Assessing auditory evoked potentials of wild harbor porpoises (Phocoena phocoena)

Maritime traffic alters distribution of the harbour porpoise in the North Sea

The North Sea is one of the most industrialised marine regions globally. We integrated cetacean-dedicated aerial surveys (2015-2022) with environmental covariates and ship positions from the Automatic Identification System (AIS) to investigate the disturbance radius and duration on harbour porpoise distribution. This study is based on 81,511 km of line-transect survey effort, during which 6511 harbour porpoise groups (8597 individuals) were sighted. Several proxies for ship disturbance were compared, identifying those best explaining the observed distribution. Better model performance was achieved by integrating maritime traffic, with frequent traffic representing the most significant disturbance to harbour porpoise distribution. Porpoises avoided areas frequented by numerous vessels up to distances of 9 km. The number of ships and average approach distance over time improved model performance, while reasons for the lower performance of predicted ship sound levels remain unclear. This study demonstrates the short-term effects of maritime traffic on harbour porpoise distribution.

Output compensation of auditory brainstem responses in dolphins and sea lions

Cochlear dispersion causes increasing delays between neural responses from high-frequency regions in the cochlear base and lower-frequency regions toward the apex. For broadband stimuli, this can lead to neural responses that are out-of-phase, decreasing the amplitude of farfield neural response measurements. In the present study, cochlear traveling-wave speed and effects of dispersion on farfield auditory brainstem responses (ABRs) were investigated by first deriving narrowband ABRs in bottlenose dolphins and California sea lions using the high-pass subtractive masking technique. Derived-band ABRs were then temporally aligned and summed to obtain the "stacked ABR" as a means of compensating for the effects of cochlear dispersion. For derived-band responses between 8 and 32 kHz, cochlear traveling-wave speeds were similar for sea lions and dolphins [∼2-8 octaves (oct)/ms for dolphins; ∼3.5-11 oct/ms for sea lions]; above 32 kHz, traveling-wave speed for dolphins increased up to ∼30 oct/ms. Stacked ABRs were larger than unmasked, broadband ABRs in both species. The amplitude enhancement was smaller in dolphins than in sea lions, and enhancement in both species appears to be less than reported in humans. Results suggest that compensating for cochlear dispersion will provide greater benefit for ABR measurements in species with better low-frequency hearing.

Variability in Anthropogenic Underwater Noise Due to Bathymetry and Sound Speed Characteristics

Oceanic acoustic environments are dynamic, shaped by the spatiotemporal variability in transmission losses and sound propagation pathways of natural and human-derived noise sources. Here we used recordings of an experimental noise source combined with transmission loss modeling to investigate changes in the received levels of vessel noise over space and time as a result of natural water column variability. Recordings were made in the Juan de Fuca Strait, on the west coast of Vancouver Island, a biologically productive coastal region that hosts several cetacean species. Significant variability in noise levels was observed due to changing water masses, tied to seasonal temperature variation and, on a finer scale, tidal movements. Comparisons of interpreted received noise levels through the water column indicated that vessel noise recorded by bottom-stationed monitoring devices might not accurately represent those received by whales in near-surface waters. Vertical and temporal differences of 3–5 dB were commonly observed in both the recorded and modeled data. This has implications in estimating the success of noise mitigation measures, and our understanding of the change in sound fields experienced by target species for conservation.

Toothed whale auditory brainstem responses measured with a non-invasive, on-animal tag

Empirical measurements of odontocete hearing are limited to captive individuals, constituting a fraction of species across the suborder. Data from more species could be available if such measurements were collected from unrestrained animals in the wild. This study investigated whether electrophysiological hearing data could be recorded from a trained harbor porpoise (Phocoena phocoena) using a non-invasive, animal-attached tag. The results demonstrate that auditory brainstem responses to external and self-generated stimuli can be measured from a stationary odontocete using an animal-attached recorder. With additional development, tag-based electrophysiological platforms may facilitate the collection of hearing data from freely swimming odontocetes in the wild.

