Fisheries, and the problem with sharks

Longline fisheries catch and often kill stunning numbers of sharks.

(Photo: Southwest Fisheries Science Center, NOAA)

When squid was used for bait, half the catch of the Hawai'i swordfish longline fishery was sharks. The number dropped to 32 percent after fish replaced squid as bait.

The number is less than 25 percent in Australian and Fiji longline fisheries—a smaller number but still significant.

Why is that a problem?

"Sharks and their relatives are much more vulnerable to overfishing and population collapse than bony fishes. They grow slower, mature later and have lower population increase rates. Therefore, methods to manage them may have to differ from traditional fishery management methods,” said Eric Gilman, of the World Conservation Union.

Gilman is the lead author of a new report, “Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries: Industry Practices and Attitudes, and Shark Avoidance Strategies.”

The report was produced by the Western Pacific Regional Fishery Management Council, along with the United Nations Environmental Programme's Regional Seas Programme, Blue Ocean Institute, Consortium for Wildlife Bycatch Reduction, New England Aquarium, Project GloBAL (Global Bycatch Assessment of Long-Lived Species), and the Gordon and Betty Moore Foundation.

Researchers were from the United States, Japan, Australia, Peru, South Africa, Italy, Fiji and Chile.

The goal of the report was to learn from fishers themselves how best to reduce the unwanted catch of sharks.

A survey of captains found that you'll catch more sharks using squid for bait, using wire leaders that the sharks can't break, and fishing at certain depths preferred by sharks.

There are ways to reduce the shark catch, but they can have other impacts. For instance, if anglers use plastic leaders that sharks can cut, they will be less likely to put weights near the hooks, for fearing of losing both. That means the bait won't sink as quickly, and may be more likely to attract and hook seabirds.

There are fisheries where the sharks are an economic benefit. The boats keep the sharks and are able to sell them, and the revenue exceeds the cost of catching sharks. But in fisheries where shark take is either not valuable or not permitted by law, the costs of fishing in such a way that you catch sharks can be high.

In Hawai'i, as an example, it is illegal to simply take the shark fins and toss the rest of the shark back.

The costs of fishing in areas where sharks are caught can include damaged and lost gear, risk of crew injury in handling sharks, lost time in taking sharks off the gear, the lost opportunity to catch valuable species on hooks occupied by sharks, and so forth.

Veteran longliners are finding that they can adjust their fishing methods to increase their catch of the fish they want, and reduce their catch of sharks. More efficient fishing methods can include carefully selecting where to fish, specific times of fishing, leaving bait in the water for only limited amounts of time, fishing at specific depths and so on.

“Beyond these strategies, the state of knowledge to reduce unwanted bycatch and depredation by sharks in pelagic longline fisheries is poor,” the report said.

It proposes a number of new strategies, among them shark deterrents, which can include chemical, magnetic and electrical measures that may cause sharks to avoid fishing gear.

The study also notes, however, that there seem to be increasing markets for shark meat, meaning that sharks, instead of being troublesome bycatch, could become sought-after fish.

"Sharks are one of the world's most valuable fishery resources. They provide an important protein source, as well as a luxury item,” said Kitty Simonds, executive director of the Western Pacific Regional Fishery Management Council. The luxury item is shark fin soup.

And that increasing demand for their flesh a problem for the future of sharks, which as a group are long-lived, and which reproduce at low rates. Sharks thus are particularly vulnerable to overfishing, and will be slow to recover from it, the study says.

The world's fishery regulators thus will need to learn a lot more about the sharp-toothed predators, in order to protect them as fishing prey.

© 2007 Jan W. TenBruggencate