Broadcast spawning marine invertebrates have been widely used as model organisms to study processes of evolution. One of these is the study of various life history stages associated with reproduction. Fertilization ecology in broadcast spawning marine organisms, i.e. the process by which sperm and egg fusion occurs once released into the ocean, has been the subject of intensive study for roughly the last 20 years, and represents thus a recent field in ecological sciences. This growth in interest was sparked by studies that showed spawning events may take place predominantly under sperm limiting conditions. More recent findings however, suggest that the occurrence of sperm competition, and the risk of polyspermy (multiple sperm entry in to the egg, which is lethal in echinoderms) can also occur during reproductive events in marine invertebrates. This has also been predicted on theoretical grounds. Even though polyspermy has been observed previously, particularly during aquaculture studies, evidence to assess the occurrence of polyspermy in situ, and the conditions under which it occurs is lacking. Simulated field studies in Evechinus chloroticus as well as laboratory studies in E. chloroticus and Coscinasterias muricata found high levels of polyspermy, even under the sperm limiting conditions that are naturally found in the field in other marine broadcast spawners. Furthermore, laboratory results in both Evechinus and Coscinasterias showed that polyspermy is most likely to increase when sperm concentrations are increased. In addition, even though increasing sperm concentrations increases monospermy to a certain extent, an increase in gamete contact times generates similar levels of monospermy. Results from these studies confirmed the mathematical model predicting some of these events. The timing of gamete release to achieve maximum fertilization success (monospermy) in a broadcast spawner is thought to be under strong selective pressures, because reproductive success is directly related to fitness. The spawning patterns in the sea urchin E. chloroticus observed in this study demonstrated that E. chloroticus spawns under varying environmental and ecological conditions. Interestingly, it appeared that highly synchronous and widespread spawning was found to occur when highly turbulent conditions existed. This may reduce the potential for polyspermy that may exist during mass spawnings under shallow and calm conditions. The demonstration of polyspermy and the confirmation of the mathematical model, suggest that polyspermy is a common and frequent occurrence during broadcast spawning events. Thus polyspermy represents an evolutionary force that may shape the evolution of reproductive phenomena at levels ranging from the gamete to the population.

Previous studies indicated the marine pyridoacridone alkaloid ascididemin possessed a unique biological profile. Investigation of synthetic routes to ascididemin led to the discovery that ascididemin precursors possessed a wide range of biological activities. One precursor possessed hollow fiber in vivo antitumoral activity and continuing in vivo studies at the NCI, using subcutaneous xenograft assays, are in progress. A crystal structure of a precursor indicates that these tetracyclic heterocycles are planar, suggesting intercalation as a mechanism of antitumoral action. Ascididemin was synthesized by two novel ring E forming reactions. The superior methodology was also useful in the preparation of analogues, such as kuanoniamine A. Ascididemin possessed promising hollow fiber in vivo antitumoral activity but was poorly active in a subcutaneous xenograft study. Ascididemin was incapable of exerting antitumoral activity at a distance and further analogues were prepared to address this problem. Ten analogues were prepared, with all the non-bromine containing analogues selected for in vivo evaluation at the NCI. Two ring A analogues were prepared by a thennew synthetic route, including the antiviral natural product 11-hydroxyascididemin. 11-Methoxyascididemin was selected for hollow fiber evaluation. Two bromine containing ring D analogues were prepared, including the antifungal natural product 2-bromoleptoclinidinone. A crystal structure on 2-bromoleptoclinidinone was the first determined on a pyridoacridone alkaloid and the molecule was planar, further supporting an intercalative mechanism of action. Due to non-selective antitumoral cytotoxicity, ring D analogues are not useful antitumoral agents. Six carbon-based ring E analogues were prepared by novel methodology. All 6-substituted analogues assessed were selected for in vivo antitumoral evaluation. Hollow fiber antitumoral activity decreased with bulk of the substituent. 