Wageningen University & Research: Using science to protect porpoises

Large numbers of porpoises swim in the Dutch waters of the North Sea these days. That does not necessarily mean they are thriving, say WUR marine biologists, who have been studying the protected marine mammal since 2006. “You can only protect an animal properly when you understand how it functions.”

At least two dead porpoises will wash up somewhere on the Dutch coast today, and another two tomorrow, followed by two more the day after. “At the moment we’re getting about eight hundred a year,” says marine biologist Mardik Leopold.

This was not always the case. In fact, this small cetacean had disappeared almost entirely from Dutch waters in the 1960s and 1970s. “If a porpoise did wash up on a beach, it made the papers,” says Leopold, who works for Wageningen Marine Research (WMR) in Den Helder. “It was still rare to see a porpoise in the sea when I started working for WMR in1991. These days, if you go sailing in good weather you will see them everywhere. We estimate there is an average of one porpoise per square kilometre swimming in the North Sea.”

Leopold and his WMR colleague Martine van den Heuvel-Greve have been studying Dutch porpoises since 2006. Although it should be ‘Dutch’ in inverted commas because this is not a separate species. “What we mean are the porpoises that swim off the Dutch coast. But they are very mobile so you can’t demarcate it that precisely.”

Human action
Why are scientists studying this marine mammal? As Leopold explains, the porpoise is a protected species. It has been under threat for years, suffering as bycatch in fishing nets and from toxic substances in the sea. That is why the porpoise has been covered by the European Union’s Habitats Directive since1992. “At the time, the Netherlands blithely signed the directive, saying it was obvious that we should protect the porpoise, but we hardly had any in our waters so the directive had few real consequences for us.”

Do the porpoises want to live here or have other areas become uninhabitable for them?
But the ink had hardly dried before the numbers of porpoises in Dutch coastal waters started to increase. “Well, then you have the commitments you made. You can only protect a species properly if you understand what drives them and what they need. What species of fish do porpoises need to eat to stay healthy? And are these fish also found in ‘our’ part of the North Sea? That is why in 2006 the Ministry of Agriculture, Nature and Food Quality asked us to start an extensive study of the porpoise.”

To comply with the rules for protected animal species, the Netherlands must monitor how many porpoises swim in Dutch waters and how they are doing. Are they healthy or diseased? What causes their deaths? “If they are dying as a result of human action, we are obliged to do everything we can to prevent it.”

What does protection involve?
The problem is that no one knows exactly what ‘protection’ means in the case of porpoises. “We don’t have a porpoise nature reserve with a fence around it. They are protected everywhere. But that is complex because if you are protected everywhere, you aren’t really protected anywhere.”

It is not easy to protect porpoises anyway. “They live far out to sea and spend most of their time underwater. And they are shy. You can compare them to roe deer. We have loads of roe deer in the Netherlands but you hardly ever see them. In fisheries research, you throw a net into the sea and count how many fish you catch. That isn’t an option for porpoises.”

So the Dutch scientists have to wait until the sea mammals wash up dead on beaches. “About 20 per cent of those animals starved to death. Another 20 per cent were attacked by seals, 20 per cent drowned in fishing nets and 20 died of various diseases. The remaining 20 per cent died from other causes, for example porpoises that got caught in a ship’s propeller.”

All those dead animals are in principle interesting subjects for Wageningen research. “But these days so many porpoises wash up that we concentrate on the fresh corpses, where we are better able to do certain measurements.”

Four studies
Why did the porpoises come back to the Netherlands? You might conclude that they find Dutch waters a good place to live, but that would be a mistake, warn the researchers. Leopold: “For over twenty years, we’ve been keeping track of the number of porpoises throughout the North Sea with large-scale counts from planes. This number has remained stable. They have simply moved location — in this case from the central and northwest parts of the North Sea. We know that thanks to those counts. So you have to wonder whether the porpoises want to live here or whether they came to the Netherlands because other areas had become uninhabitable for them.”

Reproduction is under severe pressure. That seems to be connected to their diet
That question is not easy to answer, but it is not usually a good sign when a species population moves to a new area. WUR scientists and researchers at Utrecht University are studying the porpoise’s diet, reproduction, causes of death and the concentrations of industrial compounds in their body. “It looks as if our coastal waters are a marginal habitat for porpoises. They can just about survive but it is far from ideal for them.”

The researchers discovered, for example, that Dutch porpoises have trouble reproducing. Only a small proportion of the adult females that are found on the beaches are pregnant. “Far more pregnant females are found in other parts of the world. So reproduction is under severe pressure here. That seems to be connected to their diet.”

