Does the dietary inclusion of Ulva meal improve the survival of abalone during summer on two Australian abalone farms?

A research priority identified by the Australian Abalone Growers’ Association (AAGA) in 2019, prior to the commencement of the project, was to improve the survival of commercially cultured abalone during periods of high summer water temperatures (> 22°C). Two separate on-farm commercial trials were run at Jade Tiger Abalone (Avalon, Victoria) [JTA Avalon] and Yumbah Narrawong (Allestree, Victoria). The trials were both run on a commercial basis using routine farming procedures used by each company. The trials were designed and run to investigate the potential to improve survival and growth by feeding diet containing 10% dried Ulva meal sp. compared to a commercial diet within farms. Abalone in both farm trials experienced mild summers and water temperatures. However, the JTA Avalon trial experienced several instances where water temperatures exceeded 22°C during January and February. The maximum water temperature experienced during the Yumbah Narrawong trial was 22.5°C in early January and remained relatively low compared to those recorded at JTA Avalon. Summer mortality was observed during the farm trial at JTA Avalon, whereas water temperatures were too low during the Yumbah Narrawong trial to induce summer mortality. The dietary inclusion of 10% dried Ulva sp. meal did not result in any significant improvement in survival during either farm trial. In fact, survival appeared to be lower during both farm trials when 10% dried Ulva sp. meal was included in the diets (JTA Avalon survival: commercial diet survival 96.4% vs. 93.4% for the Ulva diet; Yumbah Narrawong survival: commercial diet 98.1% vs 97.5 % for the Ulva diet). The dietary inclusion of 10% dried Ulva sp. meal did not appear to lead to any significant alteration in digestive tract or gill structure. Heat stress, in combination with commercial culture stressors, appeared to be a factor associated with histopathological alterations observed in abalone during the study, which were chiefly noted in the gill leaflet tips of Tiger Abalone. Gill leaflet tip damage scores appeared to be useful indicators of damage associated with the combination of culture stressors and heat stress in abalone. Overall, the dietary inclusion of 10% dried Ulva sp. meal did not lead to improvements in survival of Tiger or Greenlip Abalone cultured during summer. Based on the data obtained from the Yumbah Narrawong trial, improvements in growth performance (~9.6% improvement in SGR), feed and nutrient efficiency and basic sales revenue were obtained when abalone were cultured under mild summer growing conditions and fed the Ulva diet compared to the commercial control diet.