6-Methylascididemin has been selected for subcutaneous xenograft studies. The 5-substituted analogue prepared gave the best in vitro antitumoral profile of all alkaloids in this study and has been selected for in vivo evaluation. Ring E substituted N-8-deaza-ascididemin analogues possessed no antitumoral activity, highlighting the importance of the 1,10-phenanthroline-like bay region of ascididemin in antitumoral activity. Another four structurally novel, quinoid containing alkaloids have been selected tor in vivo evaluation. While ascididemin was the only compound capable of topoisomerase IIα cleavable complex stabilization, related alkaloids possessed a similar level of inhibitory action against this enzyme. This further supports intercalation as the dominant mechanism of action for pyridoacridone alkaloids. Ecological roles of four natural pyridoacridone alkaloids were assessed. Alkaloids were species specific antifeedant agents against important consumers. These alkaloids may have a long term detrimental effect on predator physiology, due to the well established ability of these alkaloids to interfere with cell proliferation. Ascididemin elicited avoidance responses in numerous marine species. Ascididemin has no antifouling activity against macrofoulers. Microbiological assessment of ascididemin, 11-hydroxyascididemin and 2-bromoleptoclinidinone indicated that modification of the ascididemin chromophore leads to the directing of antimicrobial activity towards a different phyla of parasites. Pyridoacridone alkaloids may be part of a non-antibody based immune system. All studies point to pyridoacridone alkaloids enhancing the eventual reproductive success of the organism. Biological and chemical evaluation of 29 New Zealand ascidians has been performed. Significant biological activity was detected in ten ascidians. Novel metabolites isolated were 2-(3'-bromo-4'-hydroxyphenol)ethanamine (Cnemidocarpa bicomuta) and 1,3-dimethylguanine (unidentified ascidian). Known metabolites isolated were 1,3-dimethylisoguanine (Cnemidocarpa bicomuta) and rubrolides A, B and C (unidentified ascidian). The survey highlighted the importance of overexpressed purine bases in ascidian metabolism. No physiological roles for these overexpressed purines are as yet apparent. Our study of NZ ascidians has led to the isolation of many compounds previously isolated from sponges. The widespread distribution of such metabolites gives credence to the theory that common metabolite-generating genes are present in both phyla, due to the evolutionary success of these genes. Two optically active 9-(5-S-methyl-5-sulfinyl-lyxofuranosyl)-9H-purin-6-amine (lyxosyl-MTAS) nucleosides were isolated from the nudibranch Doriopsis flabellifera. This is the first report of any lyxosyl-MTAS nucleoside as either a natural or synthetic product.

Confronting Barriers to Ecological Information Transfer in New Zealand's Fisheries Management system: Towards Developing a Trajectory for Trans-disciplinary Inquiry This thesis confronts barriers to ecological information transfer in New Zealand's fisheries management system. The investigation reveals a wider problematic within the governmental institutions of the fisheries milieu. The thesis deploys Callon's (1991) concept of translation to ask new questions regarding the conduct of fisheries science and management, which since the introduction of the Quota Management System in 1986, has been defined through the practices of commercial stakeholder groups. These stakeholders enrol ecological information through a narrowly defined and non-reflexive discourse of sustainable utilisation to serve specific purposes that suit their own needs. The thesis uses imaginaries to illustrate how sustainable utilisation has marginalised those stakeholders with different views on how ecological information could be used in fisheries management decision making. Moreover, the thesis contends that the milieu's hegemonic ontological environment prevents translation by mitigating the potential for dialogue between stakeholder groups. The thesis engages with a wide range of literature and cuts through established modes of thinking, developing a trans-disciplinary discourse that re-conceptualises the fisheries management problematic in process terms. The thesis demonstrates how trans-disciplinary dialogue that might transcend the current impasses in the New Zealand fisheries milieu will only develop through a network of previously unconnected groups. The thesis proposes a series of policy and behavioural interventions aimed at shifting the existing hierarchies of power and realigning established networks to ensure such dialogue is possible. Keywords: trans-disciplinary research, translation, network alignment, New Zealand, ecological information transfer, fisheries management.

