Question of the month: Why more often dead porpoises around offshore wind farms?

NETHERLANDS UTRECHT / KATWIJK - "How is it possible that we catch dead porpoises around offshore wind farms more often?", Asks Art-Jan van der Plas (WR 244 and KW 5). Tim Haasnoot van Vistikhetmaar finds out.

Fishermen are increasingly reporting that they are catching dead porpoises around offshore wind farms. To find an answer, marine mammal experts Mardik Leopold from Wageningen Marine Research and Lonneke IJsseldijk from Utrecht University became involved. Upon hearing the question, their interest was immediately aroused, because both have been researching porpoises in the North Sea for years. Research is now being done with dead porpoises that wash ashore on the Dutch coast. On the other hand, very little to nothing is known about dead porpoises that are caught further in the North Sea.

Art-Jan (and Nico) van der Plas (KW 5 and WR 244), Dirk Kraak (BRA 7) and Cor (and Adrie) Vonk (TX 1) agreed to obtain more information about the precise situation. While this seems to be a simple question, there is a danger of making connections too quickly between the construction of offshore wind farms and the observed dead porpoises. For example, a number of additional questions have been put to the fishermen mentioned, such as:

• What time span are you talking about when you compare it to 'before'?
• What is 'close to a wind farm'?
• What numbers of porpoises are we talking about?
• In what condition are these dead porpoises found in the net?
• Do you see differences throughout the year?

During the conversation with Art-Jan aboard the KW 5, it became clear that he was referring to 'earlier' as the time period in which his father, Nico van der Plas, fished. "You used to catch a dead porpoise every now and then, but nowadays almost every trip there is a hit," said Art-Jan. "We just want research to determine where those porpoises are dying."

Diagnosis

However, determining the cause of death is not a simple task. "When porpoises are in a late stage of decomposition, this makes it difficult to investigate the cause of death," said IJsseldijk. In pathological research we look at changes in the organs, both with the naked eye and on a cell basis under the microscope, but tissues lose color, shape and consistency when the decaying bacteria comes into play. This also applies especially to tissues around the inner ears, such as the eardrum, so that research into hearing damage can only be done on very fresh porpoises."

Father and son Van der Plas indicate that they are concerned about 'stress' in porpoises due to noise around the wind farms. IJsseldijk: "Stress is a diagnosis that we cannot cite as a cause of death, because it does not cause direct injury. For example, stress can have an effect on nutritional status or the immune system, making animals sick, lean and weak, with all the associated consequences. And we can determine such things. We will therefore have to focus the research on the overall health status of these porpoises. Ultimately we can compare this with what we find with the stranded porpoises. The question we need to ask is whether the animals that are fished around the wind farms in worse health condition than animals from the coastal area?”

Mardik Leopold adds: "Investigating the stomach contents of the dead-fished porpoises can further help us to determine whether these fished porpoises have indeed died in wind farms. Have the animals eaten fish that are concentrated in wind farms, such as pebbles? In the dead porpoise, the stomach often remains intact for a long time, so that even decayed specimens are still suitable for this research.”

It is therefore possible to test and reinforce the findings of the fishermen, but more information is needed.

Plan

The following plan has been drawn up in consultation with the fishermen involved:

• The number of dead porpoises they catch will be recorded by taking a photo of the animals and recording the location.
• The locations where dead porpoises are caught will be recorded.
• A number of dead porpoises will be placed in airtight crates.

The last point is important to prevent the carcass of the porpoise from affecting the remaining catch. Exemptions have been are arranged for the three participating fishing vessels to take the fished dead porpoises. The moment a cutter with a fished dead porpoise comes in, the researchers are notified. The airtight chest with the dead harbor porpoise is then collected upon arrival and the harbor porpoise is examined.

The first results of this pilot study are expected in early 2020. These results will be shared with the fishermen and via the 'Vistikhetmaar' column in Visserijnieuws. Fishermen who regularly catch dead porpoises in the vicinity of wind farms can also report via the address below.

Strandings

The number of porpoises washed ashore has increased enormously in recent decades. Where fewer than a hundred dead porpoises were reported each year in the 20th century, this number increased steadily. Peak stranding years were 2011 and 2013 when almost 900 stranded dead porpoises were found every year on the Dutch coast alone! The increase was the reason for the ministry to investigate the causes of death. Porpoises are a protected species. The research focuses on determining the most important threats to porpoises in Dutch waters, with the focus points being: pathology, diet and contaminants (disease-causing substances). The first takes were place at the Faculty of Veterinary Medicine (Utrecht University), the last two at Wageningen Marine Research. More info:www.uu.nl/strandingsonderzoek and https://www.wur.nl/nl/Dossiers/dossier/Bruinvissen.htm

Translation to English was assisted using Google Translate