Optimising Growth Performance in Juvenile Hapuku (Polyprion oxygeneios): An Insight into the Physiology of a Novel Finfish Aquaculture Species

Abstract

Maximising production performance (growth rate and feed conversion ratio, FCR) is essential to the long term viability of finfish aquaculture. Production efficiency can also determine the viability of any new aquaculture species being considered for commercial culture. The current study aimed to understand how certain environmental and commercial conditions affect the production performance of a novel aquaculture species in New Zealand, the hapuku Polyprion oxygeneios. Additionally, the current study aimed to construct a simple energetic model for this species by collating the metabolic costs of feeding, growth and swimming within the physiological framework set by aerobic metabolic scope (AMS). AMS represents the physiological capacity for non-maintenance activities by setting the limit for aerobic ATP production. This work was done under the hypothesis that production performance is linked the capacity to perform physiological work and that maximal growth can only occur when AMS is non-limiting. Juvenile hapuku were employed to assess whether respirometric tests and thermal preference methods could be used to resolve the optimum temperature for growth and feed conversion efficiency in novel culture species (Chapter 2). On the basis that the energetic costs of rapid growth are substantial and need to be accommodated physiologically, it was hypothesised that maximal growth and optimal feed conversion would coincide with temperatures where aerobic metabolic scope was maximised. It was further hypothesised that hapuku would behaviourally self-select temperatures that lead to the greatest level of AMS, growth and FCR performance. Acclimating hapuku juveniles to 12, 15, 18, 21 and 24 °C for 4 weeks resulted in a peak in specific growth rate (SGR) in the range of 18 °C – 21 °C with slower growth at lower and higher temperatures. AMS was also maximal between 18 °C – 21 °C and was tightly linked with SGR. The behavioural thermal preference (Tpref) range of hapuku also fell within the optimum range for growth. FCR, however, was inversely related to temperature with the most and least efficient rates of conversion occurring at 12 °C and 24 °C respectively. Though AMS and Tpref had no utility in predicting the optimal range for FCR, standard metabolic rate (SMR) showed a positive linear relationship to FCR. The conclusions of this initial study are three-fold: i) hapuku select temperatures that optimise both AMS and growth, ii) AMS and Tpref appear tightly linked with SGR and could be used to predict the optimum temperature for growth in novel species and iii) AMS and Tpref have no utility in predicting the optimum temperature for FCR. Within Chapter 3, specific dynamic action (SDA) was measured in juvenile hapuku at two temperatures and two different ration sizes: 15 ºC (0.75% body weight [BW] and 1.5% BW) and 21 ºC (1.5% and 3% BW). To resolve whether SDA is functionally related to the SGR and FCR, SDA was then compared against the SGR and FCR of hapuku raised for 6 weeks at the same temperatures and ration levels. A marked effect of temperature on SDA was found and all SDA parameters (i.e. MO2 peak, duration, time to MO2 peak, limitation of AMS, SDA energy and SDA coefficient) were significantly higher for the shared ration size (1.5% BW) at 21 ºC than 15 ºC. However, only a few SDA parameters increased in magnitude with ration size (i.e. MO2 peak and SDA energy). Although SDA is thought to reflect the cost of growth in fish, larger SDA parameters were not linked with SGR which was significantly lower at 21 ºC than 15 ºC for the shared ration size of 1.5% BW. Further observations confirmed that there is a complex and interactive effect of temperature and ration size which is driving the SDA - growth response of juvenile hapuku. Specifically, elevated growth potential through larger SDA responses were present at the higher temperature of 21 ºC, but the higher maintenance cost (i.e. SMR) of fish at this temperature constrains growth if a restricted 1.5% BW ration is delivered. FCR was largely independent of ration size and was significantly less efficient at 21 ºC, despite fish having a larger SDA response. No single SDA parameter was found to predict the SGR and FCR performance of juvenile hapuku but future research might focus on the SDA response of fish to multiple (vs. single) feeds per day as this conveyed a significant growth benefit to hapuku for reasons not yet explained by metabolism. Chapter 4 examined the applicability of exercise training for juvenile hapuku as induced-swimming can improve the growth and FCR of finfish aquaculture species, such as salmonids and Seriola sp. However, the response to exercise is not universal and some species (such as Atlantic cod Gadus morhua) show no or a negative productivity response to exercise. As a possible explanation for these species-specific differences, a recent hypothesis proposed that the applicability of exercise training, as well as the exercise regime for optimal growth gain (ER opt growth), was dependent upon the size of available AMS. This study aimed to test this hypothesis by measuring the growth and swimming metabolism of hapuku to different exercise regimes and collating the metabolic costs of swimming and growth (i.e. specific dynamic action, SDA) against AMS. Two 8-week growth trials were conducted with ERs of 0.0, 0.25, 0.5, 0.75, 1 and 1.5 body lengths per second (BL s-1). Fish on a relatively high growth trajectory showed a small but positive growth response to exercise but only in the range of 0.5 BL s-1 to 0.75 BL s-1 compared to static water controls. Slightly larger fish on a slower growth trajectory, however, showed no evidence of exercise-induced growth. Long-term exposure to water flow at 0.75 BL s-1 and 1.5 BL s-1 also yielded no difference in the swimming metabolism of fish relative to static water controls. Combining the SDA of hapuku with the metabolic costs of swimming showed that hapuku have sufficient physiological capacity to support growth and swimming at all ERs. The current study therefore suggests that exercise-induced growth was not limited by AMS and possibly varies as a function of species, life stage and/or inherent growth trajectories. As a follow on from the exercise trials, Chapter 5 examined the role of exercise in altering the flesh quality of juvenile hapuku. Consumers base the quality of fish on factors such as taste, texture, appearance, freshness and odour and it has been suggested that exercise could modify some of these factors in a way that affects consumer preferences. The effect of exercise training on the flesh firmness of juvenile hapuku was therefore investigated to determine whether exposing them to increased water currents can produce a firmer, and presumably more desirable product. Flesh samples were taken from fish that had been exercised for 8 weeks at either 0.0, 0.75 or 1.5 BL s-1 and used to determine firmness (textual analyses), muscle fibre density (MFD) and the distribution of muscles fibre sizes from prepared histological slides. MFD and flesh firmness were negatively related to ER and decreased significantly between 0.0 BL s-1 and 0.75 BL s-1 suggesting a stimulation of muscle fibre hypertrophy with continuous swimming. Fibre size distributions also showed an increase in median muscle fibre size with the increasing exercise regimes. The full effects of exercise training on flesh quality, particularly the effect on organoleptic properties, need to be determined before a definitive decision can be made on the applicability of exercise training to hapuku. Within Chapter 6, the costs of SDA and swimming were collated under the metabolic framework of AMS and presented as a model of the energetics of juvenile P. oxygeneios in culture, with particular reference to 17 °C where the costs of swimming were measured. Additionally, the kJ requirements of hapuku under different conditions were calculated and compared to ingested digestible energy (DE) from feed. The assumptions of the energetic equations, the apparent discrepancy between DE intake and energy usage and factors that would modify energy usage are all discussed in detail and compared to other species of interest. To conclude, recommendations for farming practice and future work are made based on the empirical data of the model.