Whole document restricted, see Access Instructions file below for details of how to access the print copy. / In 1986 the New Zealand system of fisheries management was reformed. The Quota Management System (QMS) augmented the existing input control regulatory instruments with a system of output controls: individual transferable quota (ITQ). This thesis examines a number of aspects of the QMS in the context of a case study of the SNA1 snapper fishery. The QMS model is contrasted with a dynamic optimization model of rights based fishing which shows the effectiveness of ITQ systems in resolving the open access problem and that proportional share ITQ prices potentially contain information about the biological and economic state of a fishery. Departures of the QMS from the theoretical model are noted, particularly the allocation mechanism, the continued application of open access input control apparatus, and the basis of total allowable catch definition. Input control is considered in a bioeconomic model of the Hauraki Gulf sub-stock of the SNA1 fishery that features a sea surface temperature recruitment sub-model. Changes to the minimum legal size regulation (MLS) and the effect of differing inter-temporal recruitment structures on rent generation are evaluated. The biological and economic inefficiency of size regulation and the unequal distribution of risk between sectors in this fishery under the current management regime are demonstrated. The potential role for discount rates as performance indicators is shown. An intra-cohort stock externality due to growth overfishing is evident. The productive capacity of the SNA1 fishery system is presently not fully utilized. Output control is examined through time series analyses of prices in SNA1 ITQ markets. To date government has relied on the results of stock assessment research when setting the allowable harvest and monitoring the performance of fisheries policy. Excessive reliance on biological data, in particular modelling outputs, has attracted criticism. An alternative, a minimum information system uses information contained in quota prices as indicators of the biological and economic health of a fishery. SNA1 quota ownership and lease markets are found to exhibit the characteristics of weak form efficiency. Prices reflect expected rents in the fishery. However, at this stage of their development prices also reflect firms' uncertainty over expectations of fishery management agency behavior. Concern has been expressed that ITQ systems may be compromised by market failure through the exercise of market power. Evidence of market power is found in the SNA1 market for ITQ lease title. Explicit non-commercial quota, a review of the effectiveness of remnant pre-QMS regulatory instruments, and a substantial role for economic information in the total allowable catch setting process and as performance indicators form the basis of recommendations for policy.

This thesis uses molecular genetics as a tool to uncover information about the population structure and genetic variation in Hector’s dolphin (Cephalorhynchus hectori), to track population declines and to assess the evolutionary origins and taxonomic status of this species. A high-resolution genetic analysis of population structure was considered important for the determination of population boundaries and delimitation of conservation management units due to potentially unsustainable fisheries-related mortality. Population structure and dispersal rates were assessed using 281 samples collected from individual Hector’s dolphins of ten population groups representing the known geographic range of this species. Variation among mitochondrial DNA sequences (ΦST = 0.545) and microsatellite allele frequencies at six loci (RST = 0.252) indicated the presence of four genetically isolated regional populations, North Island (n = 29), East Coast South Island (n = 110), West Coast South Island (n = 122) and South Coast South Island (n = 19). Significant levels of genetic differentiation were not detected within local sub-populations of the East Coast and West Coast regional populations. However, the estimated geneflow between these sub-populations fitted a one-dimensional stepping-stone model (r2 = 0.6225) suggesting a vulnerability of local populations to fragmentation. A measure of expected mtDNA diversity (Tajima’s D statistic) suggested decline in eight of the ten populations. Microsatellite heterozygosity was also lower than expected in the East Coast and North Island regions, suggesting either further regional sub-structuring (Wahlund effect), loss of diversity due to population decline or the presence of null alleles. Examination of all Hector’s dolphin museum specimens of known origin (n = 55) enabled comparison of historic (1870 - 1987) genetic diversity to contemporary (1988 – 1999) diversity in two regional populations to assess the possibility that these populations have undergone recent declines. Over the last 20 years the North Island population has been reduced from at least three lineages (h = 0.41) to a single lineage (h = 0, p < 0.05). The diversity of the East Coast, South Island population has declined significantly from h = 0.65 to h = 0.35 (p < 0.05). These results suggest that the low abundance currently observed is due to recent population declines and that the North Island population is threatened with extinction in the near future. Based on a trend analysis of the mtDNA, it can be predicted that the East Coast South Island population may lose all mtDNA diversity within the next 20 years. Alternatively, detection of a one dimensional dispersal pattern may indicate that some populations are at risk of extirpation while others may not be in decline. If this is the case