Bryde's whales and sei whales - a comparison
25 Feb 2019

Bryde's whales and sei whales - a comparison

In recent months we have had several sightings of sei whales who are a visitor to 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 very unique in their own ways.

 

    

Bryde's whale (left) and sei whale (right) seen in our trips 

 

1. Size - Adult sei whales reach up to approximately 15-18 metres in length whereas Bryde's whales are generally slightly smaller at up to 13-15 metres in length. 

 

2. Length - Both adult sei and Bryde's whales can weigh between 20-30 tons

 

3. Appearance - 

 

Sei whales and Bryde's whales look very similar!

Photo: A World With Whales

 

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 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 (see image above). 

 

Bryde's whale head (left) and sei whale head (right)

Photo: North Atlantic Society

 

4. Range -

Sei whale global range

Photo: NOAA

 

Sei whales are found in both hemispheres and can be found in more temperate waters than the Bryde's whale as far down as Antarctica in the South and Iceland in the North 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 whale global range

Photo: NOAA

 

Bryde's whales (who is also known as the 'tropical whale') are also found in both hemispheres but are thought to only inhabit warmer waters between 40° South to 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 which extend to close to the 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!

 

   

Bryde's whale lunge feeding on its right-hand side with ventral pleats extended (left) and head-slapping whilst feeding on plankton (right) 

 

 

7. Dive behaviour - Experienced whale watchers may be able to tell the species apart by looking at the dive behaviour. In the Hauraki Gulf, Bryde's whales typically surface 3-5 times over several minutes before undertaking a longer dive for approximately 5 minutes. They do not show their tail flukes when diving but do arch their back and tailstock. The sei whales, dive for a similar length of time but instead of arching their back, 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 - The Bryde's whale population in Auckland is resident and therefore can be seen in the Hauraki Gulf Marine Park year-round whilst sei whales are occasional visitors to the area.