Harbour porpoises in the Baltic Sea: "We have to solve the gillnet problem!"
Andreas Fritsch
· 29.02.2024
The Ministry of the Environment in Schleswig-Holstein recently made sailors interested in the environment sit up and take notice, as Minister Goldschmidt announced that the harbour porpoise was now so endangered that the national park on the coast of Schleswig-Holstein was urgently needed. But this is only half the truth, as the interview with Anja Gallus shows. She has been a harbour porpoise expert at the Stralsund Oceanographic Museum for many years. She knows the differences between the various whale populations on the German coast and reports on the latest research and protection approaches for the animals.
Mrs Gallus, the harbour porpoise occurs in two different populations in the Baltic Sea, one in the western Baltic Sea and one in the central Baltic Sea, i.e. off Mecklenburg-Western Pomerania, Bornholm and Sweden. The latter is considered so small that it has long been threatened with extinction. How many animals are actually still around and how are they counted?
The last time an acoustic census was carried out in the central Baltic Sea was ten years ago. To this end, all Baltic Sea states except Russia installed underwater microphones in a search grid at 305 measuring positions over a period of two years. The result was that we have around 600 animals in the central Baltic Sea. However, these differ from the animals in the Belts, which sailors encounter mainly off the German coast.
That sounds amazingly simple and precise. Is this the breakthrough for exact figures on the inventory?
Well, it's not quite that simple. We first need to gather experience: How often does a harbour porpoise echo during the day? At what depth does it do this? How far does the signal travel underwater? How large is the group that hunts there? How reliably does the measuring device pick up the signal? This is a learning process in order to draw conclusions about how many animals are really there. In any case, we will repeat the census this summer for a year almost exactly as we did back then in order to have a real comparison. The whole thing has to be analysed internationally; after all, the harbour porpoise is a migratory species and national borders don't matter to the animals.
Are there also counts for the western part?
Yes, the last count in 2022 showed 14,400 animals for the Belt Sea and western Baltic Sea. Unfortunately, this is around 3,000 fewer animals than in the last census, which was also lower than the previous one. In the western Baltic Sea as well as in the Belt and Sound, however, counts are carried out by aeroplane. This method was also tried in the past east of Rügen. Flight counts were carried out in the central Baltic Sea between Rügen and Bornholm in the early 1990s. There were three sightings. This was then extrapolated to the huge area of the central Baltic Sea. The result was around 1,900 animals. Depending on the budget, this is then repeated once a year or every two years. If you fly there and don't see any animals, it doesn't mean that there are no animals. Sometimes the sun is too reflective, sometimes the water is too murky, sometimes the animals are simply somewhere else because they have followed a shoal of herring. And of course the animals also migrate. In summer, they migrate further towards the central Baltic Sea, sometimes as far as Mecklenburg-Western Pomerania, and in winter they retreat to the Belt Sea. The declining flight counts in the western Baltic Sea also contradict our acoustic measurements, which have shown an increase in acoustic harbour porpoise signals. We do not yet know exactly how this contradiction can be explained. Do the harbour porpoises from the western Baltic Sea perhaps migrate further than before? In the North Sea, there are always significant shifts in populations, currently towards the English Channel. We simply know little about the animals' migratory behaviour. Or do they simply have to forage more often because food is more difficult to find? And of course the flight count is less accurate than measuring the actual sounds that occur, as the microphones measure 24 hours a day. But the flight count is less time-consuming and cheaper: three people are on board for a day, they count the animals seen, the number is extrapolated and that's that. With the acoustic method, the equipment is checked all year round, ships with crew have to go out and deploy and retrieve the equipment. Material costs are also added.
They say it is impossible to tell whether one animal or groups are sending signals. In how large groups do harbour porpoises actually occur?
The animals are often travelling as a mother-calf pair, and sometimes the calf from the previous year is still with them. But often alone. The young, not sexually mature animals, on the other hand, often come together as a small group, perhaps around five whales. Even if there is a lot of food available locally, several animals come together. However, they do not regularly come together in groups as large as those of dolphins or orcas.
Back in 2007, YACHT conducted an interview with a leading harbour porpoise researcher at the time. At that time, he also estimated the population in the central Baltic Sea at 600 animals. Is the population actually more stable than expected?