then the East Coast regional population is at risk of fragmentation. On a wider evolutionary scale, Hector’s dolphin is one of four species of the genus Cephalorhynchus, all of which suffer fisheries–related mortality. To describe the origin and radiation of these species, 485 bp of the mitochondrial DNA control region was sequenced from 320 individuals (including previously sequenced 200 Hector’s dolphins) representing nine of the ten species in the sub-family Lissodelphininae. The hypotheses that either Cephalorhynchus is a monophyletic genus or that the four species have arisen separately from pelagic Lissodelphine species and have converged morphologically were tested. The mtDNA phylogeny supported the monophyly of the genus and suggested that the genus Cephalorhynchus originated in the waters of South Africa and, following the West Wind Drift, colonised New Zealand and then South America. Secondary radiations resulting in two genetically isolated populations were found for the Kerguelen Island Commerson’s dolphin and the North Island Hector’s dolphin. A comparison of the genetic differentiation between the Commerson’s dolphins of the Kerguelen Islands (n = 11) and the coast of South America (n = 35), and between the North Island (n = 14) and South Island (n = 185) Hector’s dolphins, was conducted in order to assess the conservation and taxonomic status of these populations. A single fixed substitution in the mtDNA control region was diagnostic for the Kerguelen Island compared to South America (FST = 0.306, ΦST = 0.602) and the North Island compared to the South Island (FST = 0.442, ΦST = 0.495). Population differentiation of four microsatellite alleles (including unique alleles in each of the four populations) between the Kerguelen Island and South American Commerson’s dolphin (FST = 0.036, RST = 0.0493) and between the North and South Island Hector’s dolphins (FST = 0.391, RST = 0.3197) indicated restricted nuclear as well as maternal geneflow. These data, combined with additional evidence of morphological and geographic isolation, indicated that the Kerguelen Island Commerson’s dolphin and the North Island Hector’s dolphin are likely to be reproductively isolated from their alternate con-specific populations. Examination of various species concepts and definitions of conservation units leads to the conclusion that these four populations should each be considered unique at the subspecies level for the purposes of management, protection and evolutionary potential. These results lead to the conclusion that the Hector’s dolphin consists of highly subdivided populations. As a result of this and a low reproductive potential, Hector’s dolphin populations are vulnerable to extirpation through even low levels of human induced mortality. To manage such populations, it is appropriate to consider each of the two islands as separate sub-species. Within the South Island, the populations may be further subdivided into three demographically independent Management Units – the East, West and South Coasts. The South Coast management unit is vulnerable due to its low abundance and isolation and requires further investigation. Population modelling will need to reflect the fact that the local populations within the East and West coast regions share only limited dispersal with immediately adjacent populations and are thus susceptible to fragmentation. These results also show that the population declines of the East Coast South Island and the North Island populations are of recent origin thus implicating fisheries-related mortality as the principal threat to Hector’s dolphin. To prevent further decline or fragmentation of South Island populations more stringent control of inshore gillnet fisheries is required. By contrast, current decline of the North Island population may be a result of inbreeding depression. Given the low abundance and rapid decline of the North Island population, it is imperative to evaluate the potential for inbreeding depression while continuing to mitigate all human-related threats.

Various aspects of metabolism and physiology were investigated during the ontogeny of yellowtail kingfish (Seriola lalandi), a fish of growing aquaculture importance in both New Zealand and other countries. Incubation experiments between 18-24°C showed that developing eggs and larvae were heavily influenced by temperature. It appeared that at warmer temperatures larvae hatched smaller but grew on the yolk sac, whereas at cooler temperatures larvae grew inside the chorion. Oxygen consumption data supported this, with a negative correlation found between total embryonic oxygen consumption and temperature. A mechanism was proposed to explain the differential effect of temperature on ontogeny and growth. Like other marine fish with pelagic eggs, yellowtail kingfish were found to be heavily reliant on free amino acids as a source of energy. At 23°C the pattern of substrate utilisation in eggs was considerably different from that at 17-21°C, indicating that 23°C exceeded the tolerance for normal development. Inter-individual aggression by large individuals was associated with the development of size heterogeneity in juveniles. Although this aggression also affected the survival of smaller juveniles, it was not the primary agent of much of the mortality that occurs during this phase, as many of these individuals were on a degenerate developmental trajectory. Yellowtail kingfish fingerlings used for ongrowing were robust to the stressors imposed by live transport. The ontogenetic development of metabolic rate from 0.6 mg-2.2 kg did not follow the same scaling exponent as that observed for mammalian models of allometry, and has implications for interspecific studies of mass-dependent metabolism.