Cochlear apical morphology in toothed whales: Using the pairing hair cell-Deiters' cell as a marker to detect lesions

The apex or apical region of the cochlear spiral within the inner ear encodes for low-frequency sounds. The disposition of sensory hair cells on the organ of Corti is largely variable in the apical region of mammals, and it does not necessarily follow the typical three-row pattern of outer hair cells (OHCs). As most underwater noise sources contain low-frequency components, we expect to find most lesions in the apical region of the cochlea of toothed whales, in cases of permanent noise-induced hearing loss. To further understand how man-made noise might affect cetacean hearing, there is a need to describe normal morphological features of the apex and document interspecific anatomic variations in cetaceans. However, distinguishing between apical normal variability and hair cell death is challenging. We describe anatomical features of the organ of Corti of the apex in 23 ears from five species of toothed whales (harbor porpoise Phocoena phocoena, spinner dolphin Stenella longirostris, pantropical spotted dolphin Stenella attenuata, pygmy sperm whale Kogia breviceps, and beluga whale Delphinapterus leucas) by scanning electron microscopy and immunofluorescence. Our results showed an initial region where the lowest frequencies are encoded with two or three rows of OHCs, followed by the typical configuration of three OHC rows and three rows of supporting Deiters' cells. Whenever two rows of OHCs were detected, there were usually only two corresponding rows of supporting Deiters' cells, suggesting that the number of rows of Deiters' cells is a good indicator to distinguish between normal and pathological features.

Is Harbor Porpoise (Phocoena phocoena) Exhaled Breath Sampling Suitable for Hormonal Assessments?

Simple Summary

The progress of animal welfare in wildlife conservation and research calls for more non-invasive sampling techniques. In cetaceans, exhaled breath condensate (blow)—a mixture of cells, mucus and fluids expelled through the force of a whale’s exhale—is a unique sampling matrix for hormones, bacteria and genetic material, among others. Especially the detection of steroid hormones, such as cortisol, is being investigated as stress indicators in several species. As the only native cetacean in Germany, harbor porpoises (Phocoena phocoena) are of special conservation concern and research interest. So far, strandings and live captures have been the only method to obtain samples from free-ranging individuals, and novel, non-invasive monitoring methods are desirable for this small cetacean species. Hence, three different blow collection devices were tested on harbor porpoises. All samples were analyzed for cortisol using a commercially available immunosorbent assay. The most suitable protocol for sampling, storage and processing is using a sterile 50 mL centrifuge tube. This pilot study shows that cortisol can be detected in the exhale of harbor porpoises, thus paving the way for future studies and most likely successful non-invasive small cetacean health monitoring through blow.

Abstract

Over the last decades, exhaled breath sampling has been established for laboratory analysis in various cetacean species. Due to their small size, the usability of respiratory vapor for hormone assessments was questionable in harbor porpoises (Phocoena phocoena). This pilot study compared three different blow collection devices for their suitability in the field and during laboratory processing: a sterile petri dish covered by a Nitex membrane, as well as sterile 50 mL centrifuge tubes with or without manganese(II) chloride as a stabilizer. Collected exhales varied between three, five or ten, depending on feasibility. Hormones were extracted through an ether mix, followed by centrifugal evaporation and cortisol analysis using an immunoassay. Although close to the lower end of the assay’s dynamic range, the ELISA produced results (n = 110, 0.102–0.937 ng/mL). Hence, a simple 50 mL centrifuge tube was determined as the best suited blow collection device, while three consecutive exhales proved sufficient to yield results. These findings are promising regarding the suitability of exhaled breath as a matrix for future endocrine and immune system-related studies in harbor porpoises. If further advanced, blow sampling can become an important, non-invasive tool for studying and monitoring health, stress levels and diseases in harbor porpoises.

