Common porpoise
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Two common porpoises ( Phocoena phocoena ) |
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| ( Linnaeus , 1758) |
The common porpoise ( Phocoena phocoena ) is a toothed whale up to 1.85 meters long . Its color is black on top and white on the underside. It lives in the coastal waters of the North Atlantic off Europe , northwest Africa and eastern North America , in the Black Sea and in the American and Asian coastal waters of the North Pacific . Its food is fish , crustaceans and octopus .
Harbor porpoises are by far the most common whales in the North and Baltic Seas , but the population is declining. The causes are likely to be the discharge of poison into the oceans and death by suffocation in fishing nets .
In older literature in particular, the common porpoise is also called "bottlenose dolphin", "brown fish" (Dutch: bruinvis ) or "guinea pig".
Appearance
With a maximum body length of 1.85 meters, in very rare cases even over two meters, the porpoise is the smallest species of whale found in European waters . The body sizes fluctuate depending on the study area, the harbor porpoises in the Baltic Sea are only about 1.40 meters long (males) or 1.52 meters (females) on average . Its body weight averages between 50 and 60 kilograms and a maximum of about 90 kilograms, with the females being larger and heavier than the males. As with most whales, the sex is differentiated based on the location of the anus opening and the genital fold. In the females these are close together in the anus area, in the males they are clearly separated, whereby the genital fold is further forward.
Compared to other toothed whales, the skull of the animals is only slightly extended at the front , but this “ snout ”, the rostrum, is not externally recognizable due to a layer of fat on top . The body is stocky with a flat and triangular dorsal fin . The back is black and has a gray field emanating from the dorsal fin, the belly is white. A black drawing runs from the corner of the mouth to the front of the flipper .
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The dorsal fin, the pectoral fins, the caudal fin ( fluke ) and the fluke stalk are also colored black. In juvenile animals, parts of the ventral side are also black; this is called juvenile melanism . Albinism is very rare among porpoises. The dorsal fin itself does not have a concave curve, but slopes vertically at the back, the base is about twice as long as the height of the fin. The flippers are relatively short and have a pointed end. The Fluke is about two feet wide and stoutly built.
The jaws contain in the upper jaw 22 to 28 on each side, in the lower jaw 21 to 25 very short teeth , which are leaf-shaped to spatula-shaped. The rear teeth have a three-humped chewing surface.
Population and distribution
The harbor porpoise prefers shallow waters, migrating to coastal waters in spring and to areas further away from the coast in autumn. Its distribution area covers large parts of the northern hemisphere. On the American North Pacific coast you can find the animals from Los Angeles to the mouth of the Mackenzie River in the Beaufort Sea , on the Asian Pacific coast from the Yellow Sea to the Chukchi Sea . In the North Atlantic you can find it on the east coast of America from Cape Cod to Upernavik , sometimes also on the Greenland coast near Thule .
The eastern Atlantic coast is inhabited by harbor porpoises from North Africa ( Senegal , Mauritania , Morocco ) across the entire European coastline to the coast of Svalbard including the North Sea. In the Baltic Sea, the Belt Sea is settled, with populations being exchanged with the North Sea via the Kattegat . The Mediterranean does not have its own stocks, but it is not uncommon for immigrants to migrate into the western Mediterranean to Mallorca via the Strait of Gibraltar and into the eastern Mediterranean ( Aegean Sea ) from the Black Sea, where there is a separate population. More recently, harbor porpoises have been sighted again in north German rivers, after they had increasingly disappeared due to water pollution for a hundred years.
