Poor body condition associated with an unusual mortality event in gray whales

Abstract

The eastern North Pacific gray whale Eschrichtius robustus experienced an unusual mortality event (UME) in 2019-2020, with 384 whales found dead along the Pacific coasts of Mexico, USA and Canada. A similar UME in 1999-2000 was speculated to have been caused by starvation, but body condition data were not available to test this hypothesis. Between 2017 and 2019, we used unmanned aerial vehicles (drones) and photogrammetry methods to measure the body condition of gray whales in San Ignacio Lagoon, Baja California Sur, Mexico. Body condition was calculated from the residual of the relationship between body volume and length. The body condition of gray whales was significantly lower in 2018 (-11.1%, SE = 1.74, n = 531) and 2019 (-9.7%, SE = 1.76, n = 628) compared to 2017 (n = 59) for all reproductive classes (calves, juveniles, adults and lactating females). Overall, lactating females were in good body condition. The reduction in body condition of whales in 2018-2019 is unlikely to have affected their survival, but could have reduced their reproductive rate by prolonging the post-weaning recovery time. This could explain the low number of mother-calf pairs observed in the San Ignacio Lagoon in 2018 and 2019. For juveniles and adults that arrived in the lagoons with less energy reserves, their reduced body condition may have been close to their survival threshold. This could explain the high proportion of juveniles and adults among the stranded dead whales in 2019-2020. Although the underlying cause of the reduction in gray whale body condition is unknown, starvation likely contributed to the 2019-2020 UME.

Flexible use of a dynamic energy landscape buffers a marine predator against extreme climate variability

“…Tuna consumed relatively more energy than average only at times when they migrated unusually far north (figures 2 c , 3 d and 4 b ). Declines in local prey availability or quality, coupled with a lack of migratory flexibility, appear to explain why some top predators including sea lions, grey whales and some seabirds experienced starvation-induced mass mortalities in the California Current in the years immediately following this event [51–54]. Future work could complement our analyses of variation in the tunas' energy landscape by predicting the distributions of important prey species in relation to climate variability.…”

Rapid weight loss in free ranging pygmy killer whales (Feresa attenuata) and the implications for anthropogenic disturbance of odontocetes

“…With increasing awareness on the implication of non-lethal effects of human disturbance and environmental changes 52 on cetaceans 6 , 14 , mechanisms to understand the relationship between disturbance that results in disruptions in foraging, and animal health are increasingly important. Future applications of UAS-photogrammetry to detect changes in body volume for odontocetes should consider comparisons of impacted and non-impacted populations to investigate long-term implication of disturbance on body condition.…”

“…As such, disturbances that result in decreased foraging efficiency can result in reduced fitness of a population 12 . Quantifying the impacts of foraging disruptions on population health will lead to a better understanding of the biological significance of disturbance and how it affects vital rates 13 , 14 . Here we quantify the weight loss of free-ranging pygmy killer whales ( Feresa attenuata ) through repeated measurements of the same individuals during an extended disruption in foraging activity over 21 days.…”

“…Reductions in body mass as a result of fasting have been documented in migrating and breeding baleen whales 14 – 16 as well as captive harbor porpoises 17 . However, this metric is logistically difficult to obtain for free ranging odontocetes, yet represents a fundamental component of energetic-based approaches to estimating effects of disturbance, such as the Population Consequences of Disturbance (PCoD) model framework 18 .…”

Predicting Harbor Porpoise Strandings Based on Near-Shore Sightings Indicates Elevated Temporal Mortality Rates

“…Increases in mortality, including UMEs, are of interest for ecologists as well as in the context of conservation management (Ten Doeschate et al, 2018). Marine mammal mortality events can result from anthropogenic activities (Fernández et al, 2005;Schwacke et al, 2014;Sharp et al, 2019), disease outbreaks (Rubio-Guerri et al, 2013;Kemper et al, 2016), or nutritional deficiency (Trites and Donnelly, 2003;Christiansen et al, 2021). Obviously, there may be combinations of factors involved, and pinpointing one cause is not always possible (Mazzariol et al, 2011;IJsseldijk et al, 2018d).…”