Long-term trends of epibionts reflect Mediterranean striped dolphin abundance shifts caused by morbillivirus epidemics.

Since 1990, Mediterranean striped dolphins, Stenella coeruleoalba, have suffered two mass mortality events caused by the dolphin morbillivirus (DMV), but the population-level impact is unknown because abundance estimates are spatio-temporally sparse. This study investigates whether data from epibionts of striped dolphins-the barnacle Xenobalanus globicipitis, the cyamid Syncyamus aequus, and the copepod Pennella balaenoptera, with different life cycles and degrees of specificity-could provide indirect evidence on host population dynamics. To address this question, we combined empirical and theoretical approaches. First, we used Generalized Additive Models (GAMs) to examine occurrence trends of the three epibiotic species over the period 1980-2023 for both striped dolphins and other sympatric cetacean species that did not suffer DMV outbreaks. Second, we developed a two-step theoretical modeling approach to investigate the epidemiology of these DMV outbreaks (SIR model) and to link dolphin population abundance shifts with the epibiont trends observed empirically (mechanistic model). The SIR model provided coarse estimates of the impact of DMV on the striped dolphin population under two scenarios with varying virus-induced mortality and duration of the infectious period. These estimates were then used to simulate the effect of dolphin population shifts on its epibionts through mechanistic models. Models indicated that DMV-induced shifts in striped dolphin population dynamics have cascading effects on the population abundance of X. globicipitis and S. aequus, whereas the population of the less host-specific P. balaenoptera was unaffected. Together, long-term trends in the occurrence of host-specific epibionts can serve as an indicator of host abundance shifts.