Human activities are causing a global loss of plant and animal species, degrading ecosystem properties and threatening ecosystem services. One indicator of these losses is the increasing proportion of fish stocks in decline, and the Challenger oyster fishery in Tasman Bay, central New Zealand is an example of one such fishery. Anthropogenic effects from land-based activities, and towed-gear fishing have been implicated as contributors to the decline of shellfisheries and degradation of the marine ecosystem in Tasman Bay. Increased sedimentation in the bay caused by soil erosion and runoff associated with forestry, agriculture and subdivision is likely to have a range of negative effects on the benthic community. Also towed-gear fishing, dredging and trawling homogenise benthic habitat structure (reduce habitat heterogeneity) and facilitate sediment resuspension as well as causing removal and direct physical damage to benthic biota. There is an imperative to seek to mitigate these effects and look at ways to restore the benthic community including the commercial shellfish species. In this context, my central hypothesis was that enhancement of the benthic habitat by returning waste shell to the seabed would increase oyster production for the fishery. Related to this main goal of oyster fishery enhancement, a primary objective of the study was to fill knowledge gaps relating to the biology and ecology of the oyster in Tasman Bay. The second main topic of my thesis was to investigate how this form of habitat enhancement would alter the benthic community structure, and potentially aid in restoration of the wider ecosystem in the bay. I sought to link the twin goals of fishery enhancement and ecological restoration by considering potential management measures to promote a sustainable oyster fishery and at the same time facilitate ecological restoration within Tasman Bay. The investigations focused on four main themes: temporal patterns of oyster larval abundance, spatial patterns of spatfall and larval dispersal, effects of habitat enhancement on oyster population productivity, and effects of habitat enhancement on the benthic faunal community. Laboratory and field studies were conducted between October 2004 and May 2009. A peak period of oyster reproductive activity began in late spring and continued through summer in each year. Maximum rates of adult oysters brooding larvae were 17% in November 2004 and 2005, and 23 % in December 2006. Over the entire summer breeding period it was estimated that 55 to 78 % of adult oysters incubated larvae. A very low level of brooding activity (1 %) occurred during winter. Temporal trends in larval settlement closely tracked brooding patterns. Settlement on collectors deployed in Tasman Bay was greatest between November and January, and there were very low rates in winter. Results are useful in optimising the timing of substratum deployment in an enhancement program for the oyster fishery. Spat settlement density was strongly related to background adult oyster density. Spat settlement on experimental arrays deployed through the water column only occurred within a narrow vertical range very close to (<1 m above) the seabed. If suitable habitat is available for settlement, oysters tend to settle within a few hours after release, but approximately half of the larvae settled in a laboratory experiment were capable of remaining viable for several days. Oyster distribution assessed at the scale of the shellfishing industry’s annual biomass survey (median distance between sample tows ~ 1 km) is adequate to broadly predict spat settlement distribution in the subsequent settlement season, and the distribution of mature oysters is a key determinant in the placement of shell for habitat enhancement to maximise spat settlement. Deployment of waste whole scallop shell on the seabed as settlement substratum increased oyster spat density significantly. Available settlement surface on enhanced shell plots decreased by 82% in the five months after deployment, due to fouling by numerous invertebrates and sedimentation. Survival of oysters recruited to enhanced habitat was generally very low, and varied greatly among 4 experimental sites and through time. After 3+ years, survival among site/treatment combinations ranged from 0% to 0.04%. At the site where survival was greatest, the absolute density of oysters surviving to 3.41 years on enhanced habitat was estimated as 0.4 m⁻². This equated to an increase in relative density of commercial sized oysters from ~0.01 m⁻² prior to enhancement, to ~0.14 m⁻² at the end of the experiment, and demonstrated that habitat enhancement can elevate adult oyster densities to commercial levels on areas of seabed where oysters were previously below threshold densities for commercial fishing (0.02 m⁻²). Peaks in mortality occurred within experimental plots when oysters were less than one year old, and three years old. Growth modeling indicated that after 4.25 years, 98% of living oysters would attain legal size (≥ 58mm length), and 92% would attain sufficient shell depth (≥ 20 mm) to provide high grade (grade A in the industry) meat. Shell depth was a better morphometric predictor of meat weight than either shell height or shell length. The species assemblages on the shell-enhanced habitat were distinct from those on adjacent non-enhanced seabed. Measures of taxonomic and functional richness, faunal densities, and taxonomic redundancy within functional groups all increased in enhanced habitat. Beta and gamma diversity also increased due to patchiness of the habitat created within enhanced experimental sites. Large scale habitat enhancement in Tasman Bay via the deposition of waste shell on the seabed is likely to confer benefits to ecosystem function associated with those community level effects. To sustain an oyster fishery in Tasman Bay, an ecosystem-based approach to fishery management is recommended to facilitate restoration of benthic habitats and communities and to help maintain ecosystem function supporting all components of the benthic community, including the oyster population. Planning and implementation of a combination of specific management measures including habitat enhancement, rotational fishing, permanent exclusion of towed fishing gear from a network of protected areas, and integration of the management of the oyster, scallop, and finfish fisheries would provide the best chance for restoration and maintenance of a sustainable oyster fishery.

