We have all done it. You see something delicious and think, “yes I can eat that whole thing!”, only to figure out part way through that your eyes were bigger than your stomach. Well, it turns out that harbor porpoises may do that too – BUT they may actually be able to eat the whole thing!
My non-profit, Pacific Mammal Research www.pacmam.org, has been conducting photo-identification and behavioral observation research of harbor porpoises in Anacortes, WA since 2014. Over the last 6 years we have learned a lot of interesting things about this elusive species, and most recently it was about their ability to catch prey much larger than we expected!
We observed some harbor porpoises chasing and catching what looked to be salmon… and what looked like a size of prey too big for this small species to swallow! Harbor porpoises are the second smallest cetacean (the group including whales, dolphins and porpoises) in the world, at only 5-5.5 ft and averaging 150 pounds. Normally harbor porpoises are known to eat prey that is smaller than about 12 inches (or roughly 30 centimeters). Needless to say salmon, and especially adult salmon, are considerably larger than this! For salmon species common to the Salish Sea, they range from roughly 21 to 31 inches (53 – 80 centimeters). We consulted with a fish specialist and confirmed the type of fish in the photos was indeed a species of salmon (and in one particular picture, a coho).
There are risks, though – what if it gets stuck? The anatomy of harbor porpoises (like other toothed cetaceans) is unique to allowing them to swallow fish underwater without water going into the respiratory system. BUT, that adaptation means that the larynx (also called the goosebeak) can create a restriction. This can then make the prey become lodged around the larynx (sometimes causing the larynx to become dislocated) causing death by asphyxiation.
Why risk it then? Harbor porpoises have a very high metabolism, are limited in how much energy they can carry and can survive only relatively short periods without feeding. So, a bigger fish means more calories. This may be even more important for pregnant or lactating females as their energy requirements are substantially higher than other age/sex classes, and indeed some of our cases involved reproductively active females.
So the question becomes: is the risk worth the reward? That is the next question we are trying to answer. We will be looking into cases of asphyxiation in relation to age, sex, reproductive status, and prey species to see how often this behavior occurs and who is doing it.
Compared to other cetaceans we know little about harbor porpoises. We are excited to be helping to change that! This research was written into a scientific paper that you can download free here
We are excited to be learning new things about this understudied species, and we are learning more every day as we get to observe them.