Phocoena Phocoena Research Articles

How to eat a needlefish? The harbor porpoise way

How to eat a needlefish? The harbor porpoise way

Antibiotic Resistance of Bacteria in Two Marine Mammal Species, Harbor Seals and Harbor Porpoises, Living in an Urban Marine Ecosystem, the Salish Sea, Washington State, USA

The pervasive use of antibiotics in human medicine, veterinary medicine, and agriculture can result in a significant increase in the spread and environmental persistence of antibiotic resistance in marine ecosystems. This study describes the presence and distribution of antibiotic-resistant bacteria in Salish Sea harbor seals (Phoca vitulina) and harbor porpoises (Phocoena phocoena) and evaluates species, age class, and geographic differences in resistance patterns. Isolates from 95 dead-stranded animals (74 seals/21 porpoises) were tested for resistance to a suite of 15 antibiotics. Of the 95 sampled, 85 (89%) (67 seals/18 porpoises) successfully yielded 144 isolates, with 37% resistant to at least one antibiotic and 26% multi-drug resistant (24% and 39% of seal and porpoise isolates, respectively). Overall, and by study region, porpoises were significantly more likely to harbor resistant organisms compared to seals. Significant differences between age classes were noted for the antibiotics amoxicillin, cephalexin, and cefovecin. Overall isolate resistance was significantly greater in porpoises than seals for several individual antibiotics. Multiple antibiotic resistance (MAR) indices greater than 0.2 were observed in 55% of multi-drug resistant isolates, suggesting seal and porpoise exposure to anthropogenic pollution. The relatively high and disparate prevalence of antibiotic resistance in these common, but ecologically dissimilar, marine mammals reflects a potentially large environmental pool of antibiotic resistant organisms in the Salish Sea or inherently different resistance gene patterns between the two species.

Small Cetacean in a Human High-Use Area: Trends in Harbor Porpoise Abundance in the North Sea Over Two Decades

The North Sea is one of the most heavily used shelf regions worldwide with a diversity of human impacts, including shipping, pollution, fisheries, and offshore constructions. These stressors on the environment can have consequences for marine organisms, such as our study species, the harbor porpoise (Phocoena phocoena), which is regarded as a sentinel species and hence has a high conservation priority in the European Union (EU). As EU member states are obliged to monitor the population status, the present study aims to estimate trends in absolute harbor porpoise abundance in the German North Sea based on almost two decades of aerial surveys (2002–2019) using line-transect methodology. Furthermore, we were interested in trends in three Natura2000 Special Areas of Conservation (SACs), which include the harbor porpoise as designated feature. Trends were estimated for each SAC and two seasons (spring and summer) as well as the complete area of the German North Sea. For the trend analysis we applied a Bayesian framework to a series of replicated visual surveys, allowing to propagate the error structure of the original abundance estimates to the final trend estimate and designed to deal with spatio-temporal heterogeneity and other sources of uncertainty. In general, harbor porpoise abundance decreased in northern areas and increased in the south, such as in the SAC Borkum Reef Ground. A particularly strong decline with a high probability (94.9%) was detected in the core area and main reproduction site in summer, the SAC Sylt Outer Reef (−3.79% per year). The overall trend for the German North Sea revealed a decrease in harbor porpoise abundance over the whole study period (−1.79% per year) with high probability (95.1%). The assessment of these trends in abundance based on systematic monitoring should now form the basis for adaptive management, especially in the SAC Sylt Outer Reef, where the underlying causes and drivers for the large decline remain unknown and deserve further investigation, also in a regional North Sea wide context.

