Small Odontocetes Research Articles

Measurements of wild finless porpoise (Neophocaena asiaeorientalis sunameri) on-axis burst-pulse sound

Most small odontocetes utilize burst pulses and whistles as means of communication. However, limited research conducted on burst pulses produced by species that do not produce whistles. This study represents the first to measure on-axis burst pulses emitted by wild finless porpoises (Neophocaena asiaeorientalis sunameri) in Japan, which are classified as a non-whistling species. Passive acoustic monitoring was conducted in the Seto Inland Sea from June to Sepetember 2021 using A-tags and a SoundTrap 300HF. We measured eight parameters of burst pulses: apparent source level, inter-pulse interval, peak frequency, center frequency, -3dB bandwidth, pulse duration, number of pulses, and total duration. Apparent source level of the burst pulses ranged from 150 to 165 dB re 1 μPa, averaging 157 ± 3 dB re 1 μPa. -3dB bandwidth of the burst pulses ranged from 14 to 31 kHz, averaging 22 ± 5. Burst pulses had lower sound pressure levels and wider bandwidths comparing to regular clicks, which are pulsed sounds utilized for echolocation purposes. This observed disparity can be attributed to the fundamental characteristic of burst pulse, their high repetition rate. Notably, the burst pulses of finless porpoise did not exhibit any frequencies below 100 kHz.

Variations in echolocation click characteristics of finless porpoise in response to day/night and absence/presence of vessel noise.

Small odontocetes produce echolocation clicks to feed and navigate, making it an essential function for their survival. Recently, the effect of vessel noise on small odontocetes behavior has attracted attention owing to increase in vessel activities; however, the effects of the surrounding environmental factor, vessel noise, and day/night on echolocation click characteristics have not been well studied. Here, we examined the effects of vessel noise and day/night on variations in echolocation clicks and click trains parameters. Passive acoustic monitoring of on-axis echolocation clicks produced by free-ranging finless porpoises (Neophocaena asiaeorientalis sunameri) was performed at two sites in Japan, Seto Inland Sea and Mikawa Bay, in June-September 2021 and March-August 2022, using A-tag and SoundTrap 300HF. Generalized Linear Model was used to elucidate the effect of vessel noise, day/night, and surrounding environmental factors (water temperature, synthetic flow velocity, and noise level) on echolocation click and click train parameters. Echolocation click and click train parameters were strongly affected by day/night, whereas the absence/presence vessel noise did not exhibit statistically significant influence. Particularly, -3 dB bandwidth was wider, click duration was shorter, and inter-click intervals in a train were shorter at night, which may facilitate information processing at night, thereby compensating for the lack of visual information. The interaction between day/night and the absence/presence of vessel noise affected the source level of finless porpoises, with higher levels observed in the absence of vessel noise during the daytime compared to other conditions at the site with low vessel traffic. Overall, these results suggest that echolocation clicks by finless porpoise were likely to fluctuate to adapt with surrounding complex environmental conditions, especially day/night.

Sedimentological and stratigraphically controlled preservation styles and distribution of fossil cetaceans from a new Early Miocene fossiliferous locality, Patagonia, Argentina

ABSTRACT Prospecting fieldwork in a new fossiliferous locality of the marine Early Miocene Gaiman Formation (Patagonia, Argentina) results in the discovery of abundant fossil cetaceans (n = 29). We analyse stratigraphic, sedimentologic, palaeontologic, and taphonomic data with the aim of understanding the factors that control the preservation and distribution of these specimens. The cetacean sample is dominated by small odontocetes exhibiting five different preservation categories. Based on sedimentological features, three distinct facies were recognised in the stratigraphic section. Stratigraphically condensed deposits of facies C, related to a ravinement surface and subsequent transgressive deposits, enclose 52% of the specimens, grading from isolated postcranial elements to articulated and associated postcranial and cranial elements. In contrast, facies A and B exhibit predominantly isolated postcranial elements. Facies C shows higher degrees of corrosion, encrustation and fragmentation than those specimens from facies A and B, reflecting differences in preservation quality among the facies. The higher abundance and evidence of long exposure times recorded for facies C seems to be resulted mostly from physico-chemical factors after stratigraphic condensation. Finally, intrinsic biological features and ecological preferences of the specimens could explain the dominance of small-sized odontocetes in a restricted inner shelf to lower shoreface environmental setting.

