The use of seal scarers as a protective mitigation measure can induce hearing impairment in harbour porpoises.

Acoustic deterrent devices (ADDs) are used to deter seals from aquacultures but exposure of harbour porpoises (Phocoena phocoena) occurs as a side-effect. At construction sites, by contrast, ADDs are used to deter harbour porpoises from the zone in which pile driving noise can induce temporary threshold shifts (TTSs). ADDs emit such high pressure levels that there is concern that ADDs themselves may induce a TTS. A harbour porpoise in human care was exposed to an artificial ADD signal with a peak frequency of 14 kHz. A significant TTS was found, measured by auditory evoked potentials, with an onset of 142 dB re 1 µPa2s at 20 kHz and 147 dB re 1 µPa2s at 28 kHz. The authors therefore strongly recommend to gradually increase and down regulate source levels of ADDs to the desired deterrence range. However, further research is needed to develop a reliable relationship between received levels and deterrence.

Drivers of the dive response in pinnipeds; apnea, submergence or temperature?

Long and deep dives in marine mammals are enabled by high mass-specific oxygen stores and the dive response, which reduces oxygen consumption in concert with increased peripheral vasoconstriction and a lowered heart rate during dives. Diving heart rates of pinnipeds are highly variable and modulated by many factors, such as breath holding (apnea), pressure, swimming activity, temperature and even cognitive control. However, the individual effects of these factors on diving heart rate are poorly understood because of the difficulty of parsing their relative contributions in diving pinnipeds. Here, we examined the effects of apnea and external sensory inputs as autonomic drivers of bradycardia. Specifically, we hypothesized that (1) water stimulation of facial receptors would - as is the case for terrestrial mammals - enhance the dive response, (2) increasing the facial area stimulated would lead to a more intense bradycardia, and (3) cold water would elicit a more pronounced bradycardia than warm water. Three harbor seals (Phoca vitulina) and a California sea lion (Zalophus californianus) were trained to breath hold in air and with their heads submerged in a basin with variable water level and temperature. We show that bradycardia occurs during apnea without immersion. We also demonstrate that bradycardia is strengthened by both increasing the area of facial submersion and colder water. Thus, we conclude that the initiation of the dive response in pinnipeds is more strongly related to breath holding than in terrestrial mammals, but the degree of the dive response is potentiated autonomically via stimulation of facial mechano- and thermo-receptors upon submergence.

Precocious hearing in harbour porpoise neonates.

Hearing is the primary sensory modality for toothed whales, but it is not known at which age it is fully developed. For newborn calves, hearing could fill an important function in maintaining contact with the mother and to develop echolocation skills. We non-invasively measured the auditory brainstem response (ABR) in two neonate (age 1-4 days) and three adult harbour porpoises (Phocoena phocoena). The stimuli consisted of clicks centred at 130 kHz, which is within the frequency band used for echolocation and communication in this species. The temporal pattern of the neonate ABRs was indistinguishable to the adult ones. There were no significant differences between calves and adults regarding hearing thresholds and ABR latencies. The ABR amplitudes were up to more than an order of magnitude larger in newborns than in adults, most likely due to the neonates' smaller size. These results indicate that hearing is fully developed within a day after birth, which suggests that harbour porpoise neonates have the earliest hearing development of any mammal studied so far. This may be explained by the evolutionary pressures imposed by the aquatic environment for a rapid development of the key sensory system in harbour porpoises.