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The recruitment dynamics of post-settlement juvenile toheroa and the potential applications for aquaculture

Managing the recovery of threatened species due to anthropogenic disturbance is one of the most challenging problems in ecological monitoring. Researching species ecology is vital to help us improve ecosystem health and subsequently develop better management strategies in the environment. Humanity is entirely reliant on ecosystem services for food security. With the world population expected to reach ten billion by 2050, there is a growing need for more sustainable food sources. With the exploitation of many environmental resources worldwide, aquaculture has been championed as a sustainable option for future seafood production. In New Zealand, the toheroa (Paphies ventricosa) has been identified as a potentially valuable asset to the aquaculture industry and for possible conservation strategies via the implementation of reseeding populations with hatchery-reared spat. Toheroa faced inadequate resource management and exploitation in the 1900s which ultimately led to the closure of an important recreational, cultural and commercial fishery. However, toheroa populations have failed to recover despite over 40 years of protection. It has been proposed that the continued decline of toheroa populations is not due to a lack of juvenile recruitment. Very little is known about juvenile toheroa recruitment, and there is no available information that could facilitate restoration management or determine the appropriate action to take for monitoring juvenile populations. From both a conservation and an aquaculture perspective, understanding the underlying ecology will be pivotal for future endeavours. My thesis aimed to investigate the distribution dynamics of post-settlement juvenile toheroa on Ripiro Beach and the potential applications for aquaculture. Population surveys were conducted on Ripiro Beach in February and May to establish a sampling methodology that accurately captures the juvenile population both spatially and temporally. Surveys on vertical substrate stratification significantly affected both density and shell size with increasing sediment depth. Additionally, in contrast to the hypothesis formed on the basis of traditional knowledge that juvenile distribution is limited to the dunes and upper intertidal, spat appeared to have far more varied littoral distributions than previously theorised. The data from these surveys were used to make estimates of the total juvenile population on Ripiro Beach with attention to changes spatially (habitat type), temporally (seasonally), and by size cohorts (mm). My results indicated that toheroa populations are not recruit-limited. Estimates for the total February population ranged between 580 (±190) million and 650 (±200) million. By May, large abundance losses (> 90%) dropped the estimated population to between 30 (±15) million and 37 (±17) million, indicating significant bottleneck mortality during the early growth stages. However, the high abundances of small spat (< 5 mm) indicate that toheroa aquaculture is potentially viable. In aid of future aquaculture applications, I investigated how handling, harvesting, and different transportation methods could impact toheroa health, survival, or performance. This experiment aimed to mimic real-world applications wherein juvenile spat would be harvested and ongrown in hatcheries. To do this, I utilised the burrowing behaviour of juvenile toheroa as a quantitative indicator of stress. I found that if appropriate storage methods were implemented, initial handling and harvesting had a greater impact than actual transportation. The research in this thesis demonstrated that populations do not appear to be recruit-limited. Furthermore, the findings indicate that aquaculture built on the foundation of wild-harvested juveniles is potentially viable. However, further research is required in order to quantify potential annual recruitment variation both spatially and temporally. Additionally, in order to establish a conservation approach, we must first define the niche requirements of toheroa to determine the obstacle (or obstacles) inhibiting recovery.
Type of thesis
The University of Waikato
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