The effects of seismic airguns on cetaceans in UK waters

Observations undertaken during 201 seismic surveys in UK and adjacent waters were analysed to examine effects on cetaceans. Sighting rates, distance from the airguns and orientation were compared for periods when airguns were active and when they were silent, both for surveys with airgun arrays of large volume and surveys with smaller volume arrays. The results demonstrate that cetaceans can be disturbed by seismic exploration. Small odontocetes showed the strongest lateral spatial avoidance (extending at least as far as the limit of visual observation) in response to active airguns, while mysticetes and killer whales showed more localised spatial avoidance. Long-finned pilot whales showed only a change in orientation and sperm whales showed no statistically significant effects. Responses to active airguns were greater during those seismic surveys with large volume airgun arrays than those with smaller volumes of airguns. It is suggested that the different taxonomic groups of cetaceans may adopt different strategies for responding to acoustic disturbance from seismic surveys; some small odontocetes move out of the immediate area, while the slower moving mysticetes orient away from the vessel and increase their distance from the source but do not move away from the area completely.

Vertebral morphology in extant porpoises: Radiation and functional implications.

Vertebral morphology has profound biomechanical implications and plays an important role in adaptation to different habitats and foraging strategies for cetaceans. Extant porpoise species (Phocoenidae) display analogous evolutionary patterns in both hemispheres associated with convergent evolution to coastal versus oceanic environments. We employed 3D geometric morphometrics to study vertebral morphology in five porpoise species with contrasting habitats: the coastal Indo-Pacific finless porpoise (Neophocaena phocaenoides); the mostly coastal harbor porpoise (Phocoena phocoena) and Burmeister's porpoise (Phocoena spinipinnis); and the oceanic spectacled porpoise (Phocoena dioptrica) and Dall's porpoise (Phocoenoides dalli). We evaluated the radiation of vertebral morphology, both in size and shape, using multivariate statistics. We supplemented data with samples of an early-radiating delphinoid species, the narwhal (Monodon monoceros); and an early-radiating delphinid species, the white-beaked dolphin (Lagenorhynchus albirostris). Principal component analyses were used to map shape variation onto phylogenies, and phylogenetic constraints were investigated through permutation tests. We established links between vertebral morphology and movement patterns through biomechanical inferences from morphological presentations. We evidenced divergence in size between species with contrasting habitats, with coastal species tending to decrease in size from their estimated ancestral state, and oceanic species tending to increase in size. Regarding vertebral shape, coastal species had longer centra and shorter neural processes, but longer transverse processes, while oceanic species tended to have disk-shaped vertebrae with longer neural processes. Within Phocoenidae, the absence of phylogenetic constraints in vertebral morphology suggests a high level of evolutionary lability. Overall, our results are in accordance with the hypothesis of speciation within the family from a coastal ancestor, through adaptation to particular habitats. Variation in vertebral morphology in this group of small odontocetes highlights the importance of environmental complexity and particular selective pressures for the speciation process through the development of adaptations that minimize energetic costs during locomotion and prey capture.

Investigating the Diel Occurrence of Odontocetes around the Maui Nui Region Using Passive Acoustic Techniques1

The distribution of odontocetes on a daily scale is largely driven by bottom-up processes that in turn influence foraging opportunities. Environmental variables such as bathymetry may help indicate productive foraging regions and serve as useful tools when assessing dolphin spatial and temporal patterns. To begin to understand daily spatial patterns of different odontocete species relative to heterogeneous benthic habitat, passive acoustic monitoring was conducted near an understudied basin of the Hawaiian Islands, the Maui Nui region (Maui, Lāna‘i, Kaho‘olawe, Moloka‘i). Results showed that the acoustic activity of smaller species was stronger at night than day, particularly closer to shelf waters. In contrast, the acoustic activity of less common larger species tended not to follow a diel pattern, except at sites of a moderate proximity to shelf waters. These findings support previous research showing that smaller odontocetes, such as spinner dolphins (Stenella longirostris), track and feed upon the daily vertically migrating mesopelagic boundary community at night, while larger odontocetes, such as false killer whales (Pseudorca crassidens), may forage across most of the region during both the day and night. This information will help inform best management practices that account for interspecies variation in use of the Maui Nui basin.

