# Long‐term trends of epibionts reflect Mediterranean striped dolphin abundance shifts caused by morbillivirus epidemics

## TL;DR

This study shows that changes in the populations of certain parasites on striped dolphins can indicate dolphin population shifts caused by a virus.

## Contribution

The paper introduces the first SIR model for dolphin morbillivirus and demonstrates how host-specific epibionts can track host population changes.

## Key findings

- DMV-induced population shifts in striped dolphins affect host-specific epibionts like Xenobalanus globicipitis and Syncyamus aequus.
- The less host-specific Pennella balaenoptera was not affected by dolphin population changes.
- Epibiont trends can serve as indirect indicators of host population dynamics.

## Abstract

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.

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.

This original research piece demonstrates, through empirical and theoretical modelling approaches, that the epibionts of striped dolphins indicate dolphin abundance shifts caused by epidemics of dolphin morbillivirus (DMV). In addition, we provide the first SIR model to investigate the epidemiology of DMV in western Mediterranean striped dolphins. Further epidemiological data will improve the predictive power of these models.