The total number of harbor porpoises still alive today is unknown, but it is believed that it is still very high worldwide. In its monitoring report, the Federal Agency for Nature Conservation gives the number for German North Sea waters as 15,000 to 54,000, depending on the season. The total population for the North Sea is estimated at 300,000 specimens. The individual numbers of individual local populations are smaller, especially in the Black Sea and the Baltic Sea. In the Baltic Sea, the population in the western part is estimated at 800 to 2,000 animals in the eastern and central parts at 100 to 600 animals. Harbor porpoises are particularly at risk from fishing and the noisy seas from shipping and pile driving for offshore wind farms, where the noise-reducing bubble curtain is not used for reasons of cost . The number of dead finds has tripled in the last 10 years on the coast of Mecklenburg-Western Pomerania. The species is classified as endangered by the International Union for the Conservation of Nature and Natural Resources .
The harbor porpoise is under nature protection in all European countries and is listed in Appendix II of the Washington Convention on the Protection of Species . The European Union prohibits import, transport and keeping.
Way of life
The harbor porpoises prefer quiet coastal areas with a moderate depth of about 20 meters as their habitat, but are occasionally also found in deep sea waters.
Porpoises feed almost exclusively on fish, as well as on bristle worms , snails , crustaceans and squids . The food composition varies depending on the geographical conditions. Flatfish (Pleuronectiformes) represent a very large proportion in the North Sea, gobies (Gobiidae) in the Baltic Sea , and cod ( Gadus morhua ) in both waters . The fish that are eaten are usually smaller than 25 centimeters, as the porpoises cannot swallow larger fish. The foraging for food takes place mainly at the bottom of the water, where the porpoise digs up the ground. The daily ration of a whale is around 4.5 kilograms of fish.
The main natural enemies of porpoises are larger sharks and killer whales . So they found Schweinswalreste in the stomach of the Greenland Shark ( Somniosus microcephalus ) and the white shark ( Carcharodon carcharias ). The killer whale ( Orcinus orca ), however, is the main natural enemy of the porpoise compared to the sharks. In addition, other toothed whales occasionally behave aggressively towards their little relatives. Bottlenose dolphins ( Tursiops truncatus ) and common dolphins ( Delphinus delphis ) have been observed several times, inflicting severe injuries on porpoises by knocking their head into the flank.
The parasites that are particularly noteworthy are sea lampreys, as well as roundworms , flukes , tapeworms and scratches . The nematodes of the Ansisakis genus ingested through fish food are regularly found in large clusters in the whale's stomach and Stenurus minor populates the bronchi , the lungs and the cardiovascular system and can also make the whales deaf by settling in the hearing . A particularly common parasite of the gastrointestinal tract and the bile ducts is the suction worm Campula oblanga , which can cause hepatitis and cholangitis , among other things . In contrast, external parasites such as wall lice are rarely found in porpoises.
behavior
Harbor porpoises mostly live solitary or in groups of two. Larger groups of up to seven animals, called schools, have only rarely been observed so far. Several schools occasionally meet during the mating season and foraging, so that herds of over a hundred animals can arise. However, these are rare and do not last very long. Young animals always stay with their mother for a while, the exact duration is unknown. The bond between the young animal and mother is very close, and cubs left alone emit stressful sounds to call the mother.
It is also not known whether harbor porpoises form territories and defend them against intruders or whether there is a hierarchy among porpoises. However, threatening behaviors have been reported in harbor porpoises. One animal threatens another by turning its head and uttering clicks, after which there is a nod of the head and blows with the tail.
Harbor porpoises reach speeds of a maximum of around 22 km / h and very rarely jump out of the water. The maximum diving depth is about 90 meters and the diving time is about six minutes. Most of the time, the animals swim at a speed of about 7 km / h just below the surface of the water and, during normal swimming, pierce it about two to four times per minute to breathe. When surfacing, the whale bends its body into a semicircle and immediately after breathing it dives head first. Hermann Burmeister described this behavior in 1853 as follows:
“The animal first emerges with the head out of the water and panting heavily for breath; then it bends headfirst downwards, as it were, so that the neck, the back with the high fin fin and finally the back of the tail rise out of the water in a strongly curved position; but neither the broad caudal fin nor the pectoral fins come to face. "
Propulsion is almost exclusively provided by the caudal fin, which is flapped up and down. The main purpose of the pectoral fins is to control and stabilize in the water. The texture of the smooth skin and the streamlined shape of the animals have a significant influence on the speed . There are no extended periods of rest, instead the animals remain in a resting position on the surface of the water for four to six seconds several times an hour, whereby they sink and then fall back into their natural rhythm of movement.