Behavioural reactions of harbour porpoises Phocoena phocoena to startle-eliciting stimuli: movement responses and practical applications

Acoustic deterrent devices are frequently used as a mitigation method to exclude harbour porpoisesPhocoena phocoenafrom areas of potential harm, such as wind farm construction sites. However, there is increasing evidence that the devices themselves have the capacity to cause hearing damage. Here, we investigated the response of harbour porpoises to a 15 min sequence of 200 ms sound (peak frequency 10.5 kHz, range 5.5-20.5 kHz, 27 sounds total), which elicits the acoustic startle reflex. We used a duty cycle (0.6%) and sound exposure level that were significantly lower than in conventional acoustic deterrent devices. Harbour porpoises were exposed to startle sounds from a small vessel, and groups were visually tracked during 13 sound exposure sequences and 11 no-sound control trials. Porpoises showed a significant avoidance reaction during exposure, travelling a mean distance of 1.78 km (max. 3.21 km). In all cases, they left the area within 1 km of the sound source in the first 15 min after the start of the startle sequence. No avoidance was exhibited during control trials. Results are consistent with the startle reflex mediating this behaviour at low response thresholds. Our method can be used for mitigating collision risk and the risk of hearing damage from renewable energy installations, their construction and the deterrence device itself.

Echolocation activity of harbour porpoises, Phocoena phocoena, shows seasonal artificial reef attraction despite elevated noise levels close to oil and gas platforms

Abstract Harbour porpoises frequently alter their behaviour in response to underwater sound from shipping, seismic surveys, drilling and marine renewables. Less well understood is the response of porpoises to sounds emitted from oil and gas (O&G) platforms during routine operations. The responses are not easily predicted as platforms can act simultaneously and to varying degree as a source of disturbance through noise and attraction through an artificial reef effect with increased prey abundance and diversity. To investigate the presence and feeding behaviour of harbour porpoises around platforms, autonomous acoustic loggers were placed for up to 2 years, at 21 stations 0–25.6 km from the largest platform in the Danish North Sea. Harbour porpoises were detected at all distances year round in two distinct seasonal activity patterns. During July–January, porpoises were attracted to the platform as indicated by high foraging activity within 800 m of the platform. Echolocation activity levels were up to twofold higher than those observed at 3.2–9.6 km from the platform. Similar high echolocation activity was observed 200 m from neighbouring offshore installations located within 15 km, regardless of their size, during May–July. This study shows that porpoises may be attracted to offshore O&G platforms despite confirmed elevated underwater noise and are likely exploiting higher prey abundance in the vicinity of such structures. This is possibly due to increased prey availability created by the combined effect of the artificial reef formed by the underwater structure and the local protected area around all platforms where fishery is banned. Hard substrate and untouched seabed are rare and valuable habitats to many organisms in heavily trawled waters like the North Sea, and the ecological importance of these structures should be considered in the development of decommissioning strategies.

Spatio-Temporal Variability of Harbor Porpoise Life History Parameters in the North-East Atlantic

Harbor porpoises exhibit early maturation, relatively short gestation/lactation periods and a faster rate of reproduction as compared to other cetacean species. Intrinsic and extrinsic factors can influence both population vital rates and population structure, which ultimately cause changes in dynamics within and between populations. Here, we undertook a retrospective analysis of mortality data collected over a 24-year period for assessing life history traits of the North-east Atlantic harbor porpoise population. We use time-period specific models for key life history relationships that considered cause of death of individuals (as a proxy for health status), sex and management unit (MU). Sexual variation in asymptotic length, asymptotic age, average length at 50% maturity (L50) and average age at 50% maturity (A50) were observed, with females attaining a larger asymptotic length, larger L50, and delaying attainment of both sexual and physical maturity, compared to males. While females are constrained in their minimum body size due to giving birth to proportionally larger offspring, males exhibited more plasticity in size at sexual maturity, enabling re-allocation of available energy resources toward reproduction. Data were then used to compare biological parameters among two porpoise MUs in United Kingdom waters, both of which in the current study exhibited reduced reproductive rates compared to other geographic regions. In both MUs, females significantly increased their A50 and males significantly declined in their L50. An increase in the age at asymptotic length was also observed in both sexes, along with a significant decline in the Gompertz growth rate parameter that was more apparent in the female data. While availability of suitable prey resources may be a limiting factor, a combination of other factors cannot be ruled out. Porpoises in the Celtic and Irish Seas MU were significantly larger in their maximum length, asymptotic length and L50 compared to porpoises in the North Sea MU throughout the study period, suggesting limited gene flow between these two MUs. These results justify the maintenance of these harbor porpoise MUs or assessment units, as two separate units, within the range of the North-east Atlantic population, and for indicator assessments under the EU’s Marine Strategy Framework Directive.