Haematology and clinical blood chemistry in harbour porpoises (Phocoena phocoena) from the inner Danish waters

Harbour porpoises (Phocoena phocoena) in the Baltic Sea are under increasing pressure from anthropogenic activities, which affect the overall health of populations. Individuals' haematologic and biochemistry parameters are known to be linked to a population's health status and are therefore useful tools for cross-population comparisons and to assess health patterns of a population through time. However, it is often difficult to acquire data representing the full range of values and assess the influence of intrinsic and extrinsic factors. Here, we present the range of haematology and blood chemistry values obtained from 46 wild (n = 54 blood samples) individuals incidentally caught in pound-nets and five porpoises in long-term human care (n = 449 blood samples) from an outdoor semi-open facility in Denmark. Although it was not possible to formally test the differences between samples from free-ranging and captive individuals, lymphocyte values were lower for free-ranging animals whereas eosinophils and white blood cell values were higher in captive individuals. Aspartate aminotransferase and alanin aminotransferase values were also lower for captive individuals compared to free-ranging ones. Age group did not influence any of the blood parameters tested for free-ranging individuals. Sodium values were higher for males compared to females. Values were higher and lower in the fall for platelets and lactic acid dehydrogenase, respectively, compared to the other seasons. Based on samples yielded by individuals in long-term human care, haemoglobin, mean cell volume, white blood cells, absolute lymphocyte count, and alkaline phosphatase values were all influenced by health status based on clinical examination. These are therefore candidate parameters to assess health status of wild porpoises. Our results underline that it is essential to obtain ranges of reference values for all haematologic and biochemistry markers in order to assess health status of free-ranging individuals. Individuals in human care provide the opportunity to observe biological and ecological determinates (e.g. age, season) of long-term biomarker response patterns and to assess the suite of biomarkers best suited to predict individual health status.

Evoked-potential audiogram variability in a group of wild Yangtze finless porpoises (Neophocaena asiaeorientalis asiaeorientalis)

Hearing is considered the primary sensory modality of cetaceans and enables their vital life functions. Information on the hearing sensitivity variability within a species obtained in a biologically relevant wild context is fundamental to evaluating potential noise impact and population-relevant management. Here, non-invasive auditory evoked-potential methods were adopted to describe the audiograms (11.2-152 kHz) of a group of four wild Yangtze finless porpoises (Neophocaena asiaeorientalis asiaeorientalis) during a capture-and-release health assessment project in Poyang Lake, China. All audiograms presented a U shape, generally similar to those of other delphinids and phocoenids. The lowest auditory threshold (51-55 dB re 1 µPa) was identified at a test frequency of 76 kHz, which was higher than that observed in aquarium porpoises (54 kHz). The good hearing range (within 20 dB of the best hearing sensitivity) was from approximately 20 to 145 kHz, and the low- and high-frequency hearing cut-offs (threshold > 120 dB re l μPa) were 5.6 and 170 kHz, respectively. Compared with aquarium porpoises, wild porpoises have significantly better hearing sensitivity at 32 and 76 kHz and worse sensitivity at 54, 108 and 140 kHz. The audiograms of this group can provide a basis for better understanding the potential impact of anthropogenic noise.

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.

Morphological and Pathological Findings in the Middle and Inner Ears of Harbour Porpoises (Phocoena phocoena)

Hearing represents the major sense in harbour porpoises (Phocoena phocoena) and impairment of hearing has a great impact on the survival of these animals. In this communication, some anatomical and histological aspects of the tympanoperiotic complex of harbour porpoises are presented. In addition, the ears of 21 incidentally bycaught or stranded freshly dead harbour porpoises of different age groups and sex were investigated histologically. At the entrance to the middle ear cavity, mucosa-associated lymphoid tissue was present that was often hyperplastic in juvenile (9/10) and adult individuals (7/8). Solitary lymphoid follicles were additionally found in the corpus cavernosum and adjacent to the stapedius muscle in single porpoises. The nematode Stenurus minor represented the most common pathogen observed in the middle ear cavity of juvenile and adult harbour porpoises and the parasite was associated with chronic inflammation with metaplastic and hyperplastic epithelial changes. An unusual bone formation at the attachment of the corpus cavernosum to the perioticum was a common finding, even in young individuals. Whether this represents a normal structure or a metaplastic change remains undetermined. Acute haemorrhages in the cochlea and/or the tympanic cavity occurred in all animals and were most likely agonal changes. Single porpoises suffered from purulent otitis media, mycotic otitis media with osteolysis or chronically fractured tympanic bones, likely causing impairment of hearing that may have contributed to by-catch. There was no evidence that stranding in five porpoises was associated with the aural changes. Histological examination of the ears in harbour porpoises is a valuable part of the assessment of their health status. Damage to hearing structures may explain starvation due to impaired ability to catch prey or unusual behaviour such as stranding or entanglement in nets.

