Incidental catch (bycatch) in fisheries is one of the major threats to marine mammal populations worldwide. The harbour porpoise (Phocoena phocoena, Linneaus 1758) is one of the most common cetaceans in northern Europe. Previous studies have reported high bycatch numbers of harbour porpoises in gillnet fishing operations in the North Sea and Skagerrak Sea. With the aim of disentangling the driving factors behind bycatch of harbour porpoises, I looked at the relationship between occurrence of harbour porpoise bycatch and properties of fishery and porpoise density. Parameters of fishing effort, gear characteristics and operations of 951 sets of strings of cod and pollack bottom set gillnets were collected by an on-board observer programme in the Swedish Skagerrak and Kattegat Seas during 1995-1997. Porpoise density data was generated from satellite tracking locations of 116 individuals from 1997 to 2016. Generalised linear models with binomial distribution were applied to identify the relationships between the occurrence of porpoise bycatch and seven explanatory variables extracted as soak time, string length, net height, mesh size, net bagginess, water depth and porpoise density. During the observer programme, a total of 21 porpoises were bycaught during 10,174 km*h of observed fishing effort. The total bycatch was estimated to be 2.6% of the population abundance at that time, i.e. above the maximum sustainable total anthropogenic removal (1.7%). Soak time, string length and water depth were identified to be positively related to bycatch occurrence. The findings provide guidance for bycatch mitigation measures in terms of adjusting fishing operations, improving gear and establishing separate management units. Modelling can be used to predict possible spatio-temporal high-risk hot spots for harbour porpoise as well as other marine-living species to optimise future monitoring efforts.

Harbour porpoises (Phocoena phocoena) are the only cetacean residents found year-round in Swedish waters. Since in situ monitoring of cetaceans can be difficult, invasive and often costly, strandings can be used as a cost-effective alternative to continuously collect data on these elusive animals. In this study, spatiotemporal patterns, and their possible underlying causes, of harbour porpoise stranding reports in Swedish waters are investigated over the ten-year period of 2014–2023. When making spatial comparisons, for management purposes, the ten-year period is divided into two, 2014–2018 and 2019–2023. Data on 854 stranded harbour porpoises were analysed from the coasts of the Skagerrak, Kattegat, and Baltic Seas. Both significant spatial and temporal patterns could be identified, with strandings peaking in July to September and with hotspots occurring along most of the Swedish west coast, with the most frequent hotspots located around Öresund and especially the area around Kullen peninsula. The spatial patterns of strandings found in this study reflect data on porpoise abundance, prey abundance, and gillnet fisheries effort. The latter is known to be one of the primary causes of porpoise mortality. Furthermore, the coverage of the Swedish stranding network is analysed. While coverage of the stranding network overall has increased over the period, some areas still lack sufficient coverage, including the coast of Falkenberg, southern Gothenburg, northern Halmstad and certain areas around Lommabukten, north of Malmö. With this, we conclude that harbour porpoise strandings in Sweden show distinct spatiotemporal patterns that can be used as baselines for management and monitoring of these small cetaceans.

The aim of this report was to establish what category the harbour porpoise is listed as on the Swedish national redlist as well as analyze what potential sub-criteria the species fulfills besides criteria already established. The aim was also to evaluate the willingness to pay for conservation of the harbour porpoise for the first time and make a comparison of the willingness to pay between geographic places where the harbour porpoise is currently existing and where it is not. The subpopulation of the harbour porpoise in Skagerrak-Kattegat is listed as vulnerable (VU) on the national redlist and the subpopulation in the Baltic sea is listed as critically endangered (CR). Actions and precautions which have been made for conservation and protection of the harbour porpoises in Swedish waters are new Swedish laws and law proposals as well as formations of organizations such as ASCOBANS and further research. The results from the survey showed that the mean of the single time payment sum those questioned were prepared to pay to retain the porpoises at a stable population level was 273 SEK. Geographically the study showed no significant difference in geographic position and where the harbour porpoise exists but showed that the inhabitants of the south-east coast were willing to pay about half the sum, 165 SEK, of the mean sum that the north-east coast were willing to pay, 335 SEK. The results regarding the populations’ conjecture of historical threats and threats nowadays showed a significant difference from reality. Overfishing of prey had the highest amount of answers as a threat both historically and nowadays. Bycatch, which is accepted as todays biggest threat by researchers and organizations, came third place in the results from the survey study with a percentage of 14.5 %.

