Abstract
Understanding biotic interactions in estuaries provides vital information that, in turn, can be applied to conservation strategies for coastal communities to actively improve biodiversity and access to the marine environment. There is a growing body of evidence demonstrating that tuaki (Austrovenus stutchburyi) beds, which comprise the most accessible form of kaimoana, are declining throughout Aotearoa, likely due to a range of human impacts. Te Rauone Beach within Ōtākou, Dunedin provides an example of a community concerned with the decline of their local tuaki bed, but faced also with multiple other environmental issues. A tuaki biomass survey undertaken by NIWA was replicated over two years, to determine if any changes to the distribution and abundance of the population had occurred as a result of a high erosion rate on the beach and a consequent beach renourishment scheme. No significant differences in biomass between year groups were found. Alternative methods of monitoring were explored and discussed, to determine a feasible and appropriate method that could enable coastal communities to establish more autonomy over the management of their shellfish beds. The survey at Te Rauone helped establish the question that forms the focus of the present thesis - if a community wishes to take action to restore their local shellfish bed, what ecological factors must they consider?
Randomised quadrat surveys from Waitati Inlet, Dunedin, indicate that juvenile tuaki were found in higher densities when surrounded by high densities of larger adult tuaki. The pattern raised the idea that perhaps these larger adult tuaki were providing a 'pavement' of protection for juvenile tuaki, against predation from coastal predators. The prediction aligns with the concept of post-settlement Allee effects, where a low density and truncated size distributions of adult tuaki would exacerbate a decline in juvenile tuaki persistence.
In December 2020 and 2021, replicate field manipulations were undertaken to test the potential sheltering effect. 0.5x0.5m quadrats were set up and treatments with high adult tuaki density, no adult tuaki, high organic loading and no organic loading were established and crossed, and then seeded with 50 tagged juvenile tuaki in each experimental unit. The treatments were then left for two weeks, then excavated. The effects of organic loading were negligible in both years of study but in 2020, the highest juvenile retention rate was in the treatments with adult presence, suggesting that juveniles may have been more susceptible to predation or more likely to move away when adult tuaki are absent. In contrast, there was no effect of adult tuaki presence on juvenile retention in the 2021 study.
To isolate the effects of predation and determine if adult presence could be a factor in juvenile tuaki survivorship, a laboratory feeding trial was developed over a total of 60 replicates using the New Zealand paddle crab (Ovalipes catharus), an important and abundant predator of juvenile tuaki. Treatments with no adult tuaki presence, medium (400 per m2) and high adult presence (800 per m2) were set up in tanks at Portobello Marine Laboratory. A single paddle crab was left in each treatment tank over a 48 hour period, and predation rates on juveniles were tracked.
The experiment revealed that juvenile tuaki survivorship under paddle crab predation pressure is enhanced in the presence of large adults in the tanks, consistent with the hypothesised sheltering effect of adults. It was also found that predation rates changed significantly over the course of three month course of the study, which correlated with declining water temperature and female preparation for pre-copulation moulting.
The present thesis investigates the dynamics of two local tuaki beds, and determines through field surveys, manipulations and laboratory trials the importance of tuaki density and size distribution in supporting the persistence of conspecific juveniles. The work from the present thesis has important applications to monitoring, fisheries management and most importantly, potential community restoration efforts of shellfish beds.