No need to shout? Harbor porpoises (Phocoena phocoena) echolocate quietly in confined murky waters of the Wadden Sea.

Porpoise echolocation parameters may vary depending on their acoustic habitat and predominant behavior. Research was conducted in the Wadden Sea, an acoustically complex, tidally driven habitat with high particle resuspension. Source levels and echolocation parameters of wild harbor porpoises were estimated from time-of-arrival-differences of a six-element hydrophone array. The back-calculated peak-to-peak apparent source level of 169 ± 5 dB re 1 μPa was significantly lower than reported from Inner Danish Waters (-20 dB) and British Columbia (-9 dB) with narrower bandwidth. Porpoises therefore reduce their source level in the Wadden Sea under acoustically complex conditions suggesting an avoidance of cluttering.

Temporary Hearing Threshold Shift at Ecologically Relevant Frequencies in a Harbor Porpoise (Phocoena phocoena) Due to Exposure to a Noise Band Centered at 88.4 kHz

Temporary Hearing Threshold Shift at Ecologically Relevant Frequencies in a Harbor Porpoise (Phocoena phocoena) Due to Exposure to a Noise Band Centered at 88.4 kHz

Temporary Threshold Shift in a Second Harbor Porpoise (Phocoena phocoena) After Exposure to a One-Sixth-Octave Noise Band at 1.5 kHz and a 6.5 kHz Continuous Wave

Temporary Threshold Shift in a Second Harbor Porpoise (Phocoena phocoena) After Exposure to a One-Sixth-Octave Noise Band at 1.5 kHz and a 6.5 kHz Continuous Wave

Influence of environmental variability on harbour porpoise movement

The harbour porpoisePhocoena phocoenais a small marine predator with a high conservation status in Europe and the USA. To protect the species effectively, it is crucial to understand its movement patterns and how the distribution of intensively used foraging areas can be predicted from environmental conditions. Here, we investigated the influence of both static and dynamic environmental conditions on large-scale harbour porpoise movements in the North Sea. We used long-term movement data from 57 individuals tracked during 1999-2017 in a state-space model to estimate the underlying behavioural states, i.e. whether animals used area-restricted or directed movements. Subsequently, we assessed whether the probability of using area-restricted movements was related to environmental conditions using a generalized linear mixed model. Harbour porpoises were more likely to use area-restricted movements in areas with low salinity levels, relatively high chlorophyllaconcentrations and low current velocity, and in areas with steep bottom slopes, suggesting that such areas are important foraging grounds for porpoises. Our study identifies environmental parameters of relevance for predicting harbour porpoise foraging hot spots over space and time in a dynamic system. The study illustrates how movement patterns and data on environmental conditions can be combined, which is valuable to the conservation of marine mammals.

Summer Abundance of Harbour Porpoises (Phocoena phocoena) in the Coastal Waters of Iceland and the Faroe Islands

This study presents the first fully corrected abundance estimates for the harbour porpoise (Phocoena phocoena) for Iceland and the Faroe Islands. In both regions reliable estimates are needed to assess the impact of by-catch and other threats to harbour porpoises. Aerial surveys with harbour porpoise as a secondary and main target species were conducted in the summers of 2007 and 2010 in Icelandic and in Faroese coastal waters respectively. In Iceland, the cue counting procedure was used (which also produces the data required for line transect analysis), while in the Faroese, standard line transect sampling was applied, following the SCANS-II (Small Cetacean Abundance in the North Sea) survey protocol. In both surveys, identical aircraft surveyed at an altitude of 600 ft and a speed of 90–100 kn. Only data collected during Beaufort Sea States (BSS) lower than 3 and during good or moderate porpoise sighting conditions were used for abundance estimates. Abundance estimates were corrected using stratified estimates of esw (incorporating g(0)) values derived during the SCANS-II survey in 2005 as principal observers took part in this survey as well. In Iceland, realised effort in good or moderate harbour porpoise sighting conditions totalled 8,289 km in 13 survey strata, where 77 sightings (109 individuals) were made by the experienced harbour porpoise observer only. In Faroese waters, only part of the area inside the 300 m depth curve could be surveyed and 1,564 km were surveyed in good or moderate porpoise sighting conditions, yielding 39 sightings (49 individuals). The total abundance estimates were 43,179 porpoises (CV=0.45; 95% CI: 31,755–161,899) for Icelandic coastal waters and 5,175 porpoises (CV=0.44; 95% CI: 3,457–17,637) for Faroese waters.