The problem is that neither method produces completely accurate figures. We definitely need to improve this. An example of why: In the Bay of California, there is a conspecific of the harbour porpoise, the vaquita. In a census about ten years ago, the population was several hundred animals. However, as the conditions were poor for the animals, the population halved with each census, and the last census found just ten animals. This makes the extinction of this species very, very likely in just a few years. The harbour porpoise in the central Baltic Sea will then be the next species on the list.
900 harbour porpoises die in gillnets in Denmark and Sweden every year
What are the main problems facing the harbour porpoise in the Baltic Sea?
First and foremost, there are the gillnets used in fishing. They are so thin that they do not reflect echoes, so the whales cannot "see" them, so to speak, or more correctly: hear them. They swim into them, become entangled and can no longer surface to breathe, suffocating in agony.
Wasn't there once an initiative to equip all these nets with pingers to warn the whales of them?
There is an initiative by the state of Schleswig Holstein that grants fishermen advantages in return if they use these pingers: for example, they are allowed to fish longer than usual and also set longer nets. Thousands of pingers are already in use, but they are not deployed across the whole of the German or Danish coast.
Do they work well? Then it would be a good idea to simply prescribe them.
So far, this has been an incentive model: those who install the pingers are granted benefits. However, the devices are not as easy to use as everyone imagines. The first generation, for example, deterred harbour porpoises but attracted seals. At some point they learnt that where the noise is, there is a gillnet. They then swam to the nets and very skilfully ate the fish in the net without getting caught. No problem for seals. This was again counterproductive for the fishermen. Now there are new so-called PALs, "Porpoise Alert Sysstems". They send out a recorded signal from a female harbour porpoise that warns of danger. The manufacturer says it works wonderfully. However, there are also reports from Iceland that, on the contrary, even more harbour porpoises were caught there as bycatch, and only males. We simply know too little about the communication of harbour porpoises, which has hardly been researched. It is therefore impossible to say whether the PALs will really solve the problem in the long term or whether the animals will get used to them in three years, for example, and the devices will then be useless. In addition, the individual harbour porpoise populations may have their own "dialect". Those in the central Baltic Sea differ from those in the western Baltic Sea and probably also from those off Iceland. We are currently working on a study that is investigating precisely these questions, and the results will be available next year.
When this became known years ago, there was once a theory that the two German populations, which are very different in size, would not mix at all because of these differences and therefore could not strengthen. Is that the case?
Our research tends to show that the populations primarily visit completely different areas during the mating season and therefore simply do not mix. The whales from the central Baltic Sea migrate towards Gotland in the summer, while those from the western Baltic Sea stay in their region, including the Belts, perhaps even travelling as far as Mecklenburg-Western Pomerania or Bornholm. But it is true that both groups have genetic differences. It is possible that a separate subspecies will develop at some point. But that is more likely to take centuries.
How far do harbour porpoises from the western population actually migrate?
There are a few studies on this by the Danes. Some of them have other set nets, a kind of open-top ring seine that leads to the animals being led into a chamber from which they can no longer get out, but they don't die because they still come to the surface to breathe. Such animals are rescued from time to time and fitted with a transmitter with suction cups. This sends GPS data, picks up echolocation signals from the animal and registers how often it tries to catch fish. The females with their calves tend to be localised, while the young porpoises and males have other migratory areas. But we don't know that exactly.
A porpoise eats ten per cent of its own weight every day
What else did you learn about the animals?
For example, they sometimes snap at fish hundreds of times an hour, not all of which they catch, of course. But a harbour porpoise has to eat ten percent of its own body weight every day, which can be 5 to 8 kilos, in order to cover its huge energy requirements. Imagine that as a human being! It is clear from this that it is of course bad if the whale is often interrupted or distracted during this hunt.
What other problems does the harbour porpoise face?