The ability to make sounds plays an important role in the behavior of porpoises, and the spectrum is very large. The animals communicate with click sounds made up of high-frequency (110 to 150 kilohertz ) and low-frequency (around 2 kilohertz) tones. In addition, there are tones that the animals emit for echo localization, which contain both low-frequency components around 1.5 kilohertz and high-frequency components around 100 kilohertz. By examining the sounds, it was possible to identify typical sounds of exploration and orientation, dominance , solicitation for partners, assistance and warnings of dangers. For evolutionary biology , it is interesting to note that the location sounds of porpoises are outside the hearing range of the killer whale . It is believed that this difference developed as a result of the predator-prey ratio .
Reproduction and development
The females of the porpoises reach sexual maturity at around three to four years of age , the males after two to three years. The mating season in European waters is between mid-July and late August. During this time, the testicles of the male animals swell enormously, they weigh about two grams during most of the year and over 400 grams during the mating season. Mating takes place in most populations in deeper water, in others in the very shallow coastal area.
Most of the observations on mating behavior come from captured porpoises. It consists of a foreplay and the subsequent pairing. A male first pursues a selected female and tries to make initial contact with the dorsal fins. Heck (1915) described this as follows: "During the heat they are extremely agitated, they rush through the tide at lightning speed, pursue each other furiously and eagerly chase after the females."
This is followed by “stroking” and “cross-swimming” of the animals. In addition, the male shows the ventral side and nibbles on the fins of the female. The copulation takes place perpendicular to the water surface and only takes a few seconds. After that, foreplay and copulation can repeat themselves.
The pregnancy of porpoises lasts about ten to eleven months, so that the young are born in early summer between May and June. Usually only one young is born, twin births are extremely rare. There is disagreement about whether a female has a cub every year or just one every two years. The birth itself is relatively uncomplicated due to the lack of pelvic bones in the whales and takes place during normal swimming. The peristaltic birth waves last about one to two hours. The young animal and the placenta , which forms the afterbirth , separate from each other after the umbilical cord breaks off as the young animal leaves the head as the last part of the body. Immediately after birth, the young swim independently to the surface of the water and take their first breaths.
The young animal is between 65 and 90 centimeters long at birth and weighs between five and seven kilograms. The young animal is suckled by the mother for eight to nine months, but it already eats its first fish food at five months. When suckling, the mother lays on its side, allowing the young to breathe on the surface of the water. About half of the milk is made up of fat and, compared to other mammals, contains a high proportion of raw protein and minerals . With the beginning of the fishing the first teeth break through in the young animal, with about seven months the young animals have their complete set of teeth , after about a year they separate from the mother. Mothers and their young are usually much closer to the coast than their conspecifics.
The maximum age of porpoises is estimated to be around 20 years, with most animals no older than eight to ten years.
Research history
Along with dolphins, the common porpoise is one of the earliest accessible whales for research, as it could be observed from land as a resident of the flat coastal areas of Europe. Rock carvings from the Stone Age , such as those found in Roddoy and Reppa ( Norway ), show that the animals were also known to early cultures. A large part of the findings that apply to the entirety of whales or toothed whales were first discovered in the porpoise.
A first description of a porpoise was provided by Aristotle in 350 BC. BC with the information that the pregnancy of the whales he called Phokaina lasts about ten months and that the whales keep their heads above water asleep and "snore". Nevertheless, he assigned the whales to the fish. Its very precise descriptions were adopted by the Romans, but the content was mixed with the knowledge about the dolphins. Above all, Pliny the Elder should be mentioned here, who wrote a comprehensive natural history . This intermingling can also be found in the art of this and subsequent times. Since then, dolphins have been depicted with a high arched head typical of porpoises and a long snout typical of dolphins.