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.

Nematode Parasites in Baltic Sea Mammals, Grey Seal (Halichoerus grypus (Fabricius, 1791)) and Harbour Porpoise (Phocoena phocoena (L.)), from the German Coast

PurposeEndoparasitic nematodes of six harbour porpoises Phocoena phocoena and four grey seals Halichoerus grypus, stranded at the eastern coast of the Baltic Sea in Germany in winter 2019, were analysed in order to identify nematode parasites and to compare with recent studies from the same area.MethodsEndoparasitic nematodes were identified by using both morphological and molecular characters. The successfully obtained sequences of the rDNA marker regions ITS-1, 5.8S, ITS-2 from 29 anisakid and the rDNA marker region ITS-2 of 11 pseudalid nematodes were amplified.ResultsAnalyses revealed the presence of three parasite species, the anisakid nematode Contracaecum osculatum from grey seals and the pseudalid nematodes Pseudalius inflexus and Stenurus minor from the harbour porpoises. Other anisakid nematodes regularly occurring in the Baltic Sea, e.g. Anisakis simplex or Pseudoterranova decipiens, were not found.ConclusionsThe prevalence of 100% and a severe parasite load in grey seals demonstrated a very high C. osculatum infection of Baltic Sea fish as their regular prey. Prevalence of 33% for parasites in harbour porpoises and minor infection rates, combined with a distinct lack of anisakid nematodes, are typical for the current situation of the porpoise parasite fauna in the Baltic Sea. Invasive parasite species as possible indicators for climate change could not be detected.

Presentation of Binning-Based Inter-Click Interval Data from Passive Acoustic Monitoring of Free-Ranging Harbour Porpoises (Phocoena Phocoena)

C-PODs are used for Passive Acoustic Monitoring (PAM) of harbour porpoises (Phocoena phocoena) at an offshore open sea location in the German North Sea. Diel patterns of echolocation click trains are extracted from minimum inter-click interval (minICI) data by binning. The aim of this study is to reassess and refine minICI ranges of click train data with particular consideration to the binning widths. Emphasis is also placed on choosing an appropriate visualisation of these binned data. Key ecological results include presence of higher train rates during the day with intermediate minICI values defined by the range 6-28 ms and a higher train rate with short minICI values 1.25-2.00 ms at night. This indicates an increase in porpoise feeding behaviour, or change of style, at night. Click trains with long minICI values > 35 ms occur at an equal rate throughout both diel phases, suggesting a more routine behaviour, such as navigation. Results could be revealed only by judicious choice of binning widths, e.g. previously overlooked patterns within historical echolocation data. The classification methodology can be used to analyse echolocation trains from a variety of species and can be applied to any PAM data with the relevant click parameters.

Prey-Related Asphyxiation in Harbor Porpoises (Phocoena phocoena) along the U.S. West Coast: Importance of American Shad (Alosa sapidissima) on Adult Female Harbor Porpoise Mortality

Harbor porpoise (Phocoena phocoena) diets are predominantly comprised of small fish species (<30 cm) and squid. However, predation on larger species (up to 63 cm) occurs, raising the question of increased risk of asphyxiation associated with this behavior. Literature was reviewed and stranding data from 1983 to 2020 from the U.S. West Coast (including California, Oregon and Washington) were searched for cases of prey-related asphyxiation of harbor porpoises and analyzed in relation to age, sex, reproductive status and prey species. Twenty-nine cases were documented. Twenty-seven cases involved large prey; non-native American shad caused the asphyxiation in 87% of the cases where the prey species was identified. The majority (92%) of harbor porpoises were females, and at least 83.3% were pregnant or recently post-partum. Reproductively active females may be more likely to attempt potentially risky behavior in order to compensate for their increased energetic needs. Increasing numbers of non-native American shad may pose a unique danger in this region for harbor porpoises not adapted to deal with the challenges of that prey. This may be a cause for concern, as there is likely an interaction between location, age and reproductive status on the diet composition and foraging strategies of harbor porpoises.