More than a third of the earths marine mammal species are threatened. This study aims to analyze and evaluate the conservation of the proper Baltic harbour porpoisein Sweden. The population is estimated to consist of only 500 individuals and are likely to be extinct if measures are not taken. A new Swedish conservation planfor harbour porpoises will be implemented in 2021 and will be the first since 2013. This study analyzes the suggested conservation plan and aims to identify strengths and weaknesses in the actions. Through a grounded theory approach, a qualitative content analysis is performed to identify the action strengths and weaknesses. A scoring matrix was developed to create indicators of the actions effort, delivery and conservation. The findings show that the short-term goals and the actions have a multidirectional approach which lack an alignment to the vision and the long-term goal of the plan. The study presents the action efficiency, which shows the need for, and importance of prioritizing the actions with the most potential impact for conservation. Mainly, the study provides a framework for species conservation plan evaluation and by performing the plan evaluation prior to implementation, the study contributes to the field of environmental evaluations. / Över en tredjedel av jordens marina däggdjursarterär hotade. Studien analyserar och utvärderar Sveriges planer för bevarandet av Östersjötumlaren. Populationen beräknas bestå av endast 500 individer och är trolig att utrotas om förändring inte implementeras. En bevarandeplan för tumlare ska implementeras under 2021 och kommer att vara den första bevarandeplanen sedan 2013. Studien analyserar den föreslagna bevarandeplanen och identifierar styrkor och svagheter i de föreslagna åtgärderna. Genom en grundad teori genomförs en kvalitativ innehållsanalys av åtgärdernas styrkor och svagheter. En poängmatris utvecklades för att möjliggöra indikatorer för åtgärdernas ansträngning, utdelning och bevarandepotential. Studien presenterar huvudsakligen ett ramverk för utvärdering av artbevarandeplaner. Genom att utvärdera bevarandeplanen innan implementering bidrar studien till området för miljöutvärderingar.

The bycatch of cetaceans in fishing gear is considered to be one of the biggest conservation threats to these species. Gear modifications have the potential to reduce these bycatches in global fisheries but there is little available information on how such modifications may change the fishing performance of gear, or indeed the behavior of cetaceans interacting with fishing gear. Generalized linear models (GLMs) were used to identify factors related to cetacean bycatches in UK bottom set gillnets. Rigged net height had a significant positive relationship with harbour porpoise (Phocoena phocoena) bycatch in ICES Area VII suggesting that lowering the profile of gillnets may have the potential to reduce bycatch rates. Modifications to gillnets, such as changing the amount of floatation or increasing the density of the meshes, were found to have significant effects on the active fishing heights of these nets. However, results from a bycatch mitigation trial in Argentina showed that the reduced fishing profile of one experimental net did not result in a concurrent reduction in the bycatch rate of Franciscana dolphins (Pontoporia blainvillei). While there was no significant difference in the rate, length or intensity of harbour porpoise encounters in the presence or absence of gillnets, the proportion of fast echolocation click trains were significantly higher when a net was present, indicating that porpoises either increased acoustic inspection of the net or foraging in the vicinity of the net. An analysis of underwater video footage collected inside trawl nets in an Australia fishery showed that bottlenose dolphins (Tursiops sp.) were present inside nets more frequently than they were caught and were actively foraging inside these nets. The orientation of dolphins inside these nets indicates that the current design of excluder devices used in this fishery could be improved to further reduce bycatch rates.

The main purpose of this study was to produce and test the reliability of predictive models of cetacean distributions off the west coast of Scotland. Passive acoustic and visual surveys were carried out from platforms of opportunity between 2003 and 2005. Acoustic identifications were made primarily of harbour porpoises (Phocoena phocoena), delphinids, and sperm whales (Physeter macrocephalus). Generalised Additive Models (GAMs) were used to relate species’ distributions to a range of environmental variables over a range of temporal and spatial scales. Predictive models of delphinid distributions showed both inter-annual and inter-month variations. Combining all data for all months and years resulted in a model that combined the environmental influences from each monthly and yearly model. Overall, delphinids were found to associate with the deep (> 400m) warm water (10.5°C-12.5°C), and in areas of deep thermocline. Relationships between sperm whales and environmental variables were consistent over changes in grain size (9 km or 18 km), but not between areas. Although sperm whales were distributed in deep water characterised by weak thermoclines and strong haloclines in the most northerly area (Faroe-Shetland Channel), they were found in deep productive areas with cold surface temperature in the more southerly waters (Rockall Trough). Within the southern Inner Hebrides, high use areas for harbour porpoises were consistently predicted over time (in years) and with differing survey techniques (acoustic versus visual), but not over space (southern Inner Hebrides versus whole of the Inner Hebrides). Harbour porpoises were mainly distributed in areas with low tidal currents and with higher detection rates during spring tides. The use of prey as a predictor variable within models of delphinid distribution shows some promise: there were correlations between delphinid and herring (Clupea harengus) in shelf-waters in 2005 but not in 2004. These models can be used in mitigating acoustic threats to cetaceans in predicted high use areas off the west coast of Scotland.