Until the late Middle Ages, the knowledge of the Greeks and Romans about porpoises was not significantly expanded, but often only further falsified and abstracted. In the writings of Konrad von Megenberg , around 1340, one can read about the "guinea pig", an old name for porpoises:
“Porcus marinus is the name of a guinea pig and is an edible fish. He is almost entirely in the shape of a real pig. Like the common pig, its tongue is loose, but it lacks the pig's voice. It has spikes on its back that contain poison. But the bile of the fish is an antidote to the poison. The guinea pigs suffer much fear and hardship, as Pliny reports, they look for their food at the bottom of the sea and dig like real pigs in the earth. They have a trunk on their throat. "
It was only the descriptions of the 16th century that became more scientific, especially those by Conrad Gessner , Pierre Belon and Guillaume Rondelet . Rondelet began to filter the fabulous parts from the animal descriptions based on critical observations. Through dissections he was also able to study the developmental stages of the fetus and look at the anatomy of the whale brain. Belon discovered the peculiarities of the whale skeleton through sections on porpoises and dolphins. Further knowledge came from Ulisse Aldrovandi and Johannes Jonstonus ( John Johnston ) in the 17th century. In a porpoise section of the Danish researcher Thomas Bertholin even King Frederik III was. with entourage present. Bertholin himself was the first scientist to describe the larynx typical of toothed whales . Even Edward Tyson and Francis Willughby brought new insights.
One of the first anatomical descriptions of the harbor porpoise's airways comes from the year 1671 by John Ray , who nevertheless assigned the harbor porpoise to the fish, as has been customary since Aristotle.
“The tube in the head through which this species of fish draws its breath and spits water lies in front of the brain and ends on the outside in a simple hole, but on the inside it is divided by a bony septum as if it were two nostrils; but underneath it opens up again in the mouth in a cavity. "
The scientific description and the classification in the system was done by Carl von Linné in 1758 as Delphinus phocaena . He also assigned whales to mammals for the first time. Georges Cuvier created the genus Phocoena in 1816 . In the course of the 19th and 20th centuries, knowledge about whales and thus about porpoises increased massively. Especially the anatomy, the physiology and later also the behavior and the ecology of the whales have been extensively researched. Important works came from, for example, Étienne Geoffroy Saint-Hillaire , Wilhelm Ludwig Rapp and, later, especially Willy Kükenthal .
Systematics and subspecies
The harbor porpoise, together with the California harbor porpoise ( Phocoena sinus ) and the Burmeister harbor porpoise ( Phocoena spinipinnis ), belongs to the genus Phocoena within the harbor porpoise (Phocoenidae).
Just as the porpoise lives in many parts of the world, it can also look quite different, because the individual, regional populations hardly mix. The representatives in the Black Sea are consistently smaller than those in the Baltic Sea and the West Atlantic animals are larger than those in the East Atlantic off the European coast. In addition, the Baltic porpoises are darker than those in the North Sea and have a special layer of fat. The animals from the Black Sea have more tubercles in front of the dorsal fin and have physiologically adjusted to lower salinity.
The following are regarded as completely separate metapopulations and thus as subspecies:
- Phocoena phocoena vomerina in the Pacific
- Phocoena phocoena phocoena in the Atlantic
- Phocoena phocoena relicta in the Black Sea
Further, smaller-scale populations can be distinguished within these metapopulations. The harbor porpoises of the European coasts alone are divided into around ten more or less closed populations. For example, the population of the Baltic Sea seems to have a relatively large gene exchange with the population of the northern North Sea; in contrast to the harbor porpoises of the southern North Sea, there is a genetic barrier due to the different migratory behavior.
