Last November, a 14.5m bryde’s whale was found dead on Motuihe Island in the Hauraki Gulf. A necroscopy confirmed the whale had died after being hit by a vessel.
Although the Hauraki Gulf Marine Park is host to occasional visits from large migrating whales – blue whales, humpback whales, sperm whales and more – it is home to a small population of bryde’s (pronounced brooders) whales. These baleen whales feed on plankton and small fish in the gulf, breed and raise their calves there and die there.
Bryde’s whales are doing well in many other parts of the world but this small coastal population is on New Zealand’s nationally critical list. In the past 24 years, 48 dead whales have been found in the gulf, most of them bryde’s. Of the whales for which sufficient data exists, 83% were killed by ship strike and the rest were entangled in fishing gear.
Rochelle Constantine, a University of Auckland conservation biologist, has been studying the bryde’s whales. “Unfortunately, the whales’ preferred habitat overlaps with the shipping routes.” It’s a global issue. At the same time that global shipping is increasing, some whale populations are recovering from population decimation during the whaling era. “In some areas, ship strike is responsible for a third of whale mortalities.”
Whales are mammals and their response to being hit by the bow of a ship is similar to what happens when a human is hit by a car – bones are broken and muscle, cartilage, arteries and veins are damaged. “If they get hit in the head, it knocks them out. Whales are elective breathers, so if they are knocked unconscious and don’t breathe, they die.” If a whale gets hit by a ship’s propeller, “the whale will typically die from massive haemorrhage. We don’t see these injuries so much, because if their body cavity is opened up and flooded with water, they usually sink pretty fast.”
Last year, Constantine worked with international colleagues to attach multi-sensor suction tags to seven whales in the gulf. They found the bryde’s whales spent almost all their time in the top 10m of water, where they are vulnerable to ship strike, and did not vocalise, making a suggested whale-listening warning system useless. So, what can be done to stop ships killing whales?
Recent Canadian research shows that a vessel hitting a whale at 8.6 knots has a 20% chance of killing it. If the vessel is travelling at 15 knots, the likelihood of death rises to 76%. “What’s being internationally recognised is that at 10 knots you have this pay-off between the ship being able to travel at a safe and manoeuvrable speed, and a 25-30% chance, depending on the species, that if a whale is hit the ship will kill it.”
After a workshop in Auckland last year, shipping representatives agreed to reduce speed – but only when it does not affect schedules – to post watch for whales and to report all sightings to Ports of Auckland, which in December set up a system to alert vessels to all whale sightings in the gulf. But currently the best option, says Constantine, is to mandate a top speed of 10 knots in the gulf.
“We know we’re losing at least two whales a year to ship strike. We estimate there are about 59 resident whales, with a natural 4% a year mortality. If we’re adding two whales a year, that’s potentially a big disaster,” says Constantine.
“No one likes killing a whale. And I don’t like working with dead whales. Slowing down will minimise the risk of mortality to the bryde’s whales in the Hauraki Gulf.”
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