Risk factors for summer mortality in greenlip abalone (Haliotis laevigata) and hybrid abalone (H. laevigata × Haliotis rubra): A case-control study

In Australian aquaculture, abalone mortality during warm water temperatures has been termed summer mortality. Summer mortality likely involves complex interactions between biological, environmental, husbandry and nutritional factors; however, this is poorly understood. In this study, we conducted a case-control study for farmed greenlip abalone (Haliotis laevigata) and hybrid abalone (H. laevigata × Haliotis rubra) to identify risk factors associated with summer mortality. Farm mortality data from 2015 to 2021 were used to identify tanks which fit the case definition and tanks that did not. 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). 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 parameters selected in the final model were involved in one or more interactions. The model identified interactions between age and each of previous year summer mortality, size when removed from weaning tanks and feed rate. There were also interactive effects of previous year summer mortality and size when removed from weaning tanks, and of feed rate and post-grading mortality. The 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 had no effect on the risk of summer mortality. Abalone in tanks which experienced summer mortality the previous year were at a higher risk of summer mortality regardless of age. For abalone that did not experience summer mortality the year prior, the subsequent risk of summer mortality increased with abalone age. Improved understanding of summer mortality risk factors may help guide management strategies and future research to advance abalone aquaculture.