The Social Role of Vocal Complexity in Striped Dolphins

Many gregarious species require complex patterns of communication for maintaining coordinated behaviors, articulated social structure and group cohesion. In mammal species, social complexity has been considered the driving force for the development of advanced acoustic communication systems. Striped dolphins are highly social, showing large group size with females maintaining strong bonds with kin. Here, we tested the hypothesis that more complex acoustic pattern plays a key role in social activity in the striped dolphins. The production rate of clicks, whistles and burst pulses, and the acoustic features of whistles have been related to the activity context (feeding, travelling, resting and socializing). Furthermore, complex calls, consisting of a combination of frequency-modulated, and/or pulsed components were detected. Higher whistles and burst pulses production rates were recorded during socializing. Also, the social activity can be discriminated basing on the modulation of the whistle contour. Biphonic calls were especially recorded during social interaction events, suggesting that these phenomena can encode information about individual or group identity to conspecifics. Outcomes indicate the pivotal role of vocal complexity during social context and elicit further investigations of the communication system of small odontocetes from local to wider spatial scale.

Ingestion of foreign materials by odontocetes along the Catalan coast: causes and consequences.

Ingestion of abnormal materials by cetaceans has been reported worldwide, but few studies have investigated the causes of foreign material ingestion. We retrospectively analysed necropsies performed between 2012 and 2019 on 88 cetaceans stranded along the coast of Catalonia, Spain, and evaluated the association of abnormal ingested materials with 2 risk factors, namely disease of the central nervous system (CNS) and maternal separation. Abnormal materials were found in the digestive tract in 19 of 88 (21.6%) cetaceans; of these, 13 (60%) had lesions in the CNS, such as morbilliviral encephalitis, neurobrucellosis or encephalomalacia, and 3 were diagnosed as having experienced maternal separation. In a logistic regression model, CNS lesions and maternal separation were identified as risk factors for ingestion of foreign material, but with wide confidence intervals, probably due to the small sample size. In contrast, abnormal ingestion was not identified in any of the 25 (28%) cetaceans whose cause of death was attributed to interaction with humans. Abnormal ingestion should be interpreted with caution, and efforts should be made at necropsy to exclude CNS diseases through pathologic and microbiologic investigations. If disease of the CNS is a significant risk factor for ingestion of marine debris by small odontocetes, results of monitoring programmes may be biased by the prevalence of CNS disease in a specific area or population.

Spatiotemporal accumulation of fatal pharyngeal entrapment of flatfish in harbour porpoises (Phocoena phocoena) in the German North Sea.

The evolution of a permanent separation of the upper respiratory and digestive tract is one of the adaptions cetaceans evolved for their aquatic life. Generally, it prevents odontocetes from choking on either saltwater or foreign bodies during ingestion under water. Nevertheless, several sporadic single case reports from different parts of the world show that this separation can be reversed especially by overly large items of prey. This incident can have a fatal outcome for the odontocetes. The German federal state of Schleswig-Holstein has a year-round, permanent and systematic stranding network that retrieves stranded marine mammals from its shorelines and constantly enables post-mortem examinations. In 2016, with nine affected animals, a high incidence of fatal pharyngeal entrapment of flatfish in harbour porpoises (Phocoena phocoena) occurred during spring and early summer on the German North Sea island of Sylt. All flatfish were identified as common sole (Solea solea). A retrospective post-mortem data analysis over a 30-year period from the North and Baltic Sea revealed similar yearly and seasonally case accumulations on the same island in the 1990s as well as several single case events over the whole timespan. All cases except one were caused by flatfish. When flatfish speciation was performed, only common sole was identified. From 1990 to 2019, of all examined harbour porpoises, 0.3% (2/713) from the Baltic Sea and 5.5% (45/820) from the North Sea died due to fish entrapped in the pharynx. On the North Sea coast, the occurrence of fatal obstruction shows high yearly variations from 0 to 33.3%. Years that stand out are especially 1990 to 1992, 1995, as well as 2016. The majority of all cases generally occurred between April and July, indicating also a seasonality of cases. This study evaluates the occurrence of fatal pharyngeal entrapment of fish in two geographically separated harbour porpoise populations. Additionally, common sole is clearly identified as a potentially risky item of prey for these small odontocetes.

Fatal asphyxia due to laryngeal displacement by large-sized prey in a Guiana dolphin (Sotalia guianensis), Brazil

ABSTRACT: The Guiana dolphin (Sotalia guianensis) is a small odontocete distributed from Santa Catarina state, Brazil, to Honduras. Although it is currently considered “near threatened” by the International Union for Conservation of Nature, considerable knowledge on health and disease aspects of this species has been gained over the last decade. The main threats for the species are represented by multiple anthropic factors. Guiana dolphins feed on a variety of taxa, primarily teleosts, crustaceans, and cephalopods. Herein, we reported a case of severe laryngeal displacement and esophageal impaction with ulceration associated with a deglutted adult leatherjacket (Oligoplites sp.). This fish species is not a common prey item of Guiana dolphins’ diet. Severe upper aerodigestive compromise likely led to death by mechanical asphyxia (choke) in this dolphin. This report attested the occurrence of upper aerodigestive dysfunction due to overzealous feeding in Guiana dolphin and contributed to general knowledge on pathology of the species.

