Abstract:
The Australasian sea cucumber, Australostichopus mollis, is a deposit-feeding echinoderm that has attracted increasing interest for commercial fishing and aquaculture in New Zealand. According to an extensive literature review most of the information available for this species deals with the ecology and biology of adults and larvae, with only a few studies on the feeding biology and ecology of juveniles. Although this existing information can be used for aquaculture purposes in this species, there is still a need to better understand how the juveniles will respond to changes of extrinsic variables. Therefore, in this thesis the response of wild juveniles to changes in food availability and temperature was evaluated in the laboratory and used as a proxy for hatchery and grow-out scenarios. The effect of food availability in terms of organic matter content (1, 4, 12 and 20% TOM) was evaluated in terms of feeding behaviour (diurnal variation of ingestion rate and digestion), feeding biology (TOM selection from the food and absorption along the digestive tract) and growth. The effect of temperature (15, 18, 21 and 24 °C) was evaluated through changes in their metabolism (oxygen consumption and ammonia excretion), feeding behaviour, growth and macronutrient utilization (lipid, protein, and carbohydrate selection from the food and absorption along the digestive tract), with the determination of an energy budget for each temperature treatment. Finally, the effect of temperature and the presence/absence of seawater during the transport of juvenile sea cucumbers were also evaluated by measuring their subsequent feeding activity and overall well-being (i.e., skin lesions, evisceration and survival) after exposure to the different transport conditions. According to the results, the juvenile sea cucumbers feed more actively during night than day without changing their digestive activity. They are able to cope with a wide variation of TOM in their diet (from 4 to 20% TOM) in order to generate similar growth rates. The juveniles were able to compensate for the lower nutrient content and absorption in the 4% TOM treatment by increasing their selective feeding behaviour and ingestion rate. However, this was not observed in the juveniles fed with the 1% TOM treatment for which their body weight declined during the course of the experiment. Temperature was also found to be an important factor for the juveniles of this species. Temperature of around 24 °C turned out to be lethal for the juveniles, and the sea cucumbers responded negatively to increasing holding temperatures from 15 °C to 21 °C, which resulted in increased metabolism, reduced food intake and energy available for growth. The metabolism data showed that the juveniles rely more on protein as an energy source than lipid and carbohydrates regardless of the temperature. In terms of macronutrient selection and absorption, the juvenile sea cucumbers select and absorb lipid more efficiently than carbohydrate and protein at all experimental temperatures. Although selection and absorption of ingested nutrients are negatively affected by an increase of holding temperature from 15 °C to 21 °C, carbohydrate and protein remained as the dominant dietary macronutrients that are absorbed by the juveniles. Higher temperatures (i.e., above 17 °C) also proved to be negative for the successful transportation of juveniles due to an increase in their metabolism. However, lower temperatures (i.e., below 10 °C) are also detrimental for the feeding activity, increasing the appearance of skin lesions, evisceration and mortality and therefore also affect transportation success. Nonetheless, the results obtained indicate that the juveniles can be transported for short periods of time without seawater if desiccation is avoided. Transport of juveniles for longer periods of time (i.e., more than 8 hours) is possible provided they are kept with seawater and temperature is maintained within a suitable range (i.e., 12 - 15 °C). Overall, the results of the research presented in this thesis provide useful new information for the development of the aquaculture of this species, including improved methods for the transportation of sea cucumber juveniles from the hatchery to grow-out locations. Providing a better understanding of the feeding biology and growth of the juveniles and how they are affected by extrinsic factors, will help not only to improve the holding conditions of juveniles in the hatchery and nursery stage, but also help the selection of suitable aquaculture locations for grow-out, management of stocking densities, feeding regimes and the development of artificial diets.