Acoustic noise pollution. This is a very diverse area. Of course, ship propulsion systems could be quieter, and work is also being done on quieter propellers. But the ground is also being intensively explored for mineral resources using powerful echo-sounding equipment, cable routes are being laid and foundations for wind turbines are being rammed. These are still being laid using many piles, which are individually driven very loudly. This goes on for months, and work is carried out in several places at the same time. It all adds up. Once the wind farms are in place, the supply ships join in. Fast ferries are also extremely loud. But the harbour porpoise still live there, of course, because they can't get out of the way. The Kadettrinne is one of the busiest waterways in the world, and yet harbour porpoises still live there. They compensate for this to a certain extent. People also live on main roads. But that stresses both humans and animals. And the harbour porpoise is much more dependent on its hearing than we are, because it uses it to hunt and communicate. One example: the Danes were able to prove with the transmitters that harbour porpoises dive deep to the bottom when a loud ship approaches and wait quietly for it to pass. The animal then surfaces quickly, takes three deep breaths and waits until its rapidly increasing heart rate regulates itself. If this happens often, it is clear what stress this means for the animals - and that there is no time for hunting. But even jet skis or fast motorboats are a problem.
Can the noise be life-threatening?
Yes, unfortunately. For example, when ammunition remnants are blown up on the ground because they can no longer be recovered. This can lead directly to injury and death. If the inner ear is injured, the animal is no longer viable and dies almost immediately. If it is damaged and becomes hard of hearing, the animal is impaired in the medium term, sometimes it can no longer hunt optimally and starves to death.
If you were to summarise the three biggest problems of the harbour porpoise, would they be, in order, drift nets, noise emissions and ...?
Availability of food. The Baltic Sea is heavily overfished, stocks are falling dramatically and the fish are getting smaller and smaller. Cod, mackerel, herring - the harbour porpoise has to invest more and more time and energy in the search for food. This has consequences: A good half of the animals found dead in the Baltic Sea are only two or three years old, i.e. not even sexually mature yet and therefore unable to produce offspring.
Have all previous attempts at protection been completely unsuccessful?
Well, The level of pollution in the Baltic Sea, for example, has improved. In the 1980s and 1990s, this was sometimes so high that the animals were less fertile; this has improved significantly thanks to the reduction of untreated wastewater discharges into the Baltic Sea.
The three major dangers for whales: drift nets, noise, lack of food
There are currently calls for larger protected areas, such as the national park in Schleswig-Holstein. However, we keep hearing that not even the existing protected areas are currently being used in a way that would be of any benefit to the whale. What would have to happen for something to noticeably improve?
That is the problem with the so-called paper parks. They were set up because the EU demanded it and Germany had to comply. For example, there are large protected areas around Fehmarn, in the Kadettrinne or the Pomeranian Bay. These are not explicitly only for whales, sometimes they also protect reefs or important bird populations. The problem is often that there are no restrictions on fishing. There should be larger, net-free areas or finally safer fishing methods for whales, such as fish traps. That would help. The Thünen Institute in Rostock is currently researching this. It is developing a fishing gear that does not endanger seabirds and whales and still works for the desired target fish.
What was surprising was that the Schleswig-Holstein Ministry of the Environment recently reported that the harbour porpoise in the central Baltic Sea is now considered to be threatened with extinction and that action should therefore be taken to promote the Baltic Sea National Park in Schleswig-Holstein. As far as I know, the animal has been classified as endangered for years, and the population does not live off the coasts of Schleswig-Holstein, does it?
This is somewhat difficult to convey to the normal citizen, because the species was only defined as a separate population in the East as a result of the latest research findings. Before that, it was a sub-population. But it is true that the harbour porpoise was already considered to be threatened with extinction in the central Baltic Sea.
From what I have heard so far, the boat traffic of the sailors is not the problem of the harbour porpoises.
There is a big difference between talking about sailing boats and motorboats. We also have a programme in which we register reports of sightings and encounters between skippers and porpoises (www.schweinswalsichtungen.de). We repeatedly see harbour porpoises accompanying sailing boats under sail, even jumping in the bow wave and diving through the wake, sometimes for half an hour or longer. This is extremely rare for ships under engine power. Anyone who has ever held their head under water while swimming during the passage of a motorboat knows why - and harbour porpoises have much, much more sensitive hearing than we do.
So aren't speed limits perhaps a solution?
Of course, we know from car traffic how much quieter it is when you drive at 30 instead of 50 kilometres per hour, and it's the same in the sea.
Okay, in conclusion, you can see that a quick route to improvements for the harbour porpoise is complex. If you could wish for one immediate measure that would probably be the most successful, what would it be?
Find quick alternatives for the gillnets. Far too many animals simply die in them.