Transmission beam pattern and dynamics of a spinner dolphin (Stenella longirostris).

Toothed whales possess a sophisticated biosonar system by which ultrasonic clicks are projected in a highly directional transmission beam. Beam directivity is an important biosonar characteristic that reduces acoustic clutter and increases the acoustic detection range. This study measured click characteristics and the transmission beam pattern from a small odontocete, the spinner dolphin (Stenella longirostis). A formerly stranded individual was rehabilitated and trained to station underwater in front of a 16-element hydrophone array. On-axis clicks showed a mean duration of 20.1 μs, with mean peak and centroid frequencies of 58 and 64 kHz [standard deviation (s.d.) ±30 and ±12 kHz], respectively. Clicks were projected in an oval, vertically compressed beam, with mean vertical and horizontal beamwidths of 14.5° (s.d. ± 3.9) and 16.3° (s.d. ± 4.6), respectively. Directivity indices ranged from 14.9 to 27.4 dB, with a mean of 21.7 dB, although this likely represents a broader beam than what is normally produced by wild individuals. A click subset with characteristics more similar to those described for wild individuals exhibited a mean directivity index of 23.3 dB. Although one of the broadest transmission beams described for a dolphin, it is similar to other small bodied odontocetes.

Indications of mesopelagic foraging by a small odontocete

The mesopelagic layer is represented in all oceans and is of crucial importance to the pelagic communities, and in this paper it is hypothesised that the Greenlandic harbour porpoise (Phocoena phocoena) is seasonally dependent on mesopelagic prey when abandoning the ice-covered continental shelf areas and remains in offshore areas. Data from 15 harbour porpoises instrumented with satellite-linked transmitters in West Greenland were analysed with regard to foraging that may target mesopelagic prey. Contact with the porpoises was maintained for an average of 404 days where they conducted extensive offshore movements and spent an average of 90% of their time over deep waters (> 1000 m) in the North Atlantic. When entering deep water, they increased their daily travel rate significantly from 22.5 to 36.7 km d−1. Five of the 15 porpoises provided information on dive depth which suggested that dive depths > 100 m are important for these porpoises both day and night; however, the porpoises dove significantly more at nighttime compared to daytime. Harbour porpoises from West Greenland probably target vertically migrating species from the mesopelagic layer that are accessible at shallower depths at night and at lower energetic cost than during the day.

Click communication in wild harbour porpoises (Phocoena phocoena)

Social delphinids employ a vocal repertoire of clicks for echolocation and whistles for communication. Conversely, the less social and acoustically cryptic harbour porpoises (Phocoena phocoena) only produce narrow-band high-frequency (NBHF) clicks with properties that appear poorly suited for communication. Nevertheless, these small odontocetes likely mediate social interactions, such as mate choice and mother-calf contact, with sound. Here, we deployed six tags (DTAG3) on wild porpoises in Danish waters for a total of 96 hours to investigate if the patterns and use of stereotyped NBHF click trains are consistent with a communication function. We show that wild porpoises produce frequent (up to 27 bullet min−1), high-repetition rate click series with repetition rates and output levels different from those of foraging buzzes. These sounds are produced in bouts and frequently co-occur with emission of similar sounds by nearby conspecifics, audible on the tags for >10% of the time. These results suggest that social interactions are more important to this species than their limited social encounters at the surface may indicate and that these interactions are mediated by at least two broad categories of calls composed of short, high-repetition rate click trains that may encode information via the repetition rate of their stereotyped NBHF clicks.

Nasitrema attenuata (Digenia: Nasitrematidae) Infection of Long-beaked Common Dolphin (Delphius capensis) in the East Sea, Korea

The trematodes of Genus Nasitrema are the most common infection in the heads of small odontocetes. Five species of Nasitrema have been described in Asia. But those parasites has been not surveyed in the East Sea of Korean peninsula. There are no evidence of pathology associated with Nasitrema attenuate. To determine the infection ratio of N. attenuata in the dolphin, it was surveyed 9 long-beaked common dolphin, Delphinus capensis, caught incidentally from Samcheok to Pohang, East Sea, Korea. Five males and four females, estimated to be between 1 to 10 years old, were examined. The quantity of N. attenuata harvested by aspiration from pterygoid sinuses ranged from 10 to 100 with a 100% (9/9) infection with older dolphins exhibiting higher parasite counts. The body of trematoda was elongated with tegumental patches resembling the leaves of lilies and oral sucker located ventrally at the anterior terminal. Esophagus was absent and intestinal bifurcation arose immediately from the posterior of pharynx. The undulated intestinal caeca transversed the entire body and terminated at the posterior extremity of the worm. The genital pore opened anterior to the ventral sucker. This report confirms the prevalence and morphology of N. attenuata in the dolphins from the East Sea, Korea.

