Noise has recently been acknowledged as an important source of pollution in EU’s Marine Strategy Directive. Danish waters are heavily ship-trafficked, and shipping noise may affect marine mammals negatively by impeding their use of sounds for echolocation and communication. The scale of impact of anthropogenic ocean noise is difficult to quantify and still largely unknown.

In my biological project I recorded broad-band noise (25Hz-160kHz) in Aarhus Bay to investigate the effect of shipping noise on ambient noise levels. Recordings were correlated with an audiogram for harbor porpoises (phocoena phocoena) to assess the potential impact on the high-frequency species, which have their most sensitive hearing range at 100-140kHz, and produce echolocation clicks at 120-150kHz.

I found that shipping noise significantly increased oceanic noise levels across all frequencies, reaching noise levels up to 180dB re µPa (at 1kHz), and that both conventional and fast-ferries emit significant levels of high-frequency noise in the range of harbor porpoise hearing and echolocation (100-150kHz). Contrary to common beliefs the slower conventional ferry emitted the highest levels of high-frequency noise, potentially causing acoustic masking for porpoises more than 1000m away, compared to a masking zone of 500m for the fast-ferry.

In my master thesis I am recording ship noise levels more thoroughly by recording different types of vessels such as ferries, cargo ships and motorboats. I will record the vessels at varying distances to be able to approximate the attenuation of noise, and with longer timeframes to better assess the impacts of shipping noise on harbor porpoises. Approaching the scale of impact of anthropogenic noise is necessary to optimize the demands EU is sets for vessels in the Marine Strategy Directive, and highly relevant for the ongoing planning of an improved future for the Baltic Sea.

Harbour seals are very versatile predators that feed on many different prey types. So far they have been classified as raptorial feeders, meaning that they catch prey by closing their jaws around it. This feeding mechanism is typical of terrestrial animals, but less common among aquatic animals. The reason for this pattern is probably due to the difficulties connected with using raptorial feeding in water. Because water is much denser than air, pressure drag (and the bow wave) will tend to push the prey item away from the predator. Thus aquatic raptorial feeders often have long, slender pincers jaws that create less pressure drag. Another method for overcoming the obstacles of feeding in water is to use suction feeding. Suction feeders often have fewer or smaller teeth and a rounder mouth opening suited for creating suction and this mechanism is found among many aquatic animals. The question therefore is whether harbour seals are able to use suction feeding.

In a study I found that harbour seals do use suction feeding when capturing small prey items, like sand eel, but raptorial feeding when capturing bigger prey species, like herring. The physical appearance of the harbour seal, with its short jaws and raptorial teeth, supports the findings. These results demonstrate how the harbour seal is perfectly adapted to its environment and lifestyle.

In my master thesis I will search for a so-called signature jerk that resembles a feeding event in harbour seals. This will be attempted by placing an accelerometer on the heads of captive seals and possibly on wild ranging seals later. A goal could be to compare general diving profiles of harbour seals with the results obtained from the accelerometer to see if feeding does indeed occur at times when it is suspected to occur.

My project concerns the functional evolution and adaptation of auditory systems of different animals to life in air and underwater.

Sound consists of a pressure component as well as a particle motion component of the medium in which it travels. Thus, animals can hear a sound by detecting either the sound pressure or the particle motion.

In water, tissues are, as they have more or less the same impedance as water, transparent to sound pressure. In fish, the problem of detecting sound is commonly solved by having an otolith ear for detection of particle motion. The otoliths, being denser than tissue and water, have a higher impedance and are therefore accelerated differentially relative to the rest of the fish. This differential acceleration leads to hair cell deflection and thus enables fish to detect the particle motion of the sound. Further fish that have a swim bladder are able to detect the pressure component of the sound by detecting the particle motion made by the pressure-induced swim bladder oscillations.

In air it is the pressure component from a sound source that is detected. As the impedance of air differs from the impedance of tissue, most of the sound energy is reflected off when impinging on an animal. Thus, it requires adaptations for terrestrial animals to have effective sound pressure hearing. The most common adaptation to aerial hearing is a tympanic middle ear that converts sound pressure in air to particle motion in the inner ear. The sound pressure sets the tympanum in vibrations, which is conveyed through the middle ear bones to the inner ear fluid. In the inner ear, these vibrations deflect hair cells which sends signal to the brain.

In my PhD I seek to answer if it is possible for animals that lack an outer ear and a tympanum to hear sound?

