coastal fishery status and population dynamics are complex due to the broad range of stressors to which coastal species are subjected. For successful fisheries management it is essential to understand drivers of population variability so that management can be adjusted accordingly. The population dynamics, and individual traits of abalone (Haliotids) can vary on the scale of metres to kilometres. In New Zealand, the cultural keystone, pāua (Haliotis iris, blackfoot abalone) has undergone considerable stock declines, and stock enhancement efforts have had limited success. For fisheries managers, it is important to know how a population of interest has changed over time, what factors may influence a populations spatial distribution and if they can use this information to adjust management. Therefore, the aim of this thesis is to disentangle drivers of fishery status and population dynamics of H. iris. First, I examined how a H. iris population in Peraki Bay, Banks Peninsula changed after 45 years. Comprehensive historic population data from the 1970s provided a baseline of abundance, size structure, and population dynamics before exposure to a commercial fishery and a range of environmental stressors. Repeat monitoring of the H. iris population at a 45-year interval indicate that a large change in population size and structure had occurred. Since 1976, there has been an 84% decrease in estimated population size. The remaining population is restricted to where juveniles were found in the original survey. This chapter indicates that H. iris may be susceptible to multiple stressors including habitat loss, climate change, and low and inconsistent recruitment. I then examined spatial drivers of H. iris density and shell length using multiple regression modelling, and generalised linear modelling. Physical and biological habitat and fishing restrictions data were gathered across six Customary fishery Protection Areas (CPAs, Mātaitai Reserves and Taiāpure Local Fisheries) across the takiwā (tribal area) of the Te Waipounamu (South Island) iwi (tribe) Ngāi Tahu. Model averaging indicate that H. iris density and size are driven by different factors. Haliotis iris density was negatively correlated to time under fishing restrictions, and macroalgal density. Shell length of H. iris was larger in deep habitats and negatively related to the density of the gastropod, Cookia sulcata. Spatial drivers were then investigated further in the East Otago Taiāpure (EOT), a CPA in southern Te Waipounamu. In-situ and modelled wave data, and drift algae biomass were used to determine whether H. iris reach higher densities and larger shell lengths in more wave exposed environments. Principal component regression and bootstrapping indicate that H. iris density was positively related to wave bottom orbital velocity, but negatively related to days exposed to large wave events. No relationship was observed between drift algae biomass and density or mean shell length of H. iris. Lastly, I investigated the depth distribution of H. iris in the EOT in light of a proposal by the EOT Management Committee to harvest H. iris at wading depths. Wading-only harvest involves gathering in shallow waters (depths up to around knee deep) without the use of a mask and snorkel. The reasoning behind the proposal being that a wading-only fishery could keep the fishery open while providing protection for populations in deeper water. The abundance and shell length of H. iris were recorded at 0.1 m depth intervals using belt transects. Provided a wadable limit of 0.5 metres is set, 78% of the H. iris surveyed would be accessible to wading harvest at chart datum, although this percentage differed among sites from 55 – 99%. Shell length increased with depth, indicating that a depth refuge would provide protection to mature individuals. This study indicates that some sites may be suited to wading harvest while others may not, highlighting the need to consider the reef by reef implications of management decisions.
Overall, this thesis provides new insights on how H. iris populations can change on long timescales and small spatial scales. This information, can be used to make more informed management decisions for H. iris, on ecologically relevant scales, in New Zealand. The environmental drivers discussed throughout this thesis however, will likely apply to other abalone species, and may be useful to guide management decisions and restoration efforts in other abalone fisheries globally.