Physiological and behavioural responses to environmental stress in abalone : why is being a hybrid an advantage?

Australian abalone aquaculture is facing commercial and environmental challenges that may impact abalone performance and industry profitability. This calls for a more in-depth understanding on how abalone respond to environmental change to ensure that abalone farming remains sustainable. The aim of this thesis was to determine the physiological and behavioural responses of the commercially important Australian abalone species, blacklip abalone Haliotis rubra, greenlip abalone H. laevigata, and their interspecies hybrid to environmental and farm stressors. The main question centres on the most important commercial abalone, the hybrid, and asks why it has improved growth in comparison to pure parental species. The life of an abalone on Australian abalone farms begins in the hatchery where biotic and abiotic factors are relatively well-controlled. Yet, the physiological and behavioural responses of early life-stage abalone to rearing conditions are largely unknown. The influence of stocking density, oxygen availability, and light levels on oxygen consumption rate (`¬¿œÄvÑO_2`) was tested with fertilised eggs and all larvae stages of hybrids. In addition, in a second study acute thermal preference, swimming speed, and `¬¿œÄvÑO_2` were determined across an ecologically relevant temperature range for veliger larvae of H. rubra, H. laevigata, and their hybrid. Current farm conditions, i.e. stocking densities, light and oxygen levels had no influence on \\(¬¿œÄvÑO_2\\) of early-life stages of hybrids. Thermal preference of all three abalone groups increased during larval development from 16 to 20 ¬¨‚àûC for early to late veligers, respectively. Veliger `¬¿œÄvÑO_2` increased throughout the temperature range tested in all three abalone groups and `¬¿œÄvÑO_2` of hybrids reached a peak at 25 ¬¨‚àûC. These results provide support that current hatchery conditions are generally within optimal ranges for early-life performance. As juveniles, abalone are exposed to uncontrolled environmental conditions, e.g. temperature and oxygen level, in grow-out tanks. Juveniles rely frequently on anaerobic energy production and may increasingly depend on it during exposure to stressful conditions, reducing their potential for growth. I tested whether 15 month old juvenile hybrids have a distinct movement behaviour, and/or differ in their use of aerobic and anaerobic energy sources in comparison to parental species when exposed to various temperatures and oxygen levels. Enzyme activities of tauropine and lactate dehydrogenases were similar between hybrids and pure species. Hybrids tended to have an intermediate movement pattern between the more active H. rubra and the less active H. laevigata, and a lower resting `¬¿œÄvÑO_2` in comparison to both pure species. These differences were indicative but not significant to allow support of the hypothesis that hybrids have an energetic advantage over pure species. Some Australian abalone farmers notice higher mortality rates of larger abalone during summer months, suggesting that abalone may be vulnerable to warming and/or hypoxia. Movement, `¬¿œÄvÑO_2`, and heart rate were determined in 22 month old hybrids, H. rubra, and H. laevigata during acute temperature increase following acclimation to control and typical summer farm conditions. Movement of hybrids and H. laevigata was not affected by environmental condition, while H. rubra showed a strong thermal response. Heart rate- and `¬¿œÄvÑO_2`-temperature relationships of hybrids remained similar irrespective of oxygen level, while pure species adjusted both parameters. Heart rate and `¬¿œÄvÑO_2` maxima of hybrids were more stable than those of both pure species across acclimation conditions. These results support the hypothesis that hybrids are less sensitive to changes in environmental conditions, which may contribute to their growth advantage over long grow-out periods in the abiotically unstable farm environment. In summary, the studies outlined in this thesis have improved our understanding of physiological and behavioural responses of abalone to the most important environmental factors that govern abalone performance. Hatchery conditions are unlikely to negatively influence abalone performance. Yet, fluctuating temperatures and oxygen levels in the grow-out tanks lead to exposure to environmental conditions near the tolerance limits of abalone during summer months, rendering abalone vulnerable to future environmental challenges. The hybrids are more resilient than the pure species to current on-farm fluctuations, suggesting that hybrids are most suited for the aquaculture environment. The farm environment, however, is complex and abalone are likely to encounter additional stressors during culture which may result in increased metabolic constraints and thus reduced growth potentials. The successful laboratory trial, with heart rate sensors, in this thesis opens the door to future on-farm studies to address how additional stressors influence abalone physiology and underpin aquaculture production and sustainability.