Genetic approaches to management of New Zealand’s blackfoot pāua, Haliotis iris

Abstract

Reseeding is a fisheries management tool that has potential to enhance New Zealand’s iconic blackfoot abalone or pāua (Haliotis iris) fishery. This fishery is managed commercially within geographically defined areas, and in some cases through local Māori customary management tools. In this study, genetic tools (microsatellite markers) were used to address several challenges with reseeding, and to examine population dynamics relevant to the management of pāua.

Initially, the potentially harmful effects of genetic sampling (epipodial clipping) on pāua mortality and behavior were investigated. No notable differences in mortality between sampled and non-sampled pāua in both laboratory (21% and 26% respectively) and field (40% and 43% respectively) experiments over three months were observed. There were also no significant differences between sampled and non- sampled pāua for righting times, growth, and predation responses to the starfish Astrostole scabra. These findings suggest that the genetic sampling method employed in this study has no detectable effects on pāua mortality.

The utility of microsatellite markers in tracing recapture rates in reseeded pāua was investigated. Five hundred and fifty hatchery and wild pāua were reseeded onto eleven artificial reef sites. Genetic samples were collected from all individuals recaptured during three, six and thirteen month-surveys, after which population and parentage assignment approaches were used to determine the origin of recaptured individuals in relation to initial identification from a dietary mark. Parentage assignment (to hatchery broodstock) was the most robust approach enabling 98.1% of hatchery recaptures to have their origin correctly classified. This simple trial shows the applicability of genetic tools in monitoring recapture rates in commercial scale reseeding programs.

Genetic differences between broodstock, hatchery and wild pāua populations were examined to illustrate the importance of maintaining genetic diversity in hatcheries. Reductions in genetic diversity between broodstock, hatchery and wild populations were observed, as well as significant genetic differentiation between hatchery and broodstock (FST=0.021), and hatchery and wild (FST=0.032) populations. These genetic changes were attributable to large variation in reproductive success among broodstock. The numbers of broodstock required to maintain levels of genetic diversity comparable to wild stocks within a hatchery setting were then estimated, with varying levels of mean reproductive output (K) and variance in reproductive output (VK). It was shown that approximately 4000 broodstock would be required to produce a hatchery population with the same genetically effective population size as that found in Tory Channel. However the number of broodstock needed reduces significantly as VK reduces, when mating strategies such as factorial crosses are employed.

Genetic population structure was assessed in pāua populations in Tory Channel, and two customary fisheries in the South Island. Minimal population structure existed at scales of up to 300km, with weak but significant structure (ΦST = 0.201) observed over a scale of 800km. Fine-scale analyses of recruitment in Tory Channel provided evidence for local-scale larval dispersal and self-recruitment. In the customary fisheries, significant differences in size and growth between sites where observed, which were not reflected in genetic differences. These findings have implications for the scales that might be relevant for pāua management, and in defining scales relevant for broodstock sourcing and defining ‘target populations’ for reseeding.