Non-oily fish
Mardik Leopold explains that porpoises in Dutch waters mainly eat non-oily fish such as the whiting. Those are what happen to be swimming in large quantities above the sea bed. “Non-oily fish like those also swim more slowly than oily fish species and the sea is relatively shallow here, so they are easier to catch. Humans think it is good to eat food that has a low fat content, but you need that fat in nature. You need calories to keep yourself warm. That is even more important when you have a foetus growing inside you. So a healthy porpoise will prefer to eat oily species such as herring and mackerel.”

The large numbers of non-oily fish that are easy to catch may explain why the porpoise has stayed in Dutch coastal waters. Leopold: “They end up in a kind of ecological trap. If there is a lot of food here that you can easily get hold of, you go for that even if the calorific value is not so high. It’s a bit like fast food, except in this case it’s too low-fat.”

Thanks to the research by WUR, we also know that porpoises mainly eat a lot of small fish, up to about 25centimetres in length. “Porpoises don’t have front flippers like seals, so the porpoise has to eat a fish in one go. That also means porpoises are not in competition with the fishing industry because fisheries mainly want larger fish. That’s good news for both parties.”

Harmful substances
While Leopold focuses on the porpoises’ diet, his colleague Martine van den Heuvel-Greve studies the industrial compounds that accumulate in the porpoises bodies. “We’ve done extensive research on polychlorinated biphenyls (PCBs). These are compounds that don’t occur in nature and cannot easily be broken down.”

The use of PCBs has been banned since the 1980s but the PCBs have now ended up in the environment — where they will not easily disappear. “They end up in fish via algae. And because the porpoises eat the fish, they get into the porpoises’ bodies too.”

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Last year, Van den Heuvel-Greve published the results of 15 years of PCB research. She found that pregnant females pass on the PCBs in their body to the unborn offspring via the placenta. After the birth, the compounds are passed on to the porpoise calf via the mother’s milk. “PCBs mainly accumulate in the body fat. A lactating mother draws on those fat reserves and that leads to the significant transfer of PCBs from the mother to the calf.”

Infectious diseases
While young porpoises get PCBs in their bodies from the start, the concentration in the mother’s body declines. But adult males cannot get rid of the PCBs in their bodies. “They are the porpoises with the highest concentration in their bodies.”

You might wonder how much of a problem that is. “We find that porpoises with a lot of PCBs are more likely to get infectious diseases,” explains Van den Heuvel-Greve. “That means they have weaker immune systems. It’s difficult to say what is cause and what is effect, but we strongly suspect that these compounds are not good for porpoises.”

Porpoises with a lot of PCBs have more infectious diseases. That means they have a weaker immune system
And that is just one group of industrial compounds. The scientists have now started studying PFAS, another group of compounds. “They are a problem category at the moment, for example on building sites and in the Western Scheldt estuary. We are still producing and processing these compounds, so that means we can develop policies to tackle the problem.”

WUR is collaborating with Utrecht University in its various studies. “As soon as a porpoise is found on a beach, it is taken to Utrecht,” says Mardik Leopold. “The veterinary pathologists there do the autopsy. They determine the cause of death and collect all kinds of data, and they cut the organs from the carcass. I get the stomach and Martine gets a piece of blubber and liver. Then we start work on that.”

Mass deaths
Thanks to their years of research, the biologists now know the porpoise so well that they immediately spot a deviation from the norm. That was the case last August when some two hundred porpoises washed up on the Wadden islands in the space of ten days. “We had never seen anything like that before,” says Leopold. “We had all kinds of theories. Had they starved, were they badly affected by toxins, had they all been attacked by seals? But the answer was no.”

When you have mass deaths like that, it is particularly important to determine what caused the porpoise deaths. “It is only natural that porpoises die. But they should not be dying as a result of human action.”

In the end, their colleagues at Utrecht discovered a bacterium in the tissue of the mammals. Leopold: “We had only ever seen the bacterium in pigs so at first we thought someone had dumped pigs in the sea. But when we investigated further, it turned out that the bacterium is also found in the skin of some fish. Those mass deaths were unprecedented worldwide. Porpoises do not swim in large groups, so they tend to die individually.”

Icing on the cake
Leopold and Van den Heuvel-Greve hope to enrich their knowledge further still in the next few years. “This research is seriously multidisciplinary,” says Leopold. “We learn a lot from one another’s fields of expertise and from the collaboration with veterinary pathologists. Take the link between oily fish and PCBs. It is healthier for porpoises to eat oily fish but such fish can also contain more hazardous substances. So is there a ‘penalty’ for porpoises for eating nutritious fish? And what effect does that have? I would like to investigate that.”

For the time being, the biologists have plenty of work with the large number of porpoises washing up on the beaches. Van den Heuvel-Greve: “And these days if a whale is beached, we get a phone call. That’s the icing on the cake for us. Of course, you have to carry out the autopsy of such a large animal on the beach because you can’t transport a whale. But we are willing to do that in all weathers.”


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