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We are moving to a new format with our Marine Mammal Highlights series, focusing on one species of marine mammal instead of discussing three different ones. By popular demand, first up we talk about the Dall’s Porpoise!

Dall’s porpoise. Photo: NOAA.

Background

Getting its name from the researcher W. H. Dall (who actually focused his research primarily on mollusks!), the Dall’s porpoise is one of two species found in Washington waters, with the other being the harbor porpoise of course! As the largest of all porpoise species at 7.5 feet and up to 490 pounds, the Dall’s porpoise almost resembles the coloration of a mini orca: black bodied with a white belly and sporting white patches on the top of the dorsal fin and fluke. Their dorsal fin is unusually shaped when it comes to cetaceans as well! The fin will actually slope towards the front of the animal rather than the classic angling back towards the fluke we typically see in dolphins and whales, making it a clear indicator of the species along with its white patch. The body of this porpoise is also quite wide and robust with a smaller head and little to no beak.

This species is found throughout the Northern Pacific in both the open ocean and coastal environments (though not as common near the coast as the harbor porpoise). Ranging from California to Alaska in North America to Japan in the West, the population of this species is estimated to around one million individuals! Additionally, this species exhibits two subspecies: the dalli- type (Phocoenoides dalli dalli) and the truei- type (Phocoenoides dalli truei). The main difference in these subspecies is found in their coloration, where the truei- type has an elongated white patch on the belly when compared the dalli- type. Here in Washington, we only to see the dalli- type with the truei- type restricted in the waters near Japan, and that’s only if you are lucky enough to even see a Dall’s porpoise around here!

Diet & Behavior

Dalls porpoise tend to inhabit deeper waters compared to other porpoises (especially harbor porpoises), preferring depths of 600 ft or more. They have a wide array of prey they snack on, from small schooling fish like anchovies, herring and hake to mid and deep water fish like smelts and lantern fish, to cephalopods like squid and octopus and even occasionally some crustaceans like crabs and shrimp!  You won’t go hungry if you have that many options on your dinner menu.  They normally feed at night when their prey migrates towards the surface and can dive up to 1,640ft!

They swallow their prey whole, without chewing or ripping. To better grasp their often slippery prey, they have 38-56 small spade shape teeth (characteristic shape for porpoises) on each jaw.  These teeth are tiny – the size of a piece of grain or rice!  They also have rigid protruding growths between each tooth called ‘gum teeth’, which are thought to help them grasp particularly slippery prey like squid. They eat up to 28-30 lbs of food per day!

Like other porpoise species they are seen in smaller groups, from 2-12, but can be seen occasionally in groups of 100s to 1000s. But they differ in their behavior compared with other porpoise species. Sometimes they hang out with other cetaceans like Pacific white sided dolphins, short finned pilot whales and some larger whales – but why is not fully understood, and while other porpoise species avoid boats, Dalls porpoises act a bit more like dolphins in that they can be attracted to vessels and are commonly seen bow-riding, catching a free ride.

The most characteristic thing about them, beside their striking black and white coloration, is when they surface. They do it so quickly, skimming at the surface, that it creates a bow wave that arcs over their bodies and looks like a rooster’s tail – hence it is called a rooster tail splash.  Unfortunately, that means that often when you see a Dalls porpoise in the water, you will see more water than porpoise, but it is still fun to watch them zoom around, and easy to know what you are seeing!

Dall’s porpoise doing their characteristic “rooster tail” surfacing.
Photo: The Cetacean Zone.

Current Status & Threats

In the United States, Dall’s porpoise have been divided into two different management stocks by NOAA: the Alaska stock, and the California/Oregon/Washington stock.

This porpoise species is primarily threatened by entanglement in fishing gear (particularly drift nets, gill nets, and trawling gear), and accumulation of contaminants in their blubber. The other major threat to Dall’s porpoise is hunting: they are the target of the largest cetacean hunt in the world. Occurring in Japan, approximately 18,000 porpoise taken annually for meat and other byproducts.

Fun Facts: Hybrids

Dall’s porpoise occupy much of the same areas as harbor porpoise, however although they even feed on similar prey items the two species do not typically mix. In fact, they seem to oscillate: when lots of harbor porpoise are present, there are few Dall’s; when lots of Dall’s are present, there are few harbor porpoise. We’re not sure why this occurs, but it likely has to do with competition and resource-partitioning.

BUT, although they don’t typically hang around each other, Dall’s porpoise and harbor porpoise will – and do – interbreed, creating hybrid porpoises. This hybridization almost always occurs when a male harbor porpoise mates with a female Dall’s porpoise, and results in a calf that can resemble either parent (or a combination of the two), but that strikingly behaves more like a Dall’s porpoise! These hybrids have also been shown to be fertile, which is very unusual (most hybrids are infertile). Research on these hybrids is ongoing, but you can check out these resources to find out more: Canadian Journal of Zoology 2004, Pregnant hybrid porpoise stranding article.

cindy.elliser@pacmam.org

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