Spatial patterns of distribution, abundance, and species diversity of small odontocetes estimated using density surface modeling with line transect sampling

Spatial patterns of distribution, abundance, and species diversity of small odontocetes including species in the Delphinidae and Phocoenidae families were investigated using long-term dedicated sighting survey data collected between 1983 and 2006 in the North Pacific. Species diversity indices were calculated from abundance estimated using density surface modeling of line-transect data. The estimated abundance ranged from 19,521 individuals in killer whale to 1,886,022 in pantropical spotted dolphin. The predicted density maps showed that the habitats of small odontocetes corresponded well with distinct oceanic domains. Species richness was estimated to be highest between 30 and 40°N where warm- and cold-water currents converge. Simpson׳s Diversity Index showed latitudinal diversity gradients of decreasing species numbers toward the poles. Higher diversity was also estimated in the coastal areas and the zonal areas around 35–42°N. Coastal–offshore gradients and latitudinal gradients are known for many taxa. The zonal areas around 35°N and 40°N coincide with the Kuroshio Current and its extension and the subarctic boundary, respectively. These results suggest that the species diversity of small odontocetes primarily follows general patterns of latitudinal and longitudinal gradients, while the confluence of faunas originating in distinct water masses increases species diversify in frontal waters around 30–40°N. Population densities tended to be higher for the species inhabiting higher latitudes, but were highest for intermediate latitudes at approximately 35–40°N. According to latitudinal gradients in water temperature and biological productivity, the costs for thermoregulation will decrease in warmer low latitudes, while feeding efficiency will increase in colder high latitudes. These trade-offs could optimize population density in intermediate latitudes.

Biogeography of small odontocetes in relation to wide‐scale oceanographic structure in theNorthPacificOcean

AbstractInformation regarding the distributional patterns of top predators is undoubtedly important for the monitoring and management of marine ecosystems. Nevertheless, previous knowledge on the distributional patterns of cetacean species in relation to physical oceanography is usually limited to a small fraction of each species' range because of their vast distribution and infrequency of encountering many species. We carried out comprehensive analyzes on the distributions of small odontocetes covering almost the entire North Pacific Ocean. Using data from over 300 000 nautical miles (≈ 555 600 km) of dedicated sighting surveys and 7000 school sightings of 14 small odontocete species, which have accumulated for approximately a quarter of a century, we describe interspecific habitat differences in small odontocetes with respect to physical oceanography. Multivariate analyzes were used to discriminate the 14 species into four ecological groups: (i) Subtropical Domain species; (ii) Subarctic Boundary species; (iii) Transitional Domain species; and (iv) Subarctic Domain species. By comparing environmental variables in their habitats with variables indicative of water mass and frontal structures, the four species groups were further divided into several subgroups. Groups at higher latitudes comprised fewer species than those at lower latitudes, while species diversity was maximal at lower and intermediate latitudes. Our findings indicate that large‐scale oceanographic structures are important determinants for the present patterns in biogeography and species diversity of small odontocetes.

Fatal Asphyxiation in Two Long-Finned Pilot Whales (Globicephala melas) Caused by Common Soles (Solea solea).

Long-finned pilot whales (Globicephala melas) are rare visitors to the southern North Sea, but recently two individual strandings occurred on the Dutch coast. Both animals shared the same, unusual cause of death: asphyxiation from a common sole (Solea solea) stuck in their nasal cavity. This is a rare cause of death in cetaceans. Whilst asphyxiation has been reported in smaller odontocetes, there are no recent records of this occurring in Globicephala species. Here we report the stranding, necropsy and diet study results as well as discuss the unusual nature of this phenomenon. Flatfish are not a primary prey species for pilot whales and are rarely eaten by other cetaceans, such as harbour porpoises (Phocoena phocoena), in which there are several reports of asphyxiation due to airway obstruction by soles. This risk may be due to the fish’s flexible bodies which can enter small cavities either actively in an attempt to escape or passively due to the whale ‘coughing’ or ‘sneezing’ to rid itself of the blockage of the trachea. It is also possible that the fish enter the airways whilst the whale is re-articulating the larynx after trying to ingest large, oddly shaped prey. It is unlikely that the soles entered the airways after the death of the whales and we believe therefore that they are responsible for the death of these animals.

