Slowing ships in Auckland's gulf may be the only way to prevent fatal collisions with a resident whale population, as new research shows the highly endangered mammals swim in shallower water than first thought.
A workshop on Bryde's whales and ship-strike was held this week to find ways to prevent their high death toll. University of Auckland researchers estimate at least two whales die a year from collisions in shipping lanes, from a tiny Auckland population of about 50.
The gathering of Auckland Council officials, the Department of Conservation, environmental groups and shipping industry members was called after another Bryde's (pronounced "Brooders") whale was killed in late January, and in light of new research which underlined the whales' vulnerability.
Auckland University biological scientists Rochelle Constantine and Natacha Aguilar found the whales spent 90 per cent of their time in the top 12m of water.
The whales also surfaced at night, meaning they might be putting themselves in a ship's path while resting.
Various regulatory and voluntary resolutions were raised at the workshop, including speed limits for ships, new shipping corridors and technology such as underwater alarms.
Dr Constantine said a lowered speed limit was the most feasible option. This had already been introduced overseas - in the Straits of Gibraltar and off the northeast coast of the United States - and international shipping companies were accustomed to the rule.
The chances of a ship killing a whale fell by 50 per cent if the vessel was travelling below 12 knots and by 25 per cent below 10 knots.
Environmental Defence Society senior policy analyst Raewyn Peart said shipping industry representatives at the workshop were receptive to this rule change.
A new guideline for speed in the Hauraki Gulf would require approval from the Transport Minister. The ministry was unable to reply to inquiries yesterday.
Rerouting shipping traffic or creating more restricted shipping corridors were less practical options, because the range of large ships in the gulf operated at different depths.
Underwater alarms which warned whales of ships have been trialled, but have so far not proved successful. Researchers said sonar-like devices on ships simply added another sound to an already noisy marine space. In the worst cases, the alarms made the whales panic.
Ms Peart said her organisation was keen to see voluntary guidelines introduced, because they could be implemented almost immediately and reducing the incidence of ship-strike was a matter of urgency.
Dr Constantine said: "Every year we delay talking, it's potentially another two dead whales ... We've lost 41 whales in the last 16 years. Of the 18 we've looked at, 15 have died because of ship-strike."
* Options to reduce Bryde's whale collisions:
* New speed limit for ships.
* Shipping corridors or new routes
* Underwater alarms to warn whales.
* Bryde's in NZ: Fewer than 250.
* Population in Hauraki Gulf: 40-50.
* Deaths a year from natural causes: 2.
* Deaths a year by ship-strike: 1-2.