Marine mammals and sea turtles in Belgium in 2023

Orca off the coast of Koksijde, October 29, 2023 © Institute for Natural Sciences / K. Moreau

The new report 'Marine mammals and sea turtles in Belgium in 2023' summarizes the results of the monitoring and research on these animals in Belgium in 2023. Porpoises washed ashore less than in the past 20 years, but high numbers were still counted at sea. Seals continue to increase. The most remarkable marine mammal of 2023 was an Orca, and the stranding of a live Loggerhead Turtle was a Belgian first.

Kelle Moreau

Porpoises and seals

In 2023, 26 Harbour porpoises washed ashore in Belgium. Most of these animals were in an advanced state of decomposition. This was the lowest number of dead porpoises in the past 20 years, continuing the previously documented declining trend. The highest numbers date from a decade ago, when more than 100 porpoises washed ashore in some years (with a maximum of almost 150 in 2013). The reason for the declining number remains subject to speculation.

Aerial surveys of marine mammals in Belgian waters were carried out in April, June and September 2023. The observations allowed to calculate that there were 14,700, 1,400 and 2,500 porpoises present in the research area at these times, respectively. The number in April was one of the highest ever recorded. An extensive analysis of the data collected during aerial surveys in the period 2009-2022 showed that porpoises indeed occur in high densities in the Belgian part of the North Sea, especially in spring. They prefer the northernmost and westernmost parts of our waters, and appear to avoid shipping lanes.

48 dead seals washed ashore in 2023. Further investigation provided information about the cause of death for 14 of these seals. Ten of these probably died in fishing nets. The total number of dead seals washed ashore is comparable to the five previous years (with the exception of a peak in 2021). In the longer term, there is an increasing trend. Sealife cared for seven Grey and 17 Harbour seals in 2023. Almost half of the animals did not make it.

Dead Harbour porpoise on the beach of Middelkerke, January 18, 2023. © Bart Mortelmans

Orca on the border of Koksijde and De Panne

The most striking cetacean of 2023 was undoubtedly the male Orca, which first moved slowly along the coast of Koksijde on October 29, eventually ending up stranded in De Panne. The animal was very thin and weakened, and died almost immediately after stranding. For the time being it could not be determined from which population he came.

It has been since the mid-19th century that strandings of Orcas were recorded in our country. We have tried to unravel the information about those long-ago strandings. We also pay tribute to Louis François Paret, the man to whom we owe the fact that we can still admire the remains of these animals, now more than 175 years old.

Orca on the beach of De Panne, October 29, 2023 © Institute for Natural Sciences / K. Moreau

Other rare species

A dead Fin whale was brought in on the bow of a ship in the port of Antwerp on August 29, 2023. The autopsy showed that the animal had died as a result of the collision.

Some dolphins, rare for our country, washed ashore in 2023: a Common dolphin on December 22, and two very decomposed Common or Striped dolphins on July 21 and October 8.

Some sea turtles also washed ashore in 2023. A dead Leatherback turtle on October 7 died from traumatic causes. A live beached Loggerhead turtle on November 25 was the first confirmed stranding of this species in Belgium.

Live Loggerhead turtle on the beach of Bredene, November 25, 2023 © W. Rogiers

The new report 'Marine mammals and sea turtles in Belgium in 2023' is published by the Institute of Natural Sciences. The full report and previous annual reports (available since 2014) can be downloaded here (in Dutch and French).