To address this question, I have worked with snakes of the genus royal python (Python regius) to test if they are able to detect the sound pressure per se or whether it is the sound induced vibrations that they detect. Snakes cannot be trained to respond to a sound, and therefore I used evoked potentials to determine hearing thresholds. By inserting three electrodes subcutaneously on the head of the snakes, it was possible to measure the neural signals in response to sound and vibration stimulation and thereby find out whether the snakes detect the stimuli or not. By comparing the head vibrations induced by sound and vibration, it was possible to find out if they detect the pressure component per se or the induced head vibrations.

The project showed that snakes despite of the lacking tympanum and outer ears are able to hear loud sounds. But it also showed that it is not the pressure per se they detect, but rather the sound induced head vibrations. Future experiments involve hearing across the water air interface in amphibians.

My project concerns in broad terms the acoustic interaction between echolocating toothed whales and their prey, with main emphasis on how echolocation signals affect the prey. Most prey that toothed whales target is not believed to be able to detect intense ultrasonic signals emitted by toothed whales, but recent studies have shown that herring belonging to the subfamily Alosinae (shad and menhaden) can detect ultrasound. It has been suggested that Alosinae have evolved this capability to avoid predation by toothed whales. I am using playback experiments to study the behavioural response of Allis shads when exposed to ultrasounds. Experiments are carried out at INRA, Brittany, France. I have spend 3 months in Professor David Mann’s lab (USF, Florida) where I have learned to used ABR- (auditory brainstem response) and laservibrometri -techniques to gain insight into the mechanism responsible for ultrasound detection in Alosinae.

Besides working with ultrasound detection in Alosinae, I have performed playback studies on longfin squid exposing them to different types of echolocation signals, to test if squids can detect ultrasound or get stunned by intense ultrasonic sound pressure levels.

The sea surrounding Greenland is rich with nutrients and consequentially the preferred feeding grounds of many baleen whales. The seasonal abundance, migration patterns and kinematics of foraging has been only sparsely investigated. However, new techniques of passive acoustic monitoring and tagging of live whales has allowed us to better understand these aspects of the large baleen whales surrounding greenland.

Toothed whales are extremely diverse and include such different animals as deep-diving sperm whales and beaked whales, killer whales, belugas, as well as coastal harbor porpoises and even river-dwelling dolphin species. One common characteristic is that despite their varied habitats, they all use sounds for locating prey and for communicating between each other.

Bottlenose dolphins and pilot whales face different challenges to their foraging and communication capacities because their lifestyles differ in various ways. Both these species are easy and reliable animals to study in the wild using a variety of methods such as array recordings or acoustic recording tags placed directly on the animals. This greatly facilitates bioacoustic studies of these animals and makes it possible to investigate the different acoustic adaptations to their different habitats, ecology, and group structure.

The DTAG, developed by WHOI senior engineer Mark Johnson, has proven essential for studying cetaceans like the pilot whales, where much of their life is spent away from the prying eyes of researchers bound to the surface. These suction-cup tags allow us to determine the detailed underwater movement and behaviour of the whale while also revealing all the sounds that the whale emits and experiences itself. Combined with the tranquil surface behavior of these animals that allows for easy tagging of several animals, this allows us a unique insight into the acoustic world of pilot whales and their social group structure.

Up until recently most underwater biosonar research has focused on how a stationed dolphin can detect stationary objects. These experiments have shown that toothed whales can detect objects at long ranges and discriminate between targets differing not only in shape and size, but also their internal structure and composition. These studies however provide little information on the performance of biosonar systems of animals operating in their natural environments, under different noise and clutter conditions. Furthermore, as shown, inter alia, for humans and bats, auditory perception goes beyond the detection, discrimination and localization of sound stimuli. It requires organization of the extracted acoustic information to allow the listener to identify and track sound sources in the environment, through processes of both auditory scene analysis and auditory selective attention. Finally, biosonar is an active system, where the animal analyses an actively generated auditory scene using echoes of its own signals and has therefore the capability to react to the received information by adjusting both its locomotive and acoustic behaviour.

Recently a non-invasive, acoustic archival tag, the Dtag, has been developed to record the 3D movements of free ranging animals along with sounds emitted and received during echolocation, including, for the first time, echoes from prey. That has allowed researchers to tap directly into the streaming of information back to the auditory system of the tagged whale, and monitor in high resolution how the locomotor and acoustic behavior of the animal adapt to that information flow. Recent size reductions applied to the original Dtag have widened the applications of the device to smaller toothed whale species, making them available in research conducted on captive animals.

In my project, I plan to use such archival tags on captive, trained animals under controlled conditions as well as free-ranging toothed whales in a synergistic approach to address the governing principles of biosonar-based navigation, auditory scene analysis, prey detection and discrimination.