Incidental catch, or bycatch, of harbour porpoise (Phocoena phocoena) in fishing operations is an international conservation issue. The main objective of this thesis was to develop methods for determining the impact of bycatch on the state and dynamics of porpoise populations and for calculating bycatch limits that will achieve conservation objectives in the future. I applied these methods to the North Sea as a case study. First, I analysed sighting rates of harbour porpoise on seabird surveys in the North Sea during 1980-2003 to determine whether these data could provide informative time-series of relative abundance. Some general patterns and trends in sighting rates were consistent with previous studies. However, the standardised indices of abundance were relatively imprecise and thus have limited value for a monitoring framework that relies on statistical detection of trends. Second, I used a population model to integrate available data on harbour porpoise in the North Sea and to assess the dynamics of the population during 1987-2005. There was a high probability that bycatch resulted in a decrease in abundance. The estimated life history parameters suggested a limited scope for population growth even in the absence of bycatch. The model and data were not informative about maximum population growth rate or carrying capacity. The model suggested that dispersal was the most plausible explanation for observed changes in distribution within the North Sea. Third, I considered management procedures for calculating bycatch limits. I performed simulations to compare the behaviour of the procedures, to tune the procedures to specific conservation objectives and to test the robustness of the procedures to a range of uncertainties regarding population dynamics and structure, the environment, observation and implementation. Preliminary annual bycatch limits for harbour porpoise in the North Sea ranged from 187-1685 depending on the procedure, tuning and management areas used.

Genetic variation is essential for biological evolution, for maintaining viability of populations, and to ensure ecosystem resilience. Increased human exploitation and environmental change result in rapid loss of biological variation, including genetic diversity. Measures to halt this trend require that biological diversity is assessed and monitored. Assessment of biodiversity includes identifying patterns of distribution of genetic variation within individual species. This thesis focuses on spatial genetic structure and assessment of units for conservation in continuous environments without apparent migration barriers. Empirical data refer to Scandinavia and the model species are northern pike (Esox lucius), brown trout (Salmo trutta), and harbour porpoise (Phocoena phocoena). Questions regarding monitoring genetic diversity and releases of alien populations are also addressed.  The spatial genetic structure of the northern pike in the Baltic Sea is characterized by isolation by distance and continuous genetic change. Positive genetic correlation was found among pike within geographical distances of less than 150 km. This distance may be used to suggest management units in this area. For the brown trout, genetic monitoring identified two sympatric populations within a small mountain lake system. The situation is characterized by a clear genetic but no apparent phenotypic dichotomy. Scientific support for a genetically distinct Baltic harbour porpoise population is limited, and the spatial genetic structure of the harbour porpoise in Swedish waters needs to be clarified. Data for launching conservation genetic monitoring programs is available for only a few Swedish species. Millions of forest trees, fish, and birds are released annually in Sweden and the documentation on these releases is poor. To meet responsibilities of safeguarding biodiversity and surveying biological effects of releases, there is an urgent need for studies aimed at evaluating genetic diversity. / At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Incidental bycatch in gillnets is a substantial threat to small cetaceans. Using Acoustic Deterrent Devices, “pingers”, have successfully reduced bycatch of harbour porpoises in gillnets. However, seals can use pingers as “dinner-bells” to easier find gillnets in order to raid and destroy them, further aggravating the existing conflicts between seals and coastal fisheries. Therefore, in the present study, the efficiency of two alleged “seal-safe” pingers, an experimental Banana pinger “SSB” and a Future Oceans F70 pinger “FO”, in deterring harbour porpoises from the vicinity of gillnets and thereby reducing bycatch in commercial gillnet fisheries, was tested. This was done by deploying click detectors, “C-PODs”, recording Detection Positive Minutes per hour, at each end of gillnets, provided with the two pinger types or no pingers at all. Bycatch instances were recorded into logbooks by participating fishermen and verified using video footage from on-board video cameras. Results showed that video monitoring was a reliable method for verifying the number of bycatches of porpoises and seals, but not seabirds, recorded in the fishermen’s logbooks. The experimental SSB pingers and the FO pingers significantly reduced porpoise presence, measured as Detection Positive Minutes per hour in the vicinity of the nets, compared to gillnets without pingers. However, the sample size was too small to yield a significant result regarding the bycatch reducing efficiency and dinner bell effect of the experimental pingers. Nevertheless, bycatch trends suggest that pingers did in fact reduce porpoise bycatch. Although both successful, FO pingers were slightly more efficient in deterring porpoises than SSB pingers. The SSB pinger sounds had bigger directionality variations than the FO pinger, which may have affected its deterrent effects. Therefore, additional trials are needed to further investigate this aspect.