The Chilean oyster, Tiostrea chilensis, is a commercially important species that is native to New Zealand and the Pacific Coast of South America. The description of the variability in life history characteristics among populations of the Chilean oyster is fundamental to understanding its biology and may help in solving some of the problems encountered in culturing this species. Research presented in this thesis describes some aspects of the reproductive biology of the Chilean oyster from four populations, mainly in northern New Zealand, and compares the results with previous studies from elsewhere. Similar patterns of gametogenesis were found among three study populations at Manukau Harbour, Hauraki Gulf and Foveaux Strait. Oysters were protandrous, maturing firstly as males and later also producing ova. In all three populations the majority of the contents of all the gonads were male reproductive products. There was no evidence that the sexuality of oysters alternated rhythmically as has been reported in other species of larviparous oysters. For spawning females, the often concurrent release of ripe sperm indicated the possible presence of self fertilisation. In samples of oysters examined from Foveaux strait the sexuality of oysters was found to change markedly with size. This suggested that the infection of oysters by the parasite Bonamia may not be related to their sexuality as was previously thought. At two northern populations (Hauraki Gulf and Manukau Harbour) larvae were produced from young, small oysters, and a much larger proportion of the population was brooding larvae each year than has been reported elsewhere. In both populations, larvae were being brooded, released and were settling at all times of the year, unlike other populations. The mean fertility of the Manukau Harbour oysters was the highest so far reported for any population of this species. The size of larvae in all three northern populations were smaller than has been reported for all other locations in New Zealand and chile and is thought to be related to differences in water temperatures. Overall, the results highlight the importance of investigations into populations of molluscs across their geographical range for revealing variation in life history characteristics which may be of benefit for aquaculture. / Whole document restricted, but available by request, use the feedback form to request access.

The tellinid bivalve, Macomona liliana, is important to the structure and organisation of intertidal sandflat communities in Manukau Harbour. This thesis reports on features of the reproductive, larval supply and recruitment biology of Macomona liliana in relation to environmental factors in the harbour. Macomona liliana is gonochoric. In a mid-tide population sexual maturity occurred in animals ≥ 22 mm (shell length). Spawning is asynchronous and protracted from late November to March throughout the harbour and timing of the reproductive cycle is associated with seasonal changes in environmental variables: temperature, salinity and seasonal production of phytoplankton and benthic microalgae. A visual gonad index was useful for establishing the main periods of spawning activity. Favourable habitats are at mid-tide on the large exposed sandflats characterised by well sorted fine sands. Less favourable habitats are in sheltered bays characterised by moderate-poorly sorted sands. A delay in the onset of gametogenesis in small compared to large adults (July-August of. May-June) indicates size and/or age dependent resource allocation to growth and reproduction. Adult size classes are evenly represented in favourable habitats whereas in less favourable habitats size classes are dominated by small adults. Favourable habitats have an equal sex ratio or are biased towards females whereas in less favourable habitats the converse is true. Early life-history stages are described and techniques employed to facilitate their identification in plankton and sediment samples. A spatial Taylor's Power Plot (slope, b = 1.31) demonstrates thorough mixing of larvae in the main channels. Spatial autocorrelation models confirm that densities of larvae are higher to the south of Karore Bank than to the north. The recruitment period is December to early March but varies between successive years. Physical transport processes, post-settlement migration and high levels of mortality in Boccardia syrtis tube-mats dominate recruitment patterns in the high energy environments. Larval supply and active habitat selection by settling larvae may, however, be significant determinants of population structure in sheltered bays. Generalisations are made on the effect of habitat quality on the reproduction and recruitment biology of the Manukau Harbour metapopulation.

