Distinguishing between Bottlenose Dolphin and Harbour Porpoise in the field
1. Estimate the size
Bottlenose dolphins (Tursiops truncatus) are 2.5 - 3.9 metres long and weigh around
150 - 200 kg. Harbour porpoise (Phocoena phocoena) are smaller at 1.6 - 1.9 metres,
weighing about 60 - 90 kg. Porpoises can look about human size if you see much of
them, whereas bottlenose dolphins are noticeably larger, even when still quite well
submerged.
Bottlenose dolphin
At a range of even 70 metres, a porpoise moving through the water is inconspicuous
and may prompt a response of “Did I just see something?” while a bottlenose dolphin
would be very noticeable, prompting a reaction in most observers who see it clearly as
a cetacean (though members of the public have also been known to shout “Shark!”).
Here the paddle is about 2 metres long, so the animals are clearly bigger than that by
at least half a metre. The bodies seem thicker than the paddler’s, suggesting that these
are very bulky animals, which is consistent with them being bottlenose.
The porpoise in the foreground is giving a significantly smaller profile, even when
viewed from a height. The size of the auks suggests that perhaps only a fraction of a
metre is breaking the surface, while the roughish conditions are giving far more splash
from the body than is usual. Clearly not a large and powerful beast.
2. Look at the size and shape of the dorsal fin
Bottlenose dolphins have tall sickle-shaped dorsal fins while harbour porpoise tend to
have quite small triangular dorsals, though these often some shallow curvature along
the trailing edge.
Here the floats in the bottom
corner suggest that the fin is
fairly big, given that the animal
is at least another 30m further
out. As the animal is moving
slightly towards us, the sickle
shape is showing well,
indicating that it is likely to be
a bottlenose. (Seeing a faint
blow as here is quite unusual,
but this one is stronger than the
very small cone that you see
even more rarely from a
porpoise).
There can be individual variation
between animals when seen square
on, with some fins being narrower
than others. The fins are always
prominent, though, even at some
distance, and frequently look quite
pointed.
The harbour porpoise shown at close range above have much smaller, squatter fins,
being relatively narrow at the base, with a much more limited rear curvature. At a
further 30m these would become quite indistinct and would not give the strong cue
shown by the bottlenose. They are still quite characteristic of the species and are
difficult to confuse with even the smaller species of dolphin
3. Look at the head shape if it is visible
The beak is quite prominent in the bottlenose whereas the porpoise is fairly rounded at
the front end.
The beak shows
well on these
bottlenose even
when they are
moving away.
While maybe not as
pronounced on the
calves as the adult,
there is still a clear
projection.
The porpoise beak is
essentially contoured in to the
head and there is no marked
prominence (even if the
countershading on the
animal’s body makes the upper
portion more noticeable here).
The porpoise calf surfacing
alongside its mother does
show a small prominence,
but this is exaggerated by
the light and the angle of
view. It is still far less than
would be seen on even a
newborn bottlenose.
During slow swims, very little of the head can be visible in both species, though
sometimes you can just see the beak just at the water surface (as in the fin
photograph).
4. Consider the behaviour
Porpoises are most frequently seen in groups of 3-6, whereas bottlenose groups are
often larger. This is not a reliable indicator though, as locally both species can be
seen in groups of 2 – 20 or even more if there are large patches of accessible food.
Porpoises can be quite undemonstrative, rolling through the water to breath as if they
were attached to a wheel. The classic view is of about a metre of back, sometimes
rising three or four times in succession, though frequently only seen once before a
dive.
Characteristic porpoise profile
In the autumn, young porpoises can sometimes be seen breaching clear of the water,
but older animals only do this rarely e.g. when in hot pursuit of fish at the surface, and
even then they mostly only just clear the water surface.
Breaching juvenile porpoise
showing light underside.
Head has just re-entered
water, tail flukes are
horizontal and not easily
visible.
Bottlenose dolphins are well-known for their powerful acrobatics, often breaching
more than a metre in to the air, with or without somersaults. They can be splashier in
their swims and will look altogether more powerful and more energetic.
Bottlenose dolphin
profiles
It is easy to forget that in a slow swim only a rolling back with a big dorsal appears,
giving little other clue as to what lies beneath. Even during slow swims, they cover
the ground much faster than porpoises and the fins can disappear in to the distance all
too quickly. If threatened they may bunch together in tight agitated groups, visible
from a considerable distance as they charge through the water.
Additional information is available from the Irish Whale and Dolphin Group
at http://www.iwdg.ie/index.php by clicking on the Species Tab