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The Dolphin’s Shy Relative - Genetically analyzing endangered cetaceans in the Baltic Sea

Porpoise. Photo: Wikipedia.org/Ecomare.

Porpoise. Photo: Wikipedia.org/Ecomare.

You need some luck to see them. Porpoises are shy and inconspicuous. These two-meter animals flash their dark fins above the water only to immediately dive back down again. Researchers have been observing them in the North Sea and Baltic Sea for decades. Using new molecular-genetic methods, Potsdam researchers are investigating whether these animals form independent populations.

It is only a small piece of frozen tissue: A bright layer of fat beneath dark, leathery skin. What is in the petri dish in the laboratory of the Chair for Evolutionary Biology once belonged to a porpoise, which was found dead on a Baltic Sea beach and now benefits scientific research. Hundreds of these samples from various specimens are stored here at -20°C, sometimes even at -80°C for long-term storage. They are cut and digested by enzymes. The researchers are interested in the genetic information hidden in the tissue, present in the DNA in every individual body cell, because analyzing it offers insight into the animals’ relationships. The question of whether Western and Eastern Baltic Sea animals differ from one another and form independent populations is crucial for their protection.

One of the two populations in the Baltic Sea faces extinction

Ralph Tiedemann, Professor of Evolutionary Biology and Systematic Zoology, has been researching porpoises since the early 1990s. “How many of these animals are actually living in German waters?” This question concerns science now as it did back then, the researcher explains. At the German Oceanographic Museum in Stralsund, researchers measured the skulls of some porpoises and were able to identify morphological differences depending on their geographical origin. Some had longer mandibles than others, which Danish studies have corroborated. It was the first indication of two separate populations of porpoises in the Baltic Sea. Using various methods, they tried to record the reclusive animals and determine the populations’ location and size. The researchers were also interested in the migration of these mammals.

Under the direction of the German Oceanographic Museum, researchers initiated a study with underwater microphones in 2002, installing 12 hydrophones in the Baltic Sea. The researchers then installed a network of 304 submarine microphones between Rostock and Finland in 2011. The devices captured the porpoises’ typical clicks that they use to orientate themselves and hunt under water. There were also sighting campaigns by plane and genetic investigations.

The results confirmed that there are not only different populations in the North Sea and Baltic Sea but also two distinct groups within the Baltic Sea. While the population of the Western Baltic Sea, the so-called Inner Danish waters was about 10,000, there are only about 500 in the central and Eastern Baltic Sea. The latter population causes concern, as it may be in danger of extinction. Most of the porpoises found on the beaches died in fishing nets. The extensively used nets that are made of thin yet extremely tear-resistant nylon threads become a death trap for the porpoises. They will drown if they get caught in the mesh, because they would no longer be able to come up to the surface for breathing. So far, attempts to keep porpoises away from gillnets using signals have only been partially successful. Noise pollution from ship traffic and the construction of offshore wind power plants may pose additional problems to these cetaceans. Furthermore, they are at the end of the food chain – pollutants accumulate in their bodies.

Tiedemann and his team analyze the complete porpoise genome

If a dead porpoise washes ashore in Mecklenburg-Vorpommern, Michael Dähne and his team get down to business. As a curator for marine mammals at the German Oceanographic Museum, he takes care that the dead animals are first recovered and prepared for scientific studies. “Every year, we have 20-60 dead animals,” says Dähne. Fishermen who find dead whales in their nets sometimes also bring them to the Oceanographic Museum. Each specimen is recorded, given an identification number, and initially frozen. In the subsequent dissection, the researchers of the Oceanographic Museum investigate whether the animal suffered from diseases, whether it had parasites, and its cause of death. A toxicological analysis shows the level of pollution.

