High field metabolic rates of wild harbour porpoises

Abstract

Reliable estimates of field metabolic rates (FMRs) in wild animals are essential for quantifying their ecological roles, as well as for evaluating fitness consequences of anthropogenic disturbances. Yet, standard methods for measuring FMR are difficult to use on freeranging cetaceans whose FMR may deviate substantially from scaling predictions using terrestrial mammals. Harbour porpoises (Phocoena phocoena) are among the smallest marine mammals, and yet they live in cold, high-latitude waters where their high surface-tovolume ratio suggests high FMRs to stay warm. However, published FMR estimates of harbour porpoises are contradictory, with some studies claiming high FMRs and others concluding that the energetic requirements of porpoises resemble those of similar-sized terrestrial mammals. Here, we address this controversy using data from a combination of captive and wild porpoises to estimate the FMR of wild porpoises. We show that FMRs of harbour porpoises are up to two times greater than for similar-sized terrestrial mammals, supporting the hypothesis that small, carnivorous marine mammals in cold water have elevated FMRs. Despite the potential cost of thermoregulation in colder water, harbour porpoise FMRs are stable over seasonally changing water temperatures. Varying heat loss seems to be managed via cyclical fluctuations in energy intake, which serve to build up a blubber layer that largely offsets the extra costs of thermoregulation during winter. Such high FMRs are consistent with the recently reported high feeding rates of wild porpoises and highlight concerns about the potential impact of human activities on individual fitness and population dynamics.

Using Respiratory Sinus Arrhythmia to Estimate Inspired Tidal Volume in the Bottlenose Dolphin (Tursiops truncatus)

“…Proxies to estimate FMR from breaths should accurately predict f R and V T during continuous recording from free ranging animals (Fahlman et al, 2016; Rojano-Doñate et al, 2018). Such data would allow an assessment of how changes in foraging effort (duration, activity, etc.)…”

“…The RSA causes f H acceleration during inspiration, and deceleration during expiration (Mortola et al, 2015). Thus, continuous recordings of f H could allow detection of f R , which when appropriately validated provide ways to estimate field metabolic rate (Fahlman et al, 2016; Rojano-Doñate et al, 2018). Considering recent progress in the development of biologging system that allow continuous recording of the electrocardiogram (ECG) in free-ranging cetaceans (Elmegaard et al, 2016; McDonald et al, 2018), we speculated that RSA may provide a novel method to estimate V T in bottlenose dolphins.…”

Movement and Seasonal Energetics Mediate Vulnerability to Disturbance in Marine Mammal Populations

“…Our model successfully made independent predictions based on ubiquitous and low-level mechanisms, still, several simplifying assumptions were made in model development when data were lacking or for maintaining model tractability. When using the body mass and field metabolic rate relationship (Rojano-Donãte et al 2018) to calibrate the maintenance normalization constant, specifics on the storage levels and thermal environments experienced by animals were unknown, so we assumed that animals were not expending energy on thermoregulation. Any thermal costs actually incurred by observed animals would, therefore, be attributed to the estimation of maintenance costs; resulting in modelled thermal costs being present only when rates of heat loss exceed those of the observed animals.…”

Harbor Porpoise (Phocoena phocoena) Reaction to a 3D Seismic Airgun Survey in the North Sea

“…A few disturbance events are likely insignificant to the energetic status of a porpoise, but these disturbances may have fitness consequences when repeated frequently (Wisniewska et al, 2018). Porpoises are small cetaceans living in a relatively cold environment, resulting in a high demand for energy (Rojano-Donþate et al, 2018). Indeed, harbor porpoises have been shown to forage almost continuously in some areas; therefore, the disturbance of foraging activities is important to quantify and assess (Wisniewska et al, 2016).…”