While global abalone stocks have largely collapsed due to overfishing, disease and environmental stressors, New Zealand is fortunate to have one of the last remaining viable wild abalone fisheries in the world.
This fishery, focused mostly on the endemic blackfoot abalone Haliotis iris (pāua), supports valuable customary, commercial, and recreational harvests. However, the sustainability of New Zealand’s wild pāua fishery is increasingly threatened by recreational overharvesting, adverse environmental conditions and stochastic events.
This has prompted a large investment by the commercial seafood sector to enhance pāua stocks. With the exception of a small amount of scientific literature and anecdotal evidence, there are few quantitative data on the efficacy of current stock enhancement methods, how to optimise them, or if modifications might be more suitable and effective.
A recent research programme run by Seafood Innovations Limited, and sponsored by the Pāua industry Council Limited, used multi-year field surveys and experimental work to assess commercial pāua stock enhancement efforts and to test novel methods. Primarily, the focus was on assessing the 2018 commercial outplanting of 167,000 hatchery-raised “seed” pāua at sites along the Kaikōura coastline. These seed pāua were outplanted in response to the cataclysmic Kaikōura earthquake in 2016, which caused high mortality of pāua, loss of critical habitats, and a 5-year closure of the very productive fishery.
Annual field surveys of seven enhancement sites over four years provide one of the only data sets in New Zealand to follow the long-term abundance and growth of outplanted hatchery pāua. Outplanted pāua were visually identified up to three years after their release into the wild, after which they emerged from cryptic habitats and became difficult to distinguish from wild stock, or migrated to deeper habitats.
The recapture rate of seed pāua was affected by movement away from seeding sites, potential mortality, and to the unexpected, prolific natural recruitment that occurred during the study. However, seed pāua made up as much as 40 percent of the population at some sites, and averaged 12 percent of populations across all sites after three years. This suggests that the outplanted seed pāua contributed considerably to overall pāua abundance and rebuilding of pāua stocks.
Growth rates of outplanted seed pāua were typically high (33-40 mm yr-1) for the first two years, before decreasing to c. 25 mm yr-1 in subsequent years. These growth rates are greater than or equal to those of naturally recruited pāua in good habitat. After three years, many of the outplanted seed pāua that had been recaptured had potentially reached sexual maturity (>82 mm shell length). The sites with the greatest density of wild pāua, however, had the slowest growth rates for seed pāua. Furthermore, of the seven seeded sites, two were greatly affected by gravel inundation after storms, which buried juvenile habitats and caused high mortality of seed and wild pāua. This reinforces the importance of appropriate site selection for outplanting and choosing multiple sites to spread the risk associated with stochastic events. The insights from this research, which could be achieved only through detailed multi-year field studies, help clarify the benefits, limitations, utility and likely financial returns of stock enhancement efforts to the fishery using hatchery-raised juveniles.
The second component of this research programme was to experimentally test larval outplanting, the release of competent swimming pāua larvae into natural habitats, for use in stocknenhancement.
This required learning hatchery techniques, inducing mature pāua to spawn synchronously, and rearing and settling resultant larvae. Researchers spent more than two years conducting hatchery spawning trials and completed one of the only controlled experimental outplantings of swimming pāua larvae in New Zealand.
Several outplanting methods were tested in situ and assessed with follow-up surveys after three months. Most of the methods tested did not result in a detectable increase in small recruits in the immediate outplant area. However, one method that was trialled, the outplanting of small rocks pre-settled with larvae, showed some promise as a practical and scalable enhancement method that may be the most cost-effective of those trialled, and worth pursuing.
The programme has produced informative long-term data sets of commercial enhancement efforts that will help improve site selection and methods for future work and provides the details necessary to assess and model likely costs and benefits. Results from experimental larval outplanting highlight the challenges and limitations of larval work, and offer an alternative method to mitigate them. Additionally, this research offers a practical “how-to” guide of two enhancement methods, compiling insight gained from over four years of in-depth research which could prove to be a useful tool for optimising future investments in New Zealand’s pāua fishery.