How Oyster Health Indices Vary between Mass Mortality Events

Document Type


Publication Date



Coastal Sciences, Gulf Coast Research Laboratory


Ocean Science and Engineering


Two mass mortality events in 2016 and 2019 affecting the eastern oyster in the Mississippi Sound permitted an investigation of population dynamics during the collapse and recovery from systemic stressors capable of producing local extinction. In this study, metrics were reported as true population metrics with values included in proportion to the size class representation in the population. The approach was critical as trends in Dermo prevalence and weighted prevalence varied substantially between size classes, and this permitted elucidation of recovery trajectories not obvious from the population as a whole. The division of the population into larger and smaller size classes revealed a consistent differential in the time series for Dermo prevalence and weighted prevalence; these metrics did not return to levels observed before the 2016 mortality event in the 2-y interim between the 2016 and 2019 mortality events, and prevalence, in particular, remained low in the smaller size class. The time delay in the return of Dermo infection due to a substantial change in the size frequency of the population toward smaller initially uninfected animals is consistent with the differential in Dermo and young oyster cell doubling times and the required time delay, albeit relatively short, for transmission to occur. Gonadal thickness generally tracked with condition index and followed the expected seasonal cycle; any impact resulting from the 2016 mortality event on these metrics appeared to be inconsequential. The relationship between the adductor muscle diameter and shell height varied little over the time series and was much more stable than between the adductor muscle diameter and shell weight, suggesting a change in the oyster shell shape post-2016. By inference, shell weight is more variable among the three metrics in estimation of the condition index and standardization of other health indices. The ability to establish health metrics for both specific size classes and the population as a whole is essential to understanding the temporal impacts of a severe stressor or mortality event on oyster populations.

Publication Title

Journal of Shellfish Research





First Page


Last Page


Find in your library