The use of enterococci as a microbiological indicator of water quality requires an understanding of the sources, persistence, and ecology of this group of bacteria in the environment. This research describes a series of investigations undertaken to describe the abundance, occurrence, and diversity of enterococci from aquatic and terrestrial environments. A screening protocol for environmentally sourced enterococci was developed to describe species and sub-species variability. This protocol combined classical microbiological methods of selective culture and biochemical characterisation, with molecular techniques including gene probe screening for identity and RAPD-PCR for genotypic diversity. Preliminary studies on the occurrence of enterococci in terrestrial and aquatic environments suggested that these organisms are ubiquitous. However, abundance varied between individual samples taken from terrestrial (e.g. leaf litter, sand, seaweed, animal faeces) or aquatic (marine or freshwater) environments, complicating the ability to predict the enterococci load from these sources. Enumeration of enterococci from bathing beach environments indicated occasionally high levels from seaweed and sand, where levels of up to 660 CFU/100 g (wet weight) were recorded from aged and degrading seaweed but not from fresh seaweed samples. Restriction enzyme analysis (REA) of isolates from degraded seaweed indicated a dominance of clonal populations and supported the notion of replication or survival of strains. Laboratory studies conducted to investigate enterococci persistence and growth on seaweed were not conclusive, although there was some evidence to suggest enterococci replication was occurring. This was indicated by molecular fingerprinting (REA analysis), which showed that the inoculated strain persisted for the full duration of experiments (up to 28 days). The isolation of non-inoculum strains from seaweed treatments, combined with increased abundance of these strains with incubation, suggested the persistence or replication of enterococci that were naturally occurring on seaweed. To investigate the occurrence of enterococci in bathing environments a statistical analysis of Auckland Regional Council (ARC) bathing water quality data was undertaken. This analysis indicated a strong positive correlation between enterococci and turbidity, and hence turbidity may serve as a useful physical measure to indicate deteriorating water quality. Surveys of three marine bathing beaches on Auckland's North Shore (Long Bay, Mairangi Bay, and Rothesay Bay) indicated the abundance of enterococci in all bathing beach environments surveyed. These included marine and fresh water, sand, seaweed, and stream sediment, and a significant association between enterococci levels found in the sand and in the seaweed. Enterococci screening protocols were evaluated for use in describing enterococci species and sub-species diversity in bathing beach environments. This investigation showed a diversity of enterococci from all beach environmental sources, with highest levels of species diversity from marine and stream water. Enterococci diversity did not provide clues as to the sources in marine water samples. RAPD-PCR analysis and phenotypic screening of enterococci isolates did not indicate a pattern of niche-specific associations of enterococci strains, and there was no strong evidence from this study that enterococci sub-species associate with specific environments. However, the presence of identical genotypes indicated that enterococci can persist and possibly replicate in environments such as sand and seaweed. Calculation of similarity coefficients from Ent. faecalis and Ent. casseliflavus sub-species groupings indicated a greater level of sub-specific similarity between selected environments, for example, seaweed:sand, marine water:stream water, seaweed:marine water, although this was not a guarantee that environments were biologically or ecologically associated. Where an elevated level of enterococci is measured in the absence in identifiable pollution source the separation of pigmented from non-pigmented enterococci was proposed as an indicator of the environmental or faecal nature of the enterococci within the sample. Although not tested under controlled conditions, this concept was found to have good utility for discriminating sources from elevated marine bathing water samples. Enterococci from one of 13 elevated ARC marine water samples examined was shown to be environmentally derived, with 5 of the 13 samples attributed to putative human or animal faecal sources. With further validation, this concept may be a useful means of determining source.

Pacific oyster (Crassostrea gigas) farming in New Zealand has reached a point where the pressures on resources appear likely to constrain current, and future, development. To maintain industry growth, security of juvenile oyster (spat) supply and productivity gains within the existing farm leases, are becoming industry imperatives. The use of hatchery technology could achieve both of these requirements, but it is expensive to establish and maintain. The additional expense of a hatchery could be offset by the establishment of, for example, a selective breeding program to enhance oyster productivity and/or marketability. Consequently a pilot-scale oyster hatchery facility was constructed to investigate the potential for establishing hatchery production of Pacific oysters in New Zealand. This study showed that in the pilot-scale hatchery, oysters could be successfully spawned from in-season broodstock, the eggs fertilised and the resultant larvae reared through to settlement for on-growing to adult size. This process was successful for both oysters selected for morphological characteristics and those which were not. On-growing of the resultant stock indicated growth rate could be normal relative to wild caught oysters spat, although data was limited by the small scale of the experiment. However, an