Health assessment of harbour porpoises (PHOCOENA PHOCOENA) from Baltic area of Denmark, Germany, Poland and Latvia

Harbour porpoise (Phocoena phocoena), the only resident cetacean species of the Baltic Sea is formed of two subpopulations populations, occurring in the western Baltic, Belt Seas and Kattegat and the Baltic Proper, respectively. Harbour porpoises throughout these areas are exposed to a large number of human activities causing direct and indirect effects on individuals, that might also harm this species on a population level. From Latvia, Poland, Germany and Denmark 385 out of 1769 collected dead harbour porpoises were suitable for extensive necropsy. The animals were collected between 1990 and 2015 and were either by-caught or found dead on the coastline. Following necropsies, histopathological, microbiological, virological and parasitological investigations were conducted. Females and males were equally distributed among the 385 animals. Most animals from the different countries were juveniles between 3 months and 3 years old (varying between 46.5 and 100% of 385 animals per country). The respiratory tract had the highest number of morphological lesions, including lungworms in 25 to 58% and pneumonia in 21 to 58% of the investigated animals. Of those with pneumonia 8 to 33% were moderate or severe. The alimentary, hearing, and haematopoietic systems had inflammatory lesions and parasitic infections with limited health impact. 45.5 to 100% of the animals from the different countries were known by-caught individuals, of which 20 to 100% varying between countries had netmarks. Inflammatory lesions, especially in the respiratory tract were found in higher numbers when compared to control populations in areas with less human activities such as arctic waters. The high number of morphological changes in the respiratory tract and of bycatches especially among immature animals before reaching sexual maturity is of serious concern, as well as the low number of adult animals among the material. Data on health status and the causes of death are valuable for management. A next step in this regard will combine data from health and genetic investigations in order to detect differences between the two populations of the Baltic.

Composition Analysis of the Colostrum and Attempted Hand-rearing of a Neglected Harbor Porpoise (<i>Phocoena phocoena</i>)

Composition Analysis of the Colostrum and Attempted Hand-rearing of a Neglected Harbor Porpoise (<i>Phocoena phocoena</i>)

Mycotic Infections in Free-Ranging Harbor Porpoises (Phocoena phocoena)

Studies on the occurrence of fungal communities in the marine environment are still scarce, but mycotic infections in cetaceans are increasingly reported. Fungal disease following infection with Aspergillus species is most frequently reported, with the respiratory tract commonly affected in cetaceans and other taxa, like humans and birds. Infection with Aspergillus spp. is dependent on the characteristics of the fungus as well as the hosts immune status, with dissemination into other organs being relatively common. Along the southern North Sea, harbor porpoise (Phocoena phocoena) strandings increased significantly since 2005 and necropsies to determine causes of death have been conducted since 2008. Here we describe the post-mortem findings in stranded, free-ranging harbor porpoises on the Dutch coast which were diagnosed with fungal disease, to determine the prevalence of mycotic infections, and to compare them to those described in other species. A total of 18/754 (2.4%) harbor porpoises showed lesions compatible with localized or disseminated fungal disease as confirmed by histological examination. The respiratory tract was most commonly affected (67%), followed by the central nervous system (CNS, 33%), and auditory system (AS, 17%). Aspergillosis was confirmed for 11/18, by fungal growth (as A. fumigatus species complex, n = 9) and PCR (as Aspergillus spp., n = 1, and as A. fumigatus sensu strictu by sequence analysis, n = 1). One live stranded and euthanized animal presented partial hemiplegia of the blowhole and therefore an MRI was conducted, which resulted in a unique image of the extensive, fungus-induced lesion in the left cerebellar hemisphere, deforming and displacing the brainstem, and additionally affected the AS. The gross- and histologic lesions in the 18 porpoises diagnosed with fungal disease were similar to changes described in other mammalian species. The prevalence of fungal disease in free-living harbor porpoises is lower than seen in captive and rehabilitated animals, suggesting that captivity increases the risk to develop mycotic infections. Finally, fungal infection in the CNS and AS are usually considered consequences of vascular dissemination originating from pulmonary foci. However, only 1/7 cases with otitis and/or encephalitis demonstrated pulmonary aspergillosis, suggesting a different pathogenesis.

