Fin whale clearly showing its right-hand side, white lower jaw - Photo by Justin Hart

Identification of Fin whales (Balaenoptera physalus) and Sei whales (Balaenoptera borealis) - two baleen whale species we frequently see in Pico - is not always easy. Separating individuals of the two species travelling together is sometimes far from being straightforward.

The largest Fin whales are always bigger than the largest Sei whales, but we do not always encounter full-grown animals or the largest individuals. Males of both species are appreciably smaller than the females too. We may therefore encounter individuals of either species in a variety of sizes, depending on their age and sex.

For example it is very possible to encounter a large female Sei whale travelling in loose association or even close proximity to a small male Fin whale. So how do we recognise each species? Well, this is partly down to picking out one key feature of Fin whales that is never shown by Sei whales and, when this cannot be detected, using something many birdwatchers will be familiar with - something called JIZZ.

JIZZ is a pseudo-acronym for General Impression of Size and Shape and can be a very useful tool, as each species shows a variety of traits and behaviours shown less (or very infrequently) by the other.
Picking out these features in an animal can be sufficient to identify the species. There are, however, sightings that we can never be sure of their identity.

The key feature of Fin whales that is not shown by Sei whales is the colour of their lower jaw on the right-hand side. This is always white in Fin whales and black/dark in Sei whales (see photos below). It has to be the right side, as the lower jaw on the left side of Fin whales is also dark.

Fin whale - the right-hand lower jaw is white

Sei whale - the right-hand lower jaw is dark and same colour as the left hand-side

Other traits or typical Fin whale JIZZ include showing a massive, broad back when seen from behind, lunge feeding on their right side, regularly surfacing several times in succession in between dives and tending not to show the dorsal fin simultaneously with the rostrum when they surface.

Sei whale JIZZ, in contrast, includes appearing narrow in the body (particularly in between the pectoral fins), typically surfacing only once in between dives and, very importantly, Sei whales tend to show the dorsal fin simultaneously with the blowholes when they surface.

Sei whale showing blowholes and dorsal fin at the same time

Sei whales often seem to drag their dorsal fin through the surface as they slowly submerge. This appears to last longer than with fin whales and reminds of a large shark fin. They also seem to prefer to travel very near the surface and, proportionately, they seem to have a shorter rostrum than Fin whales; the rostrum also appears to down turn more at the tip (see photos below).

The rostrum of a Fin whale

The rostrum of a Sei whale

The rostrum of Fin whales (upper photo, above) is proportionately longer and have a more of a flat profile compared to Sei whale (lower photo, above).

The broad body of a Fin whale

The narrow body of a Sei whale

Sei whales (lower photo, above) have comparatively narrow bodies that appear more round or convex and less broad than Fin whales (upper photo, above).

Finally, Sei whales have often distinctive and almost unique dorsal fins fitting one of three categories (see the 3 examples below), which you are very unlikely to find in Fin whales.

Example 1: Very falcate Sei whale dorsal fin

Example 2: Erect, with quite straight trailing edge Sei whale dorsal fin example 2

Example 3: Very tall and pointy Sei whale dorsal fin example 3

If you want to maximise your chances of seeing Fin whales and Sei whales in the Azores, you should join our Giants of the Sea programme.