Stimulus bandwidth impact on auditory evoked potential thresholds and estimated upper-frequency limits of hearing in dolphins

The frequency range of hearing is important for assessing the potential impact of anthropogenic noise on marine mammals. Auditory evoked potentials (AEPs) are commonly used to assess toothed whale hearing, but measurement methods vary across researchers and laboratories. In particular, estimates of the upper-frequency limit of hearing (UFL) can vary due to interactions between the unintended spread of spectral energy to frequencies below the desired test frequency and a sharp decline in hearing sensitivity at frequencies near the UFL. To assess the impact of stimulus bandwidth on UFL measurement, AEP hearing tests were conducted in four bottlenose dolphins (Tursiops truncatus) with normal and impaired hearing ranges. Dolphins were tested at frequencies near the UFL and at a frequency 1/2-octave below the UFL, where hearing sensitivity was better (i.e., threshold was lower). Thresholds were measured using sinusoidal amplitude modulated (SAM) tones and tone-bursts of varying bandwidth. Measured thresholds varied inversely as a function of stimulus bandwidth near the UFL with narrow-band tone-bursts approximating thresholds measured using SAM tones. Bandwidth did not impact measured thresholds where hearing was more sensitive, highlighting how stimulus bandwidth and the rate of decline of hearing sensitivity interact to affect measured threshold near the UFL.

Fine-scale movement responses of free-ranging harbour porpoises to capture, tagging and short-term noise pulses from a single airgun

Knowledge about the impact of anthropogenic disturbances on the behavioural responses of cetaceans is constrained by lack of data on fine-scale movements of individuals. We equipped five free-ranging harbour porpoises (Phocoena phocoena) with high-resolution location and dive loggers and exposed them to a single 10 inch3 underwater airgun producing high-intensity noise pulses (2-3 s intervals) for 1 min. All five porpoises responded to capture and tagging with longer, faster and more directed movements as well as with shorter, shallower, less wiggly dives immediately after release, with natural behaviour resumed in less than or equal to 24 h. When we exposed porpoises to airgun pulses at ranges of 420-690 m with noise level estimates of 135-147 dB re 1 µPa2s (sound exposure level), one individual displayed rapid and directed movements away from the exposure site and two individuals used shorter and shallower dives compared to natural behaviour immediately after exposure. Noise-induced movement typically lasted for less than or equal to 8 h with an additional 24 h recovery period until natural behaviour was resumed. The remaining individuals did not show any quantifiable responses to the noise exposure. Changes in natural behaviour following anthropogenic disturbances may reduce feeding opportunities, and evaluating potential population-level consequences should be a priority research area.

Hearing thresholds of a male and a female harbor porpoise (Phocoena phocoena)

To study intra-species variability in audiograms, the hearing sensitivity of a six-year-old female and a three-year-old male harbor porpoise was measured by using a standard psycho-acoustic technique under low ambient noise conditions. The porpoises' hearing thresholds for 13 narrow-band sweeps with center frequencies between 0.125 and 150 kHz were established. The resulting audiograms were U-shaped and similar. The main difference (25 dB) in mean thresholds between the two porpoises was at the high-frequency end of the hearing range (at 150 kHz). Maximum sensitivity (47 dB re 1 μPa for the female and 44 dB re 1 μPa for the male) occurred at 125 kHz. The range of most sensitive hearing (defined as within 10 dB of maximum sensitivity) was from 16 to ∼140 kHz. Sensitivity declined sharply above 125 kHz. All five porpoises for which a valid behavioral audiogram now exists were rehabilitated stranded animals, all were tested with similar psycho-acoustic techniques, and all had similar audiograms. The present study provides further evidence to confirm that the hearing range and sensitivity of the first three harbor porpoises, which have been used in secondary research and on which policy decisions have been based, are representative of those of young harbor porpoises in general.