Environmental Controls on Withering Syndrome in Abalone: A Modeling Study

Abalone populations along the California coast have been severely impacted by the marine disease, Withering Syndrome, which is caused by a rickettsia-like organism (RLO), Candidatus Xenohaliotis californiensis. A model that includes abalone growth, host susceptibility to RLO, RLO body burden, and the influence of temperature and food availability has been developed to assess biological and environmental controls on Withering Syndrome in abalone populations. The model implementation is based on experimental and field observations made for red abalone (Haliotis rufescens). Initial simulations reproduced the disease progression observed in red abalone that were held in laboratory tanks at constant temperature for more than one year. These simulations showed that after the initial infection, disease progression is determined by RLO growth in the host and that continued RLO exposure has little effect on the disease intensity. The model was then used to simulate RLO disease progression for red abalone exposed to fluctuating seawater temperature, which included warm events that persisted for days to weeks and events that persisted for months. These simulations showed that long-term sustained warm water temperatures, as occurs during El Niño conditions, encourage RLO proliferation in the host and enhanced disease progression. Shorter term warm events have little effect on RLO and disease progression. The model was also parameterized for black abalone (Haliotis cracherodii) that are infected by RLO and by RLO with a phage (hyperparasite) that reduces the growth of the pathogen. These simulations show that the occurrence of the phage significantly reduces RLO body burden and infection intensity. This abalone model provides a tool to forecast population response to environmental variations, to guide plans for conservation and restoration of wild populations, and to inform best management practices for abalone aquaculture.