In Australian aquaculture, abalone mortality during summer has been termed summer mortality. We conducted a case-control study to identify risk factors associated with summer mortality in farmed Australian abalone. We reviewed scientific literature, ten years of abalone laboratory submissions and collaborated with Australian abalone farmers to develop a case definition for summer mortality. Farm mortality data (2015-2021) were used to identify tanks which fit the case definition (cases) and tanks which did not (controls). We constructed a model to predict summer mortality risk using logistic regression and forward variable selection. Modelling considered 61 potential risk factors (examples included water quality, husbandry, climate, biology) with the final selected model including six main effects and five two-way interactions.
Except for maximum water temperature in the week prior to the case/control date, all other parameters selected in the final model were involved in one or more interactions. The model included interactions between age and each of previous year summer mortality, size at weaning and feed rate. There were also interactive effects of previous year summer mortality and size at weaning, and of feed rate and post-grading mortality. Risk of summer mortality was two times more likely for every 2 °C increase in maximum weekly water temperature. Post-grading mortality was associated with an increased risk of summer mortality, but relative risk decreased at higher feed rates. Risk of summer mortality in younger abalone increased with higher feed rates, while increasing feed rates for 30-month abalone did not. Abalone in tanks which experienced summer mortality the previous year were at a higher risk, regardless of age. Risk of summer mortality increased with abalone age for abalone that did not experience summer mortality the year prior.
We also investigated summer mortality events in 2021 and 2022 (January to April) for primary pathogens and infectious agents. Farmers submitted 30 abalone from case tanks and 10 abalone from control tanks to veterinary diagnostic laboratories for gross and histopathological examination and microbiological culture. Vibrio spp. were detected in both case and control abalone, with a higher prevalence in case animals. Bacterial presence did not always translate to clinical disease. In some animals from case tanks, V. harveyi and Vibrio spp. caused disease as there were gross and histopathological findings consistent with vibriosis. However, mixed bacterial growth, and not one uniform pathogen, was detected in both case and control abalone. There was no evidence that V. harveyi and Vibrio spp. were consistently the primary cause of mortality. The bacterial pathogens detected were secondary (opportunistic) to primary stressor(s).