Institute for Terrestrial and Aquatic Wildlife Research (ITAW) - Effects of masking on hearing thresholds of grey seals

Projekter: ProjektAndre



The status of the German grey seal populations have gradually improved in recent years, making it the second most abundant seal species in German waters after harbour seals (Phoca vitulina). However, an increase in grey seal numbers as well as in anthropogenic activities near grey seal habitats has led to a higher risk of interactions between seals and humans. Grey seals potentially suffer a multitude of auditory and behavioural impacts from anthropogenic noise exposure both under water and in air on their haul-out sites. Limited information is available on the effects of exposure from the many different types of existing anthropogenic sound sources. As a matter of fact, baseline data on the hearing ability of grey seals is currently lacking. Such data is fundamental to determine the impact on the animals. Studies on other marine mammal species, however, have established that exposure to noise can have far-reaching consequences. Anthropogenic sound exposure can increase stress levels and alter or interrupt important behaviours of marine mammals (for example, mating or foraging).

The North and Baltic Seas are some of the marine areas worldwide that are most heavily used by humans. Human activities include oil and natural gas exploration, military training and operations, construction and operation of offshore wind energy installations, shipping traffic, fishing, transportation, and recreation (Heitmeyer et al. 2004, Hildebrand 2004, Hatch et al. 2008). Anthropogenic activities have unintended consequences to the environment with impacts that are modest to severe across widely varying spatial and temporal scales. For example, toxic contamination from run-off impacts biological processes of marine creatures and bioaccumulation in marine food sources can subsequently harm humans. Climate change is another factor influenced by human activity with impacts that occur globally. Of concern to this proposal is anthropogenic noise, or noise pollution, which has dramatically increased in the oceans over the last several decades (Hildebrand 2004).

Intensive acoustic exposure can cause temporary impairment or permanent damage to the sense of hearing, as well as negatively impact communication or interfere with the detection and identification of biologically important sounds. Estimates of the impact of anthropogenic noise on grey seals first require an understanding of the frequency range of hearing and hearing sensitivity of this species. Within a population, the characteristics of hearing can vary by age, gender, and an individual’s prior history with noise exposure, and disease and health states. Thus, it is important to determine the expected distribution of hearing variations within a population when addressing how various forms of anthropogenic sound might impact the population. For the grey seal, population-level audiometry will require the use of evoked potential methods. However, since behavioural audiometry is the accepted standard for threshold estimation, the evoked potential method needs to be benchmarked against behavioural methods within this species. Subsequently, the collection of evoked potential audiograms can be used to determine population-level variability in a manner that should relate it back to the behavioural audiometry.
For the proposed project, two captive seals will receive specialized training for voluntary participation in auditory testing using standard operant conditioning methods with positive reinforcement. First high resolution in-air and underwater audiograms will first be obtained using psychophysics. Second the same seals will be trained to voluntarily participate in electrophysiological measurement methods, called “auditory evoked potentials” (AEP), allowing for the comparison of AEP and behavioural audiometry from the same individuals. This will make it possible to relate the AEP measurements of wild seals, and will provide information on the hearing ability of the grey seal in air and under water, which will be crucial for assessing any possible effects of anthropogenic noise in this species.
Effektiv start/slut dato01/04/201931/08/2021


  • University of Veterinary Medicine Hannover, Foundation
  • SDU (leder)