By Catherine Norwood
Depredation of catches in longline fisheries is an increasing problem worldwide: whales and seals can steal up to 75 per cent of the catch from hooks as the lines are being hauled aboard. While it is still a relatively rare phenomenon in Australia’s southern waters, Nuffield Scholar Rhys Arangio has been researching preventative strategies to ensure it stays that way.
As a marine scientist working for Austral Fisheries, Rhys Arangio says the first known instance of depredation by whales in Australia’s Heard Island and McDonald Islands Patagonian Toothfish fishery was reported in 2011.
The key issue with depredation is that once one whale learns to steal the fish, other members of a group can quickly learn the behaviour. Two of the main culprits are sperm whales and killer whales, depending on the location of the fishery.
The Nuffield Scholarship that has allowed Rhys Arangio to investigate the issue was sponsored by the FRDC and Woolworths, and took him to Chile, Norway, Belgium, France, Canada and the US.
His report, Minimising whale depredation on longline fishing: Australian Toothfish fisheries, assesses efforts around the world to prevent depredation and seeks strategies suited to Australian fisheries.
These include alternatives to the current auto-line system used in Australia’s Toothfish fisheries, as well as technologies that may confuse or track whales or deter them from fishing vessels.
Rhys Arangio found that modifying fishing techniques, the use of traps and acoustic monitoring appear to be the ‘best fit’ for Australian operators to prevent depredation spreading throughout the Patagonian Toothfish fishery.
“From our knowledge, [depredation] has only occurred on the one occasion, and once the gear was hauled and the vessel moved from the area, the whales did not follow.
Though it is not currently a significant problem, with an increasing portion of longline-caught fish in the fishery, we would like to be prepared in case the whales develop this skill further and begin to have a significant impact on Toothfish caught in Australian waters,” Rhys Arangio says.
Operators in the fishery are planning to increase the proportion of their catch harvested by longline, as opposed to trawling. However, whale depredation in other Southern Ocean Toothfish fisheries has reduced longline catches by up to 75 per cent at times.
“This learned skill can alter whales’ natural foraging behaviour and diet for periods of time,” Rhys Arangio says.
“It has also been discussed in fish stock assessments, to enable scientists to assess how much fish catch has been taken by whales, and in extreme cases could lead to over or underestimating fish escapement in stock assessments.”
Depredation also increases the risk of injury to the animal targeting the line, as it brings them close to fishing vessels. At least 20 accidental marine mammal deaths related to depredation have been reported around the world.
Rhys Arangio says there are a number of factors in Australia’s favour. Depredation has only been noted once in the Heard Island and McDonald Islands fishery.
This compares to decades of depredation in some other fisheries where, he says, the skill of depredation appears to be instilled in the whales’ psyche.
The single depredation event in the Heard Island and McDonald Islands toothfish fishery was by a sperm whale, not killer whales. Fisheries affected by killer whales are generally worse off, he says. The Australian fisheries are also not in areas where whales are known to live, which is an issue for some other affected fisheries.
Australian vessels cannot discharge offal overboard, which reduces the attraction of whales. Currently only two longline vessels work in the Australian fishery, between April and October, while in other fisheries several longline vessels work up to 10 months of the year.
This creates a constant food source for the whales. It is also thought that the major cue in attracting whales is the cavitation noise of vessels’ propellers; the more vessels, the more likely it is that whales will appear.
Rhys Arangio has recommended that a whale depredation best-practice handbook for Australian Toothfish vessels be developed, along with a whale sighting catalogue for the Heard Island and McDonald Islands fishery. This will allow sightings in the fishery to be compared with those in other fisheries to determine whether the same whales are responsible for depredation.
Strategies to reduce the likelihood of depredation include reducing the length of the longlines and the depth of the lines when whales are known to be, or expected in, the area. Increasing the speed at which the line is hauled in may make it more difficult for fish to be taken off the lines, but this must be treated carefully as it also increases the risk of the line breaking.
Rhys Arangio says the best strategy, if possible, is to buoy-off gear and steam at least 40 nautical miles away when whales are sighted. “It’s important that we don’t give whales a chance to further develop their depredation skills,” he says.
He also suggests the Australian industry continues to develop the Toothfish trap fishery, which eliminates depredation, although traps trialled to date have not proved cost-effective compared to longlining and trawling.
His “next steps” include investigating passive acoustic listening devices that could be used to map the whereabouts of sperm whales at different times of year at Heard Island and McDonald Islands, and monitoring technologies such as active decoys, bubblers and the use of jamming frequencies being tested in other fisheries around the world.
FRDC Research Code: 2009-324
Rhys Arangio, 0402 883 195
The report is available from Nuffield Farming Scholars International.