Threat and protection
Fishing and whaling
The harbor porpoise has been hunted since the Middle Ages. The harbor porpoise was first mentioned in Normandy , where it has been documented since 1098. The coast was assigned to the "Walmanni" who carried out organized fishing trips.
In Middelfart on Funen (Denmark) harbor porpoises have been caught since around 1500. Here it was held by a guild of "guinea pig hunters" ("Marsvinsjaeger-Langet") who caught the porpoises with ten boats and three men each. The rules of the guild were strictly defined and regulated by royal orders. Finn wrote (1878):
"Every year on Boxing Day, after the service is over, an old man is elected by the entire guild, under the presidency of the mayor, who himself elects 4 assessors and a chair brother, whose duty it should mainly be to perceive the best of the guild and to monitor compliance with the statutes ... "
The harbor porpoise was also used commercially in Flanders , in the English Channel , on the Danish, German and Polish coasts. As in the Middelfart example above, catching was often strictly regulated. For example, every “dolphin fisherman” on the Polish coast had to pay two marks a year in tax to the incumbent fishing master. The market prices were fixed around 1379 in Königsberg . In all countries whaling was more of a side business besides regular fishing and was only operated by a few fishermen.
Up until the 19th century, around 1,000 to 2,000 animals were caught per year, and by 1944 the quota had fallen to around 320 animals. Today harbor porpoises are mainly caught commercially in the Black Sea and are banned in all European countries. Correspondingly, like other small whales , harbor porpoises occur primarily as bycatch , but sometimes in large quantities of over 4,000 specimens per year. They get tangled in fishing nets from which they can no longer free themselves, so that they suffocate due to lack of oxygen.
Today the threat to the whale lies in the fishery as bycatch, in which the animals caught with it are often injured and let into the sea and then die in agony on the beaches.
Human pollution
The main threat to harbor porpoises today is the increasing pollution of the oceans. Heavy metals such as mercury , lead and cadmium in particular are deposited in the muscles and liver of whales. Fat-soluble environmental toxins such as polychlorinated biphenyls (PCB) or (now decreasing) dichlorodiphenyltrichloroethane (DDT) are concentrated in the fat layer . Tar residues and residues of oil films lead to skin necrosis and, together with the other poisonings, to a weakening of the animals, which in turn increases the number of sick animals and animals that are heavily infected by parasites. PCB levels in excess of 70 ppm (millionths of a fraction) can lead to sterility in seals and whales, an amount that has been found in quite a few porpoises. The highest PCB concentration in a harbor porpoise so far was 260 ppm and was found in 1976.
On the basis of analyzes in 2008 a total of eighteen porpoises were categorized as bycatch along the entire German Baltic Sea coast. In 2010 only six animals were diagnosed as “suspected bycatch”, but this can no longer be clearly determined due to the often advanced state of decomposition. If one compares the bycatch data of the North and Baltic Seas with the data worldwide, the bycatch rate outside the German coasts is considerably higher. The population in the Baltic Sea is now so small that even the smallest bycatch has dramatic effects. Especially in places where the worldwide ban on bycatch is not so strictly controlled and the penalties are low, many marine mammals are deliberately caught and not released again.
The increasing noise of the oceans also endangers the harbor porpoise. In 2007, an oil company planned to search for oil and gas deposits around the Doggerbank in the North Sea using low-frequency sound waves from air cannons. Sound waves with a strength of 180 decibels were planned. Nature conservation associations feared that the animals would be driven away by the strong noise emissions. The detonation of old ammunition also poses a considerable risk to harbor porpoises. Stress for the whales that live near the coast is also caused by the noise from ship engines, which disturbs the animals' orientation.