investigation of broodstock conditioning, to achieve out-of-season spawning, was less successful. Disease occurred and condition was lost in some broodstock, suggesting they were enduring stress within the conditioning system. The microalgal food supply was examined but the clearance rate of the microalgal species suggested they were an acceptable feed supply which agreed with previous reports of successful conditioning techniques. Comparing the pilot-scale facility in this study with descriptions of facilities which reported successful broodstock conditioning suggested that the use of a microfiltered recirculating water supply, as opposed to the more common flow-through, natural seawater systems containing a range of small size particles, limited necessary nutrient and/or maturation factors and may have had a significant impact on conditioning. The nanoplanktonic (< 10 μm), food resource, which includes key microalgal species such as Chaetoceros spp. and Isochrysis spp., is generally considered the primary food resource for suspension feeding bivalves, including C. gigas. However, the picoplanktonic fraction (< 3 μm) can provide the largest proportion of this food resource in the water column in terms of abundance and biomass. Consequently, an investigation of the in situ retention of picoplankton populations (picoeukaryotes, Synechococcus-type cyanobacteria and heterotrophic bacteria) by oysters was undertaken. Flow cytometry was used to quantify the picoplankton populations in water samples taken from the inhalant and exhalant feeding currents of individual oysters, allowing retention efficiency of the particles to be calculated. Five picoplankton populations were identifiable by flow cytometry (picoeukaryotes, heterotrophic bacteria and 3 populations tentatively identified as cyanobacteria) and accounted for a large proportion (up to 97 %) of the estimated available carbon (picoplankton + microalgae) in Kerikeri Inlet water. Generally the heterotrophic bacteria accounted for the largest proportion of the biomass with up to 564 ± 51 ng C ml-1. Retention of each picoplankton population was found to be variable and not directly related to particle concentration. Cyanobacteria (Cy2 population) were retained with the greatest efficiency (up to 42 ± 4.4 %), followed by heterotrophic bacteria (up to 38 ± 4.5 %) and picoeukaryotes (up to 12 ± 3.8 %). Overall more picoplankton biomass was retained during the summer months, of which the heterotrophic bacteria made the largest contribution in either cell number or estimated carbon retained. Tracking of the condition and constituent fractions (glycogen, lipid and protein) of the subject oysters showed that in the summer months, post-spawn, these levels were lowest, indicting a period of nutritional stress. This appeared to suggest that C. gigas was able to alter its retention efficiency to expand the range of particles captured, and consequently the available nutrient pool. The retention of greater quantities of heterotrophic bacteria may allow for the acquisition of essential nutrients required for growth and later gametogenesis (such as B vitamins). However, it is also possible that the heterotrophic bacteria mediate access to otherwise inaccessible, or inefficiently accessed, nutrient resources through their degradation of, for example, crystalline cellulose. Consequently, the microbial flora associated with the oyster gut was investigated. An initial investigation, cultivating bacteria from gut contents, showed considerable variability in the numbers of colonies present within and between samples, but was inconclusive for identifying differences in species diversity. Using culture independent histological and 16S rDNA PCR/RFLP techniques to investigate the oyster gut microflora a spirochaete flora, commonly associated with bivalve crystalline styles, was clearly present. Molecular analyses provided evidence of other bacterial in the gut. A signature RFLP band pattern was found in oysters at low tide and this generally reoccurred in oysters that had been immersed for varying lengths of time up to high tide. However, the signature RFLP pattern became more dilute as immersion time/potential feeding time extended. The isolation of culturable bacteria from the oyster gut allowed characterisation and identification of a subset of the oyster gut microflora. 16S rDNA sequence analysis from selected isolates showed a predominance of Vibrio spp. These bacteria had previously been associated with marine molluscs, including as symbionts. Characterisation of these and other isolates from oyster gut showed a diversity of attributes including the ability to degrade cellulose. This suggests the bacterial production of enzymes, such as cellulases, which have been reported by other researchers as being present in ineffectual or low native levels in oysters. Consequently the bacterial presence in the oyster gut may be essential to efficient nutrient acquisition. The results of these investigations have highlighted the potential importance of the heterotrophic bacteria to C. gigas. To date, bacteria have received relatively little attention in terms of their potential nutritive contribution to oysters, primarily due to observations that they are retained with low efficiency. However, even at low retention efficiencies the potential nutritive contribution is large due to the available abundances of heterotrophic bacteria. While the mechanisms and controls of bivalve suspension feeding have yet to be fully elucidated, the published literature indicates that selective mechanisms are available to bivalves including C. gigas and this current research suggests that even pico-sized particles, retained with apparently low efficiency, can be subject to selection. The importance of the heterotrophic bacteria to C. gigas requires further investigation as it will have implications for not only hatchery production, but also farm management, public health and environmental impact monitoring.