Species Spotlight: Bryde's Whales vs Sei Whales

In recent months we have had several sightings of sei whales in the Hauraki Gulf Marine Park. They look incredibly similar to Bryde’s whales and it can be difficult to tell the two apart, but they are unique in their own ways. Here’s a guide to telling the difference between Bryde’s whales and sei whales.

1. Size

Adult sei and Bryde’s whales reach a similar size, both weighing between 20-30 tonnes.

2. Length

Adult sei whales reach up to approximately 15-18 metres in length. Bryde’s whales are generally slightly smaller, at up to 13-15 metres in length. 

3. Appearance

At first glance, Bryde’s and sei whales look almost identical, which is the reason why they are easily confused. In fact, both whalers and researchers didn’t tell the difference between the two for many years!

Theoretically, sei whales may be slightly lighter grey in colour compared to Bryde’s whales, although natural variation means that many Bryde’s whales are also light grey!

The main way to tell the difference between the two species in the field is by looking at the top of their heads. Sei whales have one ridge along the top whereas Bryde’s whales have three. In addition, the tip of the rostrum of the sei whale is slightly downturned.

4. Range

Sei whales are found in both hemispheres and can be found in more temperate waters than the Bryde’s whale. They travel as far south as Antarctica and as far north as Iceland, but but not in the polar ice seas. They tend to undertake some seasonal movements, moving towards the equator in the winter and to the cooler waters in the summer.

Bryde’s whales (also known as the ‘tropical whale’) are also found in both hemispheres but are thought to only inhabit warmer waters between 40° south and 40° north. In New Zealand, they are seldom seen further south than the North Island. Some populations of Bryde’s whales are known to migrate whereas others, like the ones in the Hauraki Gulf Marine Park, are resident. 

5. Population Size

It is estimated that the global population of sei whales stands at 50,000 making them Endangered on the IUCN Red List. For Bryde’s whales, the global population is unknown and their status is listed as Least Concern on the IUCN Red List. 

In New Zealand, Bryde’s whales are classed as ‘Nationally Critical’ under the New Zealand Threat Classification System due to their low population size of less than 200. Sei whales are classed as ‘Migrant’.

6. Diet

Both Bryde’s and sei whales are rorqual whales, which means that they have a series of long pleats under their mouth extending down towards their belly button. These pleats help the mouth and throat expand when the animal is feeding.

Most other rorqual whale species have a restricted diet by feeding on only one prey type. Bryde’s and sei whales have a more varied diet, feeding on multiple different types of food including plankton, crustaceans (such as krill) and fish. In the Hauraki Gulf all three can be found at different times of the year – Auckland’s productive waters are great for hungry whales!

7. Dive behaviour

Experienced whale watchers may be able to tell the species apart by looking at their dive behaviour. In the Hauraki Gulf, Bryde’s whales typically surface 3-5 times over several minutes before undertaking a longer dive for approximately five minutes. They do not show their tail flukes when diving but do arch their back and tailstock.

Sei whales dive for a similar length of time, but instead of arching their back they often simply sink below the surface. 

8. Swim speed

Sei whales are one of the fastest species of baleen whale, with records reaching up to 30 knots (55km/h) compared to Bryde’s whales who are slightly slower with fastest swim speeds of about 13 knots (24km/h). However, both are capable of swimming faster for short periods of time if they want to! 

9. Sightings in Auckland

Sei whales are only occasional visitors to the Hauraki Gulf Marine Park, while we have a resident Bryde’s whale population in Auckland and can see them year-round.