Effects of a morbillivirus epizootic on long-finned pilot whales Globicephala melas in Spanish Mediterranean waters

Abstract

Long-finned pilot whales Globicephala melas are a commonly encountered species in the Mediterranean Sea. In 2006−2007, an outbreak of the dolphin morbillivirus in the Western Mediterranean resulted in increased mortality of this species. The aim of this study was to determine whether survival rates differed between clusters of Spanish Mediterranean pilot whales, and how the epizootic influenced these survival rates. Photo-identification surveys were conducted between 1992 and 2009. Association indices were used to define clusters of individuals that associate with each other more frequently than with others. Based on a Cormack-JollySeber survival rate model, apparent survival rate estimates varied from 0.821 to 0.995 over 11 clusters for the 1992−2009 period. When the effect of the morbillivirus outbreak was modeled, 3 clusters with distinctly lower survival rates from previous models presented lower estimates after the outbreak (survival rate dropped from 0.919 [95% CI: 0.854−0.956] to 0.547 [95% CI: 0.185−0.866]), suggesting a negative influence of the epizootic or other unknown additive factors on certain clusters.

Epizootic effect and aftermath in a pilot whale population

“…The observed decline is likely related to the epizootic, considering the large increase of morbillivirus‐positive strandings observed during the same period (Fernández et al, ). Moreover, the survival rate decrease was similar to what was estimated for some social clusters (27%) of the neighbouring Alboran Sea pilot whales (Wierucka et al, ), although there it did not affect the entire population. Singer, Zeigenfuss, and Spicer () suggested that a terrestrial mammal population larger than 250 individuals with a large home range and migratory movements (i.e.…”

“…Worldwide, CeMV epizootics have caused higher stranding rates over a few months locally but could last for years when the virus was transmitted over larger areas (Van Bressem et al, ). However, results found in this study and in the nearby Alboran Sea (Wierucka et al, ) show that the effects, whether direct or indirect, can span over at least 5 years. Cumulative effects of some of the following hypotheses could potentially explain these results: A post‐epizootic chronic effect of the morbillivirus as reported for infected individual striped dolphins in the Spanish Mediterranean coast 4 years after the epizootics (Domingo et al, ; Soto et al, ). High immunosuppressive contaminant levels, such as polychlorinated biphenyls and dichlorodiphenyltrichloroethanes and high biomarker responses (CYP1A1 and CYP2B) (Lauriano, Di Guardo, Marsili, Maltese, & Fossi, ).…”

“…The population size estimate increased between 1999 and 2006, along with a positive population growth rate and a high apparent survival rate within the range of normal values for adults of this species in other regions (Martin & Rothery, ; Wierucka et al, ). Then, in 2006, apparent survival rate declined and population growth rate became negative.…”

“…with a large carrying capacity) is more likely to recover rapidly after an epizootic. The Alboran Sea pilot whales meet these characteristics (Cañadas & Sagarminaga, ; Verborgh et al, ; Wierucka et al, ). They could, therefore, be less impacted and may recover more rapidly than the animals from the strait, if the same logic applies to this species.…”

“…The Strait of Gibraltar population had a high survival rate and a positive annual population growth rate before the epizootic (Verborgh et al, ). In the nearby Alboran Sea, the survival rate of some pilot whale social clusters decreased by 37.2% after the epizootic event (Wierucka et al, ).…”