Although there are no authenticated records of human illness due to PSP in New Zealand, nationwide phytoplankton and shellfish toxicity monitoring programmes have revealed that the incidence of PSP contamination and the occurrence of the toxic Alexandrium species are more common than previously realised (Mackenzie et al., 2004). A full understanding of the mechanism of uptake, accumulation and toxin dynamics of bivalves feeding on toxic algae is fundamental for improving future regulations in the shellfish toxicity monitoring program across the country. This thesis examines the effects of toxic dinoflagellates and PSP toxins on the physiology and behaviour of bivalve molluscs. This focus arose because these aspects have not been widely studied before in New Zealand. The basic hypothesis tested was that bivalve molluscs differ in their ability to metabolise PSP toxins produced by Alexandrium tamarense and are able to transform toxins and may have special mechanisms to avoid toxin uptake. To test this hypothesis, different physiological/behavioural experiments and quantification of PSP toxins in bivalves tissues were carried out on mussels (Perna canaliculus), clams (Paphies donacina and Dosinia anus), scallops (Pecten novaezelandiae) and oysters (Ostrea chilensis) from the South Island of New Zealand. Measurements of clearance rate were used to test the sensitivity of the bivalves to PSP toxins. Other studies that involved intoxication and detoxification periods were carried out on three species of bivalves (P. canaliculus, P. donacina, P. novaezelandiae), using physiological responses (clearance and excretion rate) and analysis of PSP toxins in the tissues over these periods. Complementary experiments that investigated other responses in bivalves fed with the toxic cells were also carried out. These included byssus production, and the presence of toxic cells in the faeces of mussels, the siphon activity and burrowing depth in clams and the oxygen consumption in scallops. The most resistant species to PSP toxins were the mussel, Perna canaliculus and the clam, Dosinia anus. Both species fed actively on toxic dinoflagellates and accumulated toxins. The intoxication and detoxication rate of the mussel was faster than the other species of bivalve studied (P. donacina and P. novaezelandiae) which confirm mussels as a good sentinel species for early warning of toxic algal blooms. The clearance rate of the clam, Paphies donacina decreased when fed on Alexandrium species but the effect of the PSP toxins on this physiological response was not confirmed. Over the detoxification period of 8 days, this clam did not detoxify which suggests that its ability to retain high level of toxins for an extensive period may be critical for public health management. The scallop, Pecten novaezelandiae was clearly the most sensitive species to the PSP toxins and the clearance rate was significantly lower in the presence of the toxic dinoflagellate A. tamarense. Although the clearance rate was low, the scallops still fed on the toxic dinoflagellate and accumulated PSP toxins in the tissues. The scallops detoxified slowly which would affect the market for this bivalve in the presence of a toxic algal bloom. This bivalve would retain PSP toxins for longer period of time than other species such as mussels. The oyster, Ostrea chilensis, had erratic clearance rate and did not respond clearly to any of the variables tested over the time. Oysters accumulated more toxins than the sensitive species, but they had been exposed to two more days of feeding with A. tamarense; therefore this species may actually have a similar intoxication responses to P. novaezalandiae and P. donacina. The results from this thesis suggest further directions for the aquaculture sector and ongoing research in this field, which in future will lead to a better selection of suitable species for culture as well as species for monitoring of PSP toxins. In the future, research that integrates field and controlled laboratory studies will expand to other species of interest and a more complete record will in time be available in order to manage more efficiently the negative effects that harmful algal blooms may have in New Zealand.