Marine biologist Dr. Kirstin Anderson-Hansen uses suction cups to attach a DTAG (Digital Acoustic Recording Tag) on harbor porpoise, Freja at research and education facility, Fjord & Bælt in Kerteminde, Denmark. DTAGs are used to test Freja's movement, depth, sound, heart rate, and communication clicks. As harbor porpoises are extremely elusive and challenging to study in the wild, scientists hope to discover more about them by maintaining a consistent record of Freja's physiology.

Dr. Kirstin Anderson-Hansen trains harbor porpoise Freja to hold onto a target within a metabolic dome floating on the water at Fjord&Baelt, a research an education center based in Kerteminde, Denmark. The training session prepares Freja to partake in an experiment conducted by scientists at the Danish Research Center for Marine Mammals, which measures the metabolic rate of harbor porpoises by monitoring carbon dioxide and oxygen levels in their breath. The goal is to understand how these cetaceans function in the wild and predict how anthropogenic threats may impact their survival.

Animal care worker, Marielle Staal, takes a blood sample from rescued juvenile harbor porpoise, Norah at porpoise rescue and rehabilitation facility, SOS Dolfijn based in Anna Paulowna in the Netherlands. As minor changes in the animal's condition can be detrimental towards recovery, the rescue team maintains weekly protocols of each patient and works closely with a veterinarian, who is specialized in the treatment of harbor porpoises. A key component thereof is regular blood work, drawn from the fluke, which reveals important information about the health status of the animal. Discovered emaciated on Dutch shores in March 11th, 2023, Norah was treated for an esophagus ulcer, gastritis, fungal infection, pneumonia, and given regular physical therapy for her muscle spasms. Norah was released back into the Dutch sea in Den Helder on September 4th, 2023. SOS Dolfijn is the only rescue and rehabilitation facility in Europe specialized on harbor porpoises. It is registered as an Institution for General Benefit (ANBI) in the Netherlands under number 8152.70.896 and its operational permit is provided by the Dutch Government (Rijksdienst voor Ondernemend Nederland) under permit number FF/75/2012/036. The account number of the policy holder with the Dutch Government is 203773202.

Animal care worker, Hester Simons and volunteer Marcus Bierman perform a weekly examination of juvenile harbor porpoise, Norah at rescue and rehabilitation facility, SOS Dolfijn, based in Anna Paulowna in the Netherlands. Blood work entails a crucial component of the health examination, as it provides information on the well-being and recovery of the patient. Prior to taking blood from the fluke, the team coats it with an orange-colored antiseptic. Discovered emaciated on Dutch shores in March 11th, 2023, Norah was treated for an esophagus ulcer, gastritis, fungal infection, pneumonia, and given regular physical therapy for her muscle spasms. Norah was released back into the Dutch sea in Den Helder on September 4th, 2023.

The research team and volunteers at the German Oceanographic Museum span a non-catching fishing net in the Great Belt at Fyns Hoved, Denmark, for harbor porpoise (Phocoena phocoena) bycatch mitigation research. They equip the net with Porpoise Alerting Devices (PALs), which emit specific acoustic warning signals intended to alert porpoises to the presence of the net, thereby encouraging them to avoid the area and reducing the risk of entanglement. PALs work by taking advantage of the porpoises' keen echolocation abilities, producing sounds that can be easily detected and recognized as a deterrent. To measure the effectiveness of these devices, the research team employs a combination of visual and acoustic monitoring techniques. Visual observations are conducted from land and with the aid of drones, which provide a broad and real-time perspective on porpoise movements and behaviors around the nets. Underwater, hydrophones and F-PODs are strategically placed on the seabed. This comprehensive data collection helps assess the porpoises' reactions to PALs and guides the development of long-term strategies for reducing bycatch, ultimately aiding in the conservation of this species.

Specially trained sports diver, Susanne Bordin is lowered into the icy Baltic Sea water from a fishing vessel, where she will descend 30 meters (100 feet) alongside two team members to free a ghost-net caught in a shipwreck. The retrieval operation is organized by The Society for the Rescue of Dolphins on the island of Ruegen in Germany, with the aim of contributing towards a reduction of marine by-catch in fishing nets.

A specialized team of sports divers and volunteers at GRD (Society for the Rescue of Dolphins) work together to retrieve an old ghost net from a shipwreck in the Baltic Sea near the island of Ruegen in Germany. Nets are located using a sonar, which can scan strips up to 100 meters wide. Locations, where a suspected object is identified, are viewed, documented and verified by divers using an app called GhostDiver. The ghost net retrieval operation is led by seasoned diver Wolfgang Frank. Divers descend 30 meters (100 feet) underwater in groups of three. As visibility is nonexistent in the murky depths of the Baltic Sea, one diver is responsible for lighting the scene, while the other two free the net and attach lift bags to it. The lift bags are then inflated, so that the net can safely float to the surface, where it is pulled out onto a fishing vessel. Annually, between 5,000 and 10,000 fishing nets land in the Baltic Sea alone. Accidental by-catch in fishing nets is estimated to kill 4,000 harbor porpoises in the North Sea and Belt Sea annually.

