The harbour porpoise (Phocoena phocoena) is probably one of my favourite cetacean species. (Cetacean = whales, dolphins and porpoises.) There is just something cute about seeing the small black triangle of its dorsal fin skimming the surface of the sea. Seeing a harbour porpoise always put a smile on my face. It is not strange that the harbour porpoise is one of my favourite species, for it is the only cetacean species that can be found on a regular basis in the Dutch coastal waters. Harbour porpoises can often been seen from the Zuidpier of IJmuiden.
Harbour porpoise as seen from the Zuidpier IJmuiden.
In the end of 2020 and beginning of 2021 two interesting researches got published about the harbour porpoises of the North Sea. One of these was Dutch and the other German, however what was mostly interesting is the seemingly contradicting results of harbour porpoises population in the North Sea. According to Dutch research their numbers are increasing, however according to the German research their numbers are declining. Before we go into these researches in a little bit more detail, lets first introduce the harbour porpoise.
The harbour porpoise is one of the smallest marine mammals with an average length of 1.5m and weigh between 45-70kg. When born the harbour porpoise calves are 70-90cm in length and weigh 5-6kg. They reach sexual maturity after 3 to 4 years, the gestation (pregnancy) period is between 10-11 months. Their lifespan is between 8-15 years, but the oldest harbour porpoise recorded was 24 years old. The diet consists of small schooling fish, squid and crustaceans. The harbour porpoise can only be found in the northern hemisphere and mainly in relatively shallow coastal waters like; bays, estuaries, fjords, tidal channels and sometimes even in harbours. The North sea is a shallow continental sea and perfect for the harbour porpoise, thus the harbour porpoise is one of the most common cetacean species in the North Sea.
Fun fact: the name porpoise comes from the Medieval Latin word porcopiscus which loosely translate into pigfish.
Increased or declining.
Reading these researches and comparing them was really interesting, I will put links to the researches underneath the post. The German research was published in English, the Dutch research only in Dutch. The basis of both the researches are the same, both researches looked at harbour porpoise numbers in their own parts of the North Sea called the Exclusive Economic Zone (EEZ) by means of aerial surveys. The German research was conducted between 2002 and 2019 and the Dutch research was conducted between 1990 and 2019. These long term studies are important to get an a idea of population dynamics and can be of help for marine conservation and legislation. The German research found that the numbers are declining overall by 1.8% per year. Where the Dutch see an increase of numbers in the last 30 years. What do these two seemingly contradicting results mean?
First thing to note is that both these researches were done in their own part of the North Sea. This means that these researches cannot say anything about the entire North Sea and are just an indicator. Now let’s look at the North Sea itself. The North Sea is one of the busiest seas in the world, with human activity in the North Sea increasing in the past two centuries. Most notable human activities in the North Sea, which could potentially have an impact on the harbour porpoise population, are the increase of shipping, fishing and offshore structures, such as oil rigs and wind farms. Further because the preferred habitat of shallow coastal seas and the shy nature of harbour porpoises, they are also under threat of pollution, underwater noise, overfishing (loosing prey) and unfortunately also of bycatch of the fishing industry. When you look at all these activities, there is a possible explanation for the decline and increase of porpoises in the German and Dutch parts of the North Sea. Which is a possible distribution shift. This distribution shift could be caused by any of the activities that take place in the North Sea. It is actually very likely that the ‘German’ harbour porpoises have moved to the Dutch part of the North Sea. There have been some signs that there is less prey available in the northern part of the North Sea, it is however unknown if the prey availability has increased in the southern part of the North Sea. What I also shortly want to point out is the unfortunate increase of stranding cases of dead harbour porpoises. For the Dutch research also looked at stranding data as an additional source of information. Most of these animals have died due to drowning in fishing gear, infections, starvation, ship strikes and attacks of grey seals (Halichoerus grypus). This is definitely a sign that human activity in the North Sea is a threat to harbour porpoises and marine life in general. But the effect of human activities on marine life is a subject that needs to be talked about in a separate post.
These two researches give an insight into the harbour porpoise population in their own parts of the North Sea. To be able really state anything about their numbers, increasing or declining, an international research needs to be done for the entire North Sea. Which is a research I would definitely be interested in.
Sources:
Handbook of whales, dolphins and porpoises by Mark Carwardine
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