Nutrition and Reproductive Condition of Wild and Cultured New Zealand Scallops (Pecten novaezelandiae)
Wong, Ka Lai Clara
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The New Zealand native scallop, Pecten novaezelandiae, is a species with a high economic value as a wild catch and has good potential for cultivation. As a mean to enhance the future of this growing shellfish industry, this thesis set out to investigate the nutritional requirements of P. novaezelandiae in relation to reproductive conditions, and determined the physical and biological factors that affect the condition of this scallop species in the wild and cultivated environments. Adult scallops (Pecten novaezelandiae) were sampled from six populations in the Hauraki Gulf (Auckland, New Zealand) in the spawning season (October 2014), in order to evaluate the scallop reproductive condition and nutritional state across the populations. Results showed a spatial variation in reproduction condition (VGI and gonad index), with a higher number of mature scallops in populations closer to the shoreline, where higher food availability may be found. Conversely, nutrient content in scallop somatic tissues (adductor muscle carbohydrates and digestive gland lipids) did not vary across the populations, but was strongly associated with reproductive status of individual scallops (VGI). Nutrient (carbohydrates, proteins and lipids) storage and utilization were investigated within scallops from two sites in the Hauraki Gulf, bi-monthly over a year (2012−2013). In addition, sediment samples were also taken to evaluate the potential for re-suspended nutrients as a food source for scallops. Water samples were collected for seston and chlorophyll a analyses. Isotope analyses (carbon and nitrogen) and proximate analyses were conducted for the gonad, adductor muscle and digestive gland of wild P. novaezelandiae, sediment samples and the seston (1.2−5μm, >5μm). Isotope analyses revealed distinctly different signatures in suspended sediment and scallop tissues, indicating that re-suspended nutrients were unlikely to contribute to the diet of scallops. Nevertheless, seston (particularly the small fractions) signatures were closely related to scallop tissue samples, suggesting that it is likely to be the main food source for the wild P. novaezelandiae. Scallops from the two sampling sites exhibited similar reproductive cycles and utilization of nutrients. Gametogenesis started in winter, and took place at the expense of carbohydrates stored in adductor muscles. Spawning events were recorded in spring (October−November) and summer (January−March), and the energy demand required during spawning events was supported by digestive gland protein. Gonad re-maturation between spring and summer spawnings were supported by the utilization of digestive gland lipids. The reproductive condition and nutrient content of scallops were then studied during the spawning season (October 2013) in wild populations and within experimental conditions (fed with a commercial microalgal diet; Shellfish Diet 1800®) in an aquaculture laboratory, in order to identify condition and nutrient requirements for scallop cultivation in New Zealand. Field scallops (feeding on natural food sources) spawned just before the end of the experiment, while experimental animals reached gonad maturity at the end of the experiment, but did not spawn. The trend in gonad maturation for field and experimental animals indicates that there was a lag time of about 2 weeks, and that this lag is likely due to nutritional stress associated with the shift from natural food sources to the mixed microalgal formulated diet provided in the laboratory. Results indicate that experimental scallops had lower nutrient (carbohydrates, protein, lipids and total energy) reserves stored in adductor muscle tissues compared to wild animals, but both field and experimental animals utilized muscular reserves (especially carbohydrates and protein) to support reproductive activity. The fatty acid profiles revealed that polyunsaturated fatty acids (PUFA) were found in significantly lower quantities in gonad tissues of scallops from the laboratory compared to those in the field. This thesis shows that P. novaezelandiae utilizes energy reserves from both adductor muscle and digestive gland to cover the full cost of gametogenesis. In addition, cultivation environments using microalgal diets are conducive to condition P. novaezelandiae, but the optimal nutrient requirements for an efficient aquaculture production of this species needs further investigation. It is recommended by this thesis that future investigation on the conditioning requirements for P. novaezelandiae will be the next step for New Zealand scallop fisheries.