In addition, there has recently been the underwater noise that arises when driving certain foundations of offshore wind farms . During the foundation work for BARD Offshore 1 , individual event levels were measured at a distance of 750 meters from the pile, which were well above the UBA limit of 160 decibels. Currently (2012), however, protective measures such as bubble veils and cofferdams are being tested, which are to be used in further projects; these are already being used in the first projects. For example, bubble curtains were successfully used to reduce noise during the construction of the Trianel wind farm in Borkum . In addition, other foundation options are being investigated in which the noisy ramming of the monopiles can be dispensed with. According to the Federal Environment Agency, the upcoming intensive expansion of offshore wind energy is expected to result in irreversible hearing impairments or even deafness. It is possible that this species will be completely driven out of the habitats around the offshore wind turbines.
National and international conservation efforts
In large parts of their range, harbor porpoises are protected by national laws. An example of this is the Marine Mammal Protection Act in the USA.
In addition, there is international protection status in the countries of the European Union through directives such as the Flora-Fauna-Habitat Directive or the Marine Strategy Framework Directive . In northern and western Europe, states are also endeavoring to improve the conservation status of populations by working together under the Specialized Convention for the Conservation of Small Whales in the North Sea, the Baltic Sea, the Northeast Atlantic and the Irish Sea ( ASCOBANS ).
The population in the Black Sea is covered by the Convention for the Protection of the Whales of the Black Sea, Mediterranean and Adjacent Atlantic Zones ( ACCOBAMS ).
To draw attention to the threat to the porpoise, which gave German Post AG with the initial issue date January 2, 2019, a postage stamp in the denomination out of 45 euro cents. The design comes from Ingrid Hesse from Munich.
attitude
Because of its relatively small size, attempts have been made by aquariums and animal parks to keep harbor porpoises in captivity. However, due to its environmental demands and high susceptibility, it is not well suited for keeping. Most of the animals caught for keeping perish after a few weeks. For these reasons, harbor porpoises are now kept almost exclusively for scientific studies or to care for accidentally caught and injured animals.
The first known attempts at husbandry took place in London in 1862 and in the zoological garden in Hamburg in 1864, further in 1914 in Brighton and in 1935 in Berlin. In all of these experiments the animals perished after a few days. In London an attempt was made to fortify the animal, which was already severely weakened by the transport, with brandy , but it died after a few hours. It was not until the 1970s that interest in harboring porpoises rose again in the general boom in dolphinariums . They were held in New York (1970), Copenhagen (1970), Duisburg (1979), Constanța (since 1971) and in various other places, usually only for a few weeks. The lifespan of harbor porpoises in captivity is usually only two to three years, but some of the animals last longer, for example a female rescued in 1997 in Fjord og Bælt Centeret in Denmark. All harbor porpoises currently kept in Europe have been in human care for at least 7 years.
Currently (2020) seven porpoises are kept in Europe (2 males, 5 females). With the exception of one female born in human care, all of them are rescued animals that, due to their condition, cannot be released into the wild. A female lives in Fjord og Bælt Centeret , Kerteminde Denmark. Since 1997 rescued porpoises have been kept there in a demarcated part of the harbor basin and in August 2007 the first harbor porpoise born in human care saw the light of day there. Since February 2012 porpoises have been kept permanently in a separate tank at Ecomare (Texel, Netherlands). The two male specimens specimens were found as stranded young animals and cannot be released again due to their state of health. Two females rescued in 2001 and one female born here in 2012 live in the Dolfinarium Harderwijk. One female also lives at SEAMARCO in the Netherlands.
literature
- M. Carwardine: whales and dolphins. Delius Klasing, Bielefeld 2008 (high quality guide). ISBN 978-3768824736
- R. Kiefner: Whales and Dolphins worldwide. Year-Top-Special, Hamburg 2002. (Guide to the magazine “tauchen”, very detailed). ISBN 3-86132-620-5
- J. Niethammer, F. Krapp (ed.): Handbook of mammals in Europe. Volume 6. Marine Mammals, Part 1A Part 1 - Whales and Dolphins - Cetacea. Aula, Wiesbaden 1994. (standard scientific work). ISBN 3-89104-559-X
- Randall R. Reeves, Brent S. Stewart, Phillip J. Clapham, James A. Powell: Sea Mammals of the World. A Complete Guide to Whales, Dolphins, Seals, Sea Lions and Sea Cows. Black, London 2002, ISBN 0-7136-6334-0 (guide with numerous pictures).