De part l'importance et la diversité des pressions anthropiques en mer du Nord, les mammifères marins en tant que prédateurs supérieurs se trouvent de plus en plus exposés à ces activités (trafic maritime, activités industrielles et portuaires, surpêche, pollution chimique, etc.). Les campagnes d'observation SCANS et SCANS II mises en place en 1994 et 2005 pour estimer l'abondance des petits cétacés, ont mis en évidence un changement majeur dans la distribution du marsouin commun (Phocoena phocoena) en mer du Nord avec un déplacement du Nord vers le Sud. Ce changement peut être lié à une migration de ses proies préférentielles en mer du Nord et/ou à une afaptation de son régime alimentaire par rapport à la disponibilité des proies. Parallèlement, à ces modifications de distribution spatiale, un nombre croissant d'échouage de marsouin commun en Manche Orientale et sur les côtes belges a été observé depuis une dizaine d'années avec une augmentation conséquente ces deux dernières années. Pour étudier les causes responsables de ces échouages, un des objectifs de cette étude a été d'évaluer l'état de la contamination chimique des animaux échoués en relation avec l'état sanitaire des individus. L'analyse de deux familles de composés chimiques (éléments traces métalliques et polluants organiques persistants) sur des animaux échoués sur la période 2006-2013 révèlent des concentrations significativement plus élevées dans les organies des animaux présentant des pathologies que celles obtenues chez les animaux sains. Cette tendance a déjà été observée en Atlantique Nord pour le marsouin. Les comparaisaons des niveaux de concentration mesurées avec ceux d'études antérieures effectuées sur des marsouins échoués dans la même zone ou dans le Golfe de Gascogne suggèrent que l'augmentation du nombre de marsouins échoués n'est pas liée à une dégradation du milieu en terme de pollution chimique. Pour déterminer le régime alimentaire du marsouin commun, trois techniques complémentaires ont été utilisées : l'analyse des contenus stomacaux, des isotopes stables (carbone et azote) et des acides gras dans différents tissus. Pour ces deux dernières méthodes, les signatures obtenues pour le marsouin ont été comparées à celles de leurs proies potentielles. Les résultats ont mis en évidence la présences des gobies, merlans, lançons, sprats, trisopterus sp., harengs et sardines comme proies potentielles. Le déplacement des marsouins du nord jusqu'au sud de la mer du Nord a été attribué à la baisse de l'abondance du lançon dans le nord, ainsi qu'à la ré-invasion du sud de la mer du Nord par la sardine, probablement suite au changement climatique. Enfin, cette étude confirme la nécessité d'utiliser une approche multi-analyses qui intègre des informations complémentaires à différentes échelles de temps pour étudier le régime alimentaire de ces prédateurs supérieurs. / The North Sea is heavely impacted by human activities such as overfishing and pollution. Due to their position as top predators in the ocean, marine mammals are becoming increasingly affected by anthropogenic activities. The large-scale surveys SCANS in 1994 and SCANS II in 2005 that were held in the North Sea to estimate the abundance of small cetaceans highlighted a major shift in the distribution of harbour porpoises (Phocoena phocoena) from the northern parts of the North Sea to its eastern parts. Alongside, over the past few decades harbour porpoises stranding has increased in the southern North Sea particularly along the French and Belgian coastal waters. Since the contaminant exposure presents, among others, a potential threat to harbour porpoises inhabiting the North Sea, the first objective of the present study was to assess the contamination status of this species in the southern North Sea. On the other hand, the distribution and abundance of marine mammals is expected to follow the distribution of their main prey species. Hence, the second objective of this study was to investigate whether the changes in the distribution of porpoises in the southern North Sea may be a result of the changes in prey availability. Moreover, the third objective was to evaluate the interest of combining three methods to investigate the diet of harbour porpoises : stomach contents, stable isotopes (carbon and nitrogen) and fatty acids analyses. First, the contamination status was evaluated through the determination of two components of chemical contaminants (metals and persistent organic pollutants) in tissues of harbour porpoises stranded along the southern North Sea between 2006 to 2013. Several chemical contaminants presented higher concentrations in diseased animals compared to healthy animals. In addition, some metallic contaminants showed bioaccumulation with age. Comparison with previous study suggests that the population status of harbor porpoises in term of chemical concentration has been stable from 1994 to 2013. This work suggested that the increase in the number of stranded individuals is not related to the decline in the quality of the environment. Secondly, the shift in the abundance of harbour porpoises was evaluated and interpreted in the light of prey species abundance. Three techniques were used in order to determine the diet of porpoises. Results highlighted the presence of gobies, whiting, sandeel, sprat, trisopterus sp., herring and sardine as potential preys. The shift of the abundance of porpoises form the northern parts of the North Sea to its southern parts was attributed to the sandeel abundance decline in the northern parts of the North Sea along with the re-invasion of the southern North Sea by the sardine species, probably in response to climate change. Finally, the value of a multi-approach dietary analysis was evaluated. Besides overcoming the limitations of each method, combining different techniques that integrate diet over days and weeks allowed gaining more complete understanding of harbour porpoise's diet.