The Meerwind Süd-Ost offshore wind farm is located 23km (14 miles) north of the island of Helgoland in the German North Sea and consists of a total of 80 wind turbines that supply around 360,000 German households with electricity. Built in April 2014, it was the first commercially operated wind farm to be completed in Germany and has since been regarded as one of the country's most promising offshore projects. However, offshore projects like Meerwind Süd-Ost have faced strong criticism from scientists due to their detrimental impacts on wildlife. These effects include habitat disruption, collision risks for birds and bats, and underwater noise pollution that can disturb marine mammals such as whales and dolphins. Furthermore, the construction and operation of offshore wind farms can lead to changes in the marine ecosystem, affecting fish populations and other marine organisms. While offering a cleaner alternative to fossil fuels, marine experts emphasize the need to mitigate the potential environmental impacts associated with these projects.

Ranger Jaap van der Hiele is part of the Marine Rescue Team Foundation, which collaborates with harbor porpoise rescue, rehabilitation and research facilities across Dutch coastlines. As the first port of call, he examines the porpoise on site and notifies Utrecht University or SOS Dolfijn of the stranding, depending on whether the cetacean is dead or alive. If the cetacean is still alive, a team of rescue volunteers trained by SOS Dolfijn arrive on site and bring the animal to the rehabilitation center. A deceased porpoise is photographed, measured, and sexed to determine whether it is suitable for necropsy. If these criteria are fulfilled, it is collected and transported to the pathological department in Utrecht University. As harbor porpoises are considered to be critically endangered, this stranding network allows marine biologists in the Netherlands to collect valuable data on the species, determine how anthropogenic threats and impacting population sizes, and provide conservation recommendations.

Research and Education Assistant, Manon Lock, and intern Eva Schotanus perform an external examination on a deceased harbor porpoise at the University of Utrecht in the Netherlands. After measuring and weighing the animal, they photographically document any surface lesions, net imprints, and wounds on the cetacean. The aim of necropsies is to examine the cause of death and/or stranding of porpoises and determine what percentage of anthropogenic impacts was responsible for their decease. Necropsies of harbor porpoises in the Netherlands are commissioned by the Ministry of Agriculture, Nature and Food Quality, and since 2016 embedded under the Legal Research Tasks Nature & Environment unit of Wageningen UR (project reference numbers 140000353, WOT-04-009-045).

Research and Education Assistant, Manon Lock, performs a necropsy on a deceased harbor porpoise at the pathological division of the University of Utrecht in the Netherlands. After measuring and weighing the animal, she photographically documents any surface lesions, net imprints, and wounds on the cetacean. Thereupon, she opens the cetacean to extract samples of tissue, bone, organs, and excretions. The aim of necropsies is to examine the cause of death and/or stranding of porpoises and determine what percentage of anthropogenic impacts was responsible for their decease. Necropsies of harbor porpoises in the Netherlands are commissioned by the Ministry of Agriculture, Nature and Food Quality, and since 2016 embedded under the Legal Research Tasks Nature & Environment unit of Wageningen UR (project reference numbers 140000353, WOT-04-009-045).

Ph.D. candidate, Lisa Klemens, examines the contents of a harbor porpoise stomach at the German Oceanographic Museum in Stralsund, Germany. The content of harbor porpoise stomachs is used to ascertain the nutritional composition, specifically the type, quantity, size and age of the fish that the cetaceans consume. Harbor porpoises swallow their fish whole and their stomach acids are efficient at breaking down fish skeleton. Therefore, oval-shaped structures called otoliths are used to conduct dietary research, as they consist of calcium carbonate and are the last to be decomposed. Otoliths are a hearing and balancing organ located in the inner ear of the fish. Just like trees and teeth, annual growth rings in otoliths allow the determination of the age of the fish. Moreover, the varying shapes and textures of otoliths serve to distinguish fish types. By conducting this research, scientists aim to determine the abundance of food sources available to harbor porpoises in the sea and discern to what extent fishermen are in competition with the cetaceans. The German Oceanographic Museum has a research permit issued by the Federal Ministry for the Environment, Nature Conservation, Construction and Nuclear Safety (Bundesministerium für Umwelt, Naturschutz, Bau und Reaktorsicherheit) in Germany.

At 28 years old, Freja is the most well-studied and oldest harbor porpoise on Earth. Since 1997, she has been based at Fjord & Bælt, after being by-caught in a fisherman’s net and deemed non-releasable. Freja is trained to partake in non-invasive behavioral experiments, such as this one, in which her eyes are covered by suction cups and she is asked to discern between two targets in the water. Thereby, scientists hope to better understand how harbor porpoises use echolocation to hunt in the wild.