- G. Schulze: The porpoises . Die neue Brehm-Bücherei 583. Ziemsen, Wittenberg 1987, Westarp, Magdeburg 1996. (2nd extended edition, detailed monograph). ISBN 3-7403-0048-5 , ISBN 3-89432-379-5
- G. Soury: The Great Book of Dolphins . Delius Klasing, Bielefeld 1997. (detailed illustrated book). ISBN 3-7688-1063-1
- M. Würtz, N. Repetto: Underwater world: Dolphins and Whales . White Star Guides, Vercelli 2003. (illustrated book, identification book). ISBN 88-8095-943-3
Web links
- The harbor porpoise in the Baltic Sea - BUND Mecklenburg-Western Pomerania
- Phocoena phocoena in the endangered Red List species the IUCN 2006. Posted by: Cetacean Specialist Group, 1996. Retrieved on 11 May, 2006.
Individual evidence
- ↑ Netzeitung: Harbor porpoises in German rivers ( Memento from May 5, 2008 in the Internet Archive ).
- ↑ Monitoring reports of the BfN ( Memento from November 3, 2014 in the Internet Archive )
- ↑ Baltic Sea Whale is dying out: Bottlenose dolphins in great need
- ↑ Whales stranding in the Baltic Sea
- ↑ a b Susanne Prahl: Investigations into the construction of the epicranial airways in harbor porpoises (Phocoena phocoena Linnaeus, 1758). Dissertation to obtain the doctoral degree of the Department of Biology of the Faculty of Mathematics, Computer Science and Natural Sciences of the University of Hamburg, Hamburg 2007 (PDF cannot be linked due to blocking filter), pp. 6–7.
- ^ J. Ray : An account of the dissection of a porpess. In: Philosophical Transactions of the Royal Society of London , Volume 6, 1671, pp. 2274-2279
- ↑ (See, inter alia, Wehrmeister, 2013: 11)
- ↑ taz article from April 3, 2007 on the use of air cannons in the search for oil / natural gas in the North Sea .
- ↑ Detonating old ammunition
- ↑ Hannes Koch: Offshore energy, wind farm boom threatens harbor porpoises in Spiegel Online Date 23 January 2011 Accessed: 24 January 2011
- ↑ Archived copy ( Memento from May 16, 2016 in the Internet Archive )
- ^ Bubble veil and cofferdam . In: Deutschlandfunk , June 28, 2012. Retrieved November 5, 2012.
- ↑ Hannah Petersohn: New procedure to protect whales - Latest news from Lower Saxony - WESER-KURIER. In: weser-kurier.de. October 7, 2012, accessed March 9, 2019 .
- ↑ C. Schulte (Federal Environment Agency, Department Head II 2), K. Blondzik (Federal Environment Agency, Section II 2,4): Waters in Germany: condition and assessment . Ed .: Federal Environment Agency. August 2017, ISSN 2363-832X , p. 121 .
- ↑ Ceta Base | Captive Cetacean Database - Fjord & Baelt Center • Denmark. Retrieved May 13, 2020 (American English).
- ↑ http://www.ecomare.nl/nl/dieren-van-ecomare/bruinvissen/
- ↑ Ceta Base | Captive Cetacean Database - Dolfinarium Harderwijk • Netherlands. Retrieved May 13, 2020 (American English).
- ↑ Ceta Base | Captive Cetacean Database - Sea Mammal Research Co. • Netherlands. Retrieved May 13, 2020 (American English).