The researchers also take numerous tissue samples for further scientific studies. “An animal is almost completely utilized,” says Dähne. Ribs and muscle tissue go to the Belgian University of Liège, where they determine the porpoises’ food web through isotope analysis. Other institutions examine the samples for bacteria and viruses. The porpoise’s skeleton remains in the oceanographic museum and is studied morphologically. The complete dissection of a porpoise takes 2-4 hours. A portion of the skin and muscle samples also goes to Potsdam, to Ralph Tiedemann’s genetic laboratory. The carcass must be as fresh as possible for the molecular-genetic investigations that are carried out here, because only intact hereditary material can be used for genome-scale analyses. Since decomposition begins immediately after death, it is a race against time. Parallel to the studies on individual animals, Tiedemann and his team are currently analyzing the complete porpoise genome. Whether the two groups of porpoises in the Baltic Sea are two separate populations may seem unimportant to the layperson. In fact, the answer could decide whether the group in the central Baltic Sea will survive the coming decades. “If you consider the animals in the entire Baltic Sea to be one population, you could argue that there are still 10,000 animals and a few hundred more do not matter,” Tiedemann explains. "Regarding the animals in the central Baltic Sea as a separate population, however, means that they are in great danger. Then you have to discuss how big the difference between the populations must be to be relevant for nature conservation.”

This is precisely the question asked by the Potsdam researchers, who want to use molecular-genetic methods to determine in what ways the two Baltic Sea groups are distinct. It is known that the areas of the two populations overlap, but they seem to stay there at different times. While the large group from the northwestern Baltic Sea is in the Pomeranian Bay between June and September, the small group is mainly in Swedish and Polish waters. The researchers assume that this vulnerable population spreads throughout the Baltic Sea during winter. Whether the two porpoise populations intermix has not yet been able to be ruled out. The Potsdam researchers are investigating this with molecular-genetic methods. Their analyses even allow them to determine the degree of relationship of individual animals.

Michael Dähne emphasizes, “To know where the animals’ habitat is and how they migrate seasonally is the most important prerequisite for their protection.” This is only possible in cooperation with fishermen. “Methods have to be developed to reduce bycatch.” This could be achieved with alternative fishing methods, such as longlines or fish traps.

The occurrence of single mutations – so-called single nucleotide polymorphisms (SNPs) – of 400 animals is being evaluated. “Small but consistent differences” have already become apparent, Tiedemann explains. The paths of the two populations continually cross each other during seasonal migrations, but the animals may not mate with each other and always return to their original areas. This provides new, pressing arguments for the protection of the smaller Baltic Sea population. “The porpoise is the only whale species that reproduces in German waters,” Tiedemann emphasizes. His conclusion, however, is disillusioning. “If the population in the central Baltic Sea dies out, there will probably be no more porpoises in the foreseeable future."

PORPOISE

The small bottlenose dolphin, brown fish, or mereswine: There are many names for the harbor porpoise in the literature. Like dolphins and other whales, the approximately 1.5- to 2-meter mammals come to the surface of the water to breathe. The common porpoise lives in the shallow coastal waters of the North Atlantic – off the coast of Europe, northwest Africa, and eastern North America. The animals also live in the Black Sea and the coastal waters of the North Asian and North American Pacific and feed on fish, crustaceans, and squid. The porpoise is the only whale species native to the Baltic Sea. The population in the inner Baltic Sea is facing extinction. Many animals suffocate in gillnets; pollutants and noise also endanger their survival.

THE RESEARCHERS

Prof. Dr. Ralph Tiedemann studied biology, computer science, and Icelandic at the universities of Kiel and Reykjavík. He earned his doctorate with population-genetic studies on eider ducks and wading birds. After working as a visiting scholar at the Free University of Brussels (ULB), he received his habilitation in zoology in 2000. Since 2002, he has been Professor of Evolutionary Biology and Systematic Zoology at the University of Potsdam and researches population differentiation, species-formation processes, and adaptation phenomena in various organisms, e.g. whales, birds, amphibians, fish, and rotifers.

Universität Potsdam
Institut für Biologie und Biochemie
Karl-Liebknecht-Str. 24 – 25
14476 Potsdam
Email: tiedeman@uni-potsdam.nomorespam.de

Dr. Michael Dähne studied applied geography and environmental protection at the University of Rostock. Since 2015, he has been Curator for Marine Mammals at the German Oceanographic Museum in Stralsund and researches the population sizes and distribution patterns of marine mammals in the North and Baltic Sea.

Deutsches Meeresmuseum
Katharinenberg 14 – 20
18439 Stralsund
Email: michael.daehne@meeresmuseum.nomorespam.de

Text: Heike Kampe
Translation: Susanne Voigt
Published online by: Daniela Großmann
Contact for the online editorial office: onlineredaktion@uni-potsdam.nomorespam.de