Anisakid nematode species identification in harbour porpoises (Phocoena phocoena) from the North Sea, Baltic Sea and North Atlantic using RFLP analysis.

Harbour porpoises (Phocoena phocoena) are the only native cetacean species in the German North and Baltic Seas and the final host of Anisakis (A.) simplex, which infects their first and second gastric compartments and may cause chronic ulcerative gastritis. Anisakis simplex belongs to the family Anisakidae (Ascaridoidea, Rhabditida) as well as the phocine gastric nematode species Pseudoterranova (P.) decipiens and Contracaecum (C.) osculatum. These nematode species are the main causative agents for the zoonosis anisakidosis. The taxonomy of these genus with life cycles including crustaceans and commercially important fish is complex because of the formation of sibling species. Little is known about anisakid species infecting porpoises in the study area. Mature nematodes and larval stages are often identifiable only by molecular methods due to high morphological and genetic similarity. The restriction fragment length polymorphism (RFLP) method is an alternative to sequencing and was applied to identify anisakid nematodes found in harbour porpoises from the North Sea, Baltic Sea and North Atlantic to species level for the first time. In the study areas, five gastric nematodes from different harbour porpoise hosts were selected to be investigated with restriction enzymes HinfI, RsaI and HaeIII, which were able to differentiate several anisakid nematode species by characteristic banding patterns. Anisakis simplex s. s. was the dominant species found in the North Sea and Baltic porpoises, identified by all three restriction enzymes. Additionally, a hybrid of A. simplex s. s. and A. pegreffii was determined by HinfI in the North Sea samples. Within the North Atlantic specimens, A. simplex s. s., P. decipiens s. s. and Hysterothylacium (H.) aduncum were identified by all enzymes. This demonstrates the value of the RFLP method and the chosen restriction enzymes for the species identification of a broad variety of anisakid nematodes affecting the health of marine mammals.

Assessing the Effects of Banana Pingers as a Bycatch Mitigation Device for Harbour Porpoises (Phocoena phocoena)

Bycatch is a significant cause of population declines of marine megafauna globally. While numerous bycatch mitigation strategies exist, acoustic alarms, or pingers, are the most widely adopted strategy for small cetaceans. Although pingers have been shown to be an effective measure for numerous species, there are some concerns about their long-term use. Bycatch is recognised as a persistent problem in waters around Cornwall, United Kingdom, where several cetacean species are resident, with harbour porpoises (Phocoena phocoena) being the most-commonly sighted. In this study, we assessed the effects of a Banana Pinger (Fishtek Marine Limited) on harbour porpoises in Cornwall between August 2012 and March 2013. Two passive acoustic loggers (C-PODs; Chelonia Limited) were deployed 100m apart to record cetacean activity during cycles of active and inactive pinger periods. Harbour porpoises were 37% less likely to be detected at the C-POD near the pinger when the pinger was active, while they were only 9% less likely to be detected 100m further away. The effect of the pinger was constant over the study period at both C-PODs despite the temporal variation in harbour porpoise detections. In addition, we found no evidence of reduced pinger effect with changing environmental conditions. Furthermore, harbour porpoise detections at the C-POD near the pinger did not depend on the time elapsed since the pinger turned off, with harbour porpoises returning to the ensonified area with no delay. Together these results suggest that (1) harbour porpoises did not habituate to the pinger over an eight-month period, (2) the pinger effect is very localised, and (3) pinger use did not lead to harbour porpoise displacement over the study period, suggesting an absence of long-term behavioural effects. We suggest that the deployment of pingers on fishing nets would likely reduce net-porpoise interactions, thereby mitigating bycatch of harbour porpoises and potentially other cetacean species. As the small-scale fishery dominates in UK waters, there is an acute need for cost-effective mitigation strategies with concurrent monitoring to be implemented rapidly in order to address the problem of harbour porpoise, and more generally, cetacean bycatch.