Big, baleen and beautiful: Survival secrets of the Gulf’s largest resident marine mammal

One of Tīkapa Moana / Hauraki Gulf’s best kept secrets is massive – we’re talking as much as 15 metres long and as heavy as 7 elephants combined. People almost never pronounce its name properly, and it is the odd one out of its species sub-group, as a laidback and adaptable mammal that loves warmer waters year-round. Hint: it’s World Whale Day. 😊

It is of course, the Bryde’s whale, pronounced ‘Broo-dahs’ for the Norwegian whaler Johan Bryde, who established early whaling stations in South Africa. Bryde’s belong to the rorqual family, the largest group of baleen whales. What sets rorqual whales apart from other baleen whales is a series of throat ‘pleats’, which are longitudinal folds of skin that expand during feeding to greatly increase the volume that the whale’s mouth can hold, folding back into a streamlined shape when the whale isn’t feeding. These pleats run from below the mouth to various points back towards the naval depending on the species.

Bryde’s, the slenderest of rorqual whales, have a couple of distinguishing features – three rostral ridges that run from the tip of the snout to the twin blowholes (of which all baleen whales possess), and a hooked dorsal fin. They are dark grey or black on their back and almost white on the underside.

Unlike most other baleen whales that migrate seasonally to polar regions, these whales stick to warmer waters, typically living within 40° either side of the equator. While some populations move within this range, others stay put – including the semi-resident population of 140 whales located off the north-eastern coast of the North Island. At the centre of their range is the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa. Here they pop in and out to eat and breed, while a small hard-core group of about 50 stay mostly put inside the Gulf.

Why? Because the Hauraki Gulf is bountiful, due to the upwelling of nutrients from deep water along the edge of the northeast continental shelf. This is caused by offshore winds that blow warmer coastal water offshore, drawing colder nutrient-rich water up to the surface, which combined with light provides the ideal conditions for phytoplankton to grow. Phytoplankton are eaten by zooplankton and small fish which, in turn, feed the Bryde’s.

Taking eating to new heights (and depths)

Bryde’s ability to eat either zooplankton (including krill) or small fishes (anchovies, pilchards, sprats, and mackerels) is rather unusual among baleen whales. Most prefer one or the other. In fact, studies have revealed the diet of New Zealand’s Bryde’s whales has transitioned over the last decade from predominantly fish to mostly zooplankton. The reasons for this are unknown but could potentially be due to habitat degradation attributed to coastal development, fishing pressures, and global climate change which among other impacts, affects the productivity of phytoplankton.

Bryde’s forage during daylight hours in the Gulf and depending on what they are feeding on, they use either lunges or ‘chin slaps’ to capture their prey.

Lunge-feeding, which is typical of many rorqual whales, is described as an energetic thrusting of the whale’s body, open mouthed through the water into a school of prey. The throat pleats rapidly expand as they gulp a huge amount of water containing fish or zooplankton. The water is expelled as the throat pleats contract, leaving the prey caught behind in the whale’s 300 or so baleen plates which act like a sieve, straining their food from the water. What’s more, Bryde’s perform this technique obliquely, vertically, and laterally – a fancy way of saying at all different angles!

Bubble-net feeding is a very clever way of capturing food only known to be performed by humpback, fin and Bryde’s whales. Many humpback groups have perfected this technique by working together, but Bryde’s in the Hauraki Gulf feed alone creating columns of bubbles in a circular formation by swimming slowly around in circles under the surface while releasing chains of bubbles from their blowholes. This creates a curtain of bubbles to encircle their prey and concentrate them together. The whale then lunges up through the middle of this ‘net’ to consume their prey in one big gulp.

The Gulf’s Bryde’s have also been observed ‘chin slapping’ to concentrate their prey.

With all that eating, you would think the blubber on a Bryde’s would be pretty generous. In fact, it is very low compared to other baleen whales. As a result, historically, they were not hunted in great numbers and therefore little is known about them. Luckily for us, various studies on the Hauraki Gulf’s Bryde’s have been undertaken to shed light on this species.

Learn about the locals

There are approximately 140 Bryde’s found in the Hauraki Gulf – roughly equal in numbers between the sexes. You might assume the genetic pool of this small population would be narrow, but in fact there is high genetic variation suggesting these whales breed with whales that visit from other regions. In New Zealand they are classified as ‘nationally critical’ due to their small population size, although they are reported in offshore waters.

Despite this status, they are the most commonly sighted whales in the Hauraki Gulf Marine Park, helped by the fact that they spend 90% of their time between the surface and a depth of just 12 metres.

You are most likely to see these majestic creatures alone or loosely aggregated near a prey source, anywhere and everywhere in the Gulf.

Reduce speeds save lives

Unfortunately, their preference to stick to near the surface and the inner middle Gulf coincides with Gulf’s shipping channels. Historically, just over two Bryde’s whales a year were found dead with injuries attributed to ‘vessel strike’. Thankfully, following extensive research by Dr Rochelle Constantine, a science-informed collaborative approach involving the Ports of Auckland, the shipping industry, Department of Conservation, University of Auckland, the Hauraki Gulf Forum, Environmental Defence Society, Maritime New Zealand and others was embraced.

Through research it was discovered that by dropping from 14 knots to 10 knots the likelihood of death for Bryde’s reduced from 70% to 26%. In just 18 months, the shipping industry introduced a voluntary protocol to reduce the number and impact of collisions between Bryde’s and large vessels in the Gulf. It worked, with the last recorded death by vessel strike recorded in 2014. As a result, at the end of 2020, 14 Bryde’s whales were alive that may have died from ship-strike if the ships hadn’t slowed down – an amazing conservation outcome.

While this acute threat has been resolved, human curiousity can cause stress and displacement of marine mammals. As such, all marine mammals are protected under the Marine Mammals Protection Act 1978 which outlines very clear and easy guidelines. For whales (including those commonly known as whales – orca, pilot, beaked and false killer whales) these are:

• stay at least 50 m away from any whale
• stay at least 200 m away from any baleen or sperm whale mother and calf
• do not swim with whales.

The best way to see a Bryde’s whale in all its majesty is on a whale watching tour with our conservation partner, Auckland Whale and Dolphin Safari (AWADS), who also host marine scientists undertaking research on the species that make the Hauraki Gulf Marine Park so special.

Love it, Restore it, Protect it.