Determining the detection function of passive acoustic data loggers for porpoises using a large hydrophone array

Click loggers such as C-PODs are an important tool to monitor the spatial distribution and seasonal occurrence of small odontocetes. To determine absolute density, information on the detection function, the detection probability as a function of distance, and derived from this, the effective detection radius (EDR), is needed. In this study a 15 channel hydrophone array, deployed next to 12 C-PODs, was used to localize porpoises and determine their geo-referenced swim paths using the ship’s GPS and motion sensors. The detection function of C-PODs was then computed using the distance between the animals and each C-POD. In addition to this, the acoustic detection function of C-PODs has been measured by playing back porpoise-like clicks using an omni-directional transducer. The EDR for these porpoise-like clicks with a source level of 168 dB re 1 μPa pp varied from 41 to 243 m. This variation seemed to be related to the sensitivity of the devices; however, season and water depth also seemed to have an influence on detectability.

Aquatic mammal science in Latin America: a bibliometric analysis for the first eight years of the Latin American Journal of Aquatic Mammals (2002-2010)

We conducted a meta-analysis of the publication statistics for Vols. 1-8 of the Latin American Journal of Aquatic Mammals (LAJAM), the joint scholarly publication of the Sociedad Latinoamericana de Especialistas en Mamíferos Acuáticos and the Sociedad Mexicana de Mastozoología Marina, with the following purposes: (a) identifying the main patterns in the authorship and content published between 2002 and 2010, and (b) assessing the contributions of these scientific societies in the Latin American and global contexts. With the caveat that the results are only representative of the researchers that chose to publish in LAJAM during the period covered by the study, the metadata from 168 articles indicated that most of the research was conducted on small odontocetes (Sotalia, Pontoporia, Tursiops) and pinnipeds (Arctocephalus, Otaria, Mirounga) of coastal habits. Rorqual whales (Balaenoptera, Megaptera) and oceanic odontocetes (Stenella, Mesoplodon, Orcinus, Delphinus) also were well represented. Studies of distribution (including first records) were the most common, followed by those related to feeding, strandings, health and bycatch. Seventeen countries were represented in the primary affiliation of the lead author, but just five dominated the contribution: Brazil (52%), Argentina (10%), México (7%), Uruguay (5%) and USA (5%). Among institution types, a university was reported as the primary affiliation type by 50% of the authors, while 26% reported a NGO, 17% a government agency and 7% another type of organization. A social network analysis of 404 authors identified a large, well-connected cluster of 263 authors. Within this cluster, 13 authors from Brazil, Perú, Argentina and Colombia were among the most collaborative. The female to male ratio was 1:1.6 among lead authors and 1:3.2 among lead authors that published more than one article, suggesting a gender disparity within this scientific community. According to Google Scholar™, 91 articles in LAJAM were cited in other publications through January 2012, with an average of 7.5 citations per article. The 15 most cited articles had between 13 and 15 citations, were predominantly from Brazil, and were mainly about small cetaceans.

Methodology and results of calibration of tonal click detectors for small odontocetes (C-PODs)

Static acoustic monitoring (SAM) is one major technology for observing small cetacean species. Automatic click loggers deployed for long time periods (>2 months) with a single hydrophone are a standard solution. Acoustic properties, like detection thresholds of these instruments, are essential for interpretation of results, but have nevertheless received little attention. A methodology for calibrating tonal click detectors in small tanks consisting of the determination of the horizontal directivity pattern and detection thresholds including a transfer function is presented. Two approaches were tested to determine detection thresholds by (a) determining the 50% detection threshold and (b) fitting a linear regression model to the recorded relative amplitudes. The tests were carried out on C-PODs (Cetacean PODs, tonal click detectors), the most commonly used instrument for SAM in Europe. Directivity and threshold were tested between 60 and 150 kHz. Directivity showed a maximum variation of 8.5 dB in the horizontal plane. Sensitivity is highest between 80 and 130 kHz and linear (± 3 dB) in this frequency range for most of the instruments tested. C-PODs have a detection threshold (calculated with the linear model) of 114.5 ± 1.2 (standard deviation) dB re 1 μPa peak-peak at 130 kHz.