When we think about harbour porpoises we think about them as predators, top predators actually, feeding on several species of fish and crustaceans. But, have you ever thought about them as prey?
Harbour porpoises in Danish shores - by Helöise Hamel
Humans have hunted harbour porpoises in the past – and in some areas they might still be caught (legally or illegally). The biggest threat to their survival nowadays, though, is being entangled in fishing nets… but that’s for another blog!
As babies, even as adults, harbour porpoises do have 'natural' predators – the most obvious are sharks, especially mako sharks and great whites. These events are rarely seen, but it may be just because of how difficult it is to be at sea for long periods – one has to be lucky to be in the right place at the right time, while not disturbing the natural behaviour of the animals involved. But there are videos out there. It’s not pretty. If you have the strength, dive into youtube. There are enough videos to stop your heart. But sharks feed on many things, right? So, we don’t know how important porpoises are in their diet.
Now… Did you ever think harbour porpoises might be fatally attacked and even eaten by dolphins...? Yes, orcas are dolphins (the largest of them) and some populations are known to feed on other marine mammals, either exclusively or as part of a larger menu. For example, in the Pacific coasts of the US and Canada there are orcas especialised in feeding on marine mammals who occasionally feed on harbour porpoises. There are also orcas that feed on marine mammals in Norwegian waters, both in the very north of the country, as well as the very south. These orcas, though, seem to feed especially on seals, as well as different fish species. Orcas hunting a harbour porpoises is a rather dramatic event. Believe me, I witnessed it! Porpoises may be pushed from below and sometimes they can be seen 'flying' several metres above the water. Orcas are so much bigger and stronger than porpoises, there is no escape.
Orca hunting a harbour porpoise in Hardangerfjord, Norway - by Olve Erda. Click to see on Twitter.
And what about other dolphins? In 1994, in the west coast of Canada, off British Columbia, a pair of Pacific white-sided dolphins were seen interacting with a neonate harbour porpoise. Although the dolphins were not visibly aggressive, they harassed the porpoise for several hours, which eventually died. More recently, in the early 2010s, another harbour porpoise calf died as a result of interacting with dolphins on the other side of Canada. In this case there were no observations of the event, but researchers were able to do some detective work and figure out the cause of death – it was a young animal (< 1 year old) who was stranded off Quebec. The large number of teeth markings on the porpoise body and the characteristics (e.g., separation between the teeth marks) indicated the “aggressors” were Atlantic white-side dolphins.
Probably the most unexpected dolphin on this list is the bottlenose dolphin (remember flipper?). They are responsible for dozens of harbour porpoise deaths every year, for reasons that are not yet well understood. Around UK waters, especially Scotland, death by bottlenose dolphin represent about 25% of all cases. Direct observations suggest that the dolphins involved in these attacks are juveniles – they do not compete with porpoises for food and do not feed on them either. There are also cases reported off of California – but not in every area where both species coexist. Or maybe we have not heard about that yet. The impact on the population is unknown but is expected to be small when the porpoise population is large but it may be a problem in populations such as the Baltic Proper one, as they are just few hundred animals left.
And have you ever heard about seals feeding on porpoises?
I remember the first time I heard it. The idea was presented by researchers at a conference I was attending about a decade ago. The audience was not convinced - that is, until they actually saw the evidence. The first reports were purely based on the wounds and characteristics of grey seal jaw and teeth, but since then there have been direct observations of seals feeding on porpoises. The phenomenon seems to be rather new – many countries in the North Sea have kept good records of stranded animals for decades yet these wounds have only started to appear from the early 2000s. We now have reports from Belgium, the UK, France, and the Netherlands.
Grey seals in Scottish shores - by Mel Cosentino
Even I, as a scientist, have a hard time thinking that this is what happens in nature. I guess animals are both predators and prey...