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Entanglement: The Threat of Commercial Fishing to Dolphins and Whales

| Mark J. Palmer
Topics: Cetacean Habitat, Dolphin Safe Tuna, Dolphins, Tuna Industry, Vaquita, Whales

Several species of whales and dolphins are seriously endangered in oceans around the world.  One of the main culprits, in many instances, is commercial fishing gear, including nets (especially gill nets) and other fishing gear.

Gill nets have long been recognized as a serious threat to dolphins and whales.  Old-style gill nets were made of woven rope and apparently were not a serious threat to cetaceans, who could detect the nets with their own echo-location hearing – some speculate that the woven ropes held air bubbles which bounced off sounds sent out by dolphins and whales to navigate and find food.  

But in the 1980’s, widespread use of single-strand plastic gill nets came into vogue.  Lighter, stronger and longer-lasting than rope gill nets, not to mention much less expensive, these plastic strands were rendered invisible to the sonar of cetaceans, and large numbers died, and continue to die, by becoming entangled in such nets.

Gill nets, with some measuring 20 miles or more, are used in oceans around the world. After a successful campaign, in which the International Marine Mammal Project and other organizations participated,  the biggest of the nets were banned  from the high seas in 1991 by the United Nations. However, use of smaller gill nets continues inside the 200-mile-ocean-zone of many countries, despite the threat to cetaceans and other marine mammals.

Other fishing gear can also cause serious problems to marine mammals.  The use of long strands of rope and nets can entangle whales and dolphins at a high enough level that populations can become threatened - and can have an especially significant impact in the case of endangered species.

A study by the Natural Resources Defense Council (NRDC) in 2014 notes that an estimated 650,000 marine mammals (including whales and dolphins) die in foreign fisheries every year due to being hooked or entangled in nets and other fishing gear.  

This past year, a record number of endangered right whales, previously decimated by commercial whaling, have died becoming entangled, mostly in fishing lines used to link crab traps to surface buoys.  With a population estimated at only 450 total whales in the Western North Atlantic Ocean, seventeen died in 2017 - amounting to a huge and devastating loss  According to the US National Marine Fisheries Service, the population has been steadily declining since 2010,, with the highest toll  being  among female right whales who are obviously crucial for the survival of the species.  In addition to entanglement in fishing gear, vessel strikes have also killed several right whales, as well as other species.

The rare little vaquita porpoise (related to the more common harbor porpoise),  who live in the upper reaches of the Gulf of California, as well as the Maui and Hector’s dolphins confined to the coast of New Zealand, are declining due to the use of entangling fishing nets, despite world-wide concerns and largely ineffectual   attempts by the governments of Mexico and New Zealand to reducethreats.  There  are only an estimated 30 vaquitas left in Mexico, and  a mere 55 Maui’s dolphins remained in New Zealand waters in 2012.  At these extremely low numbers, it takes just a few deaths to threaten the whole species.

There has been some effort put into trying to reduce the kill by modifying fishing gear.  In the case of gill nets, experimentation has tested the use of “pingers”: noise-making devices that alert dolphins and whales to the presence of gill nets.  While showing promise in some experiments, pingers have not been widely adopted, nor do they seem to work as well in actual fisheries, with a few limited exceptions, as opposed to experimental trials.

A new technology for crab traps would eliminate the entangling ropes entirely.  Automatic releases allow traps to surface by themselves, eliminating the need to haul them in by ropes.  The fishermen can release the traps electronically from the seafloor.   However, outfitting fishermen with these kinds of new  traps would be expensive - something that may prevent the widespread adoption of the technology.

Eliminating the nets and lines by closing fisheries is an extreme, but effective technique.  In the 1980’s and early 1990’s, Californians eliminated gill net fisheries in state waters (out to 3 miles from shore) through  legislation and citizen initiative, putting an end to the threats faced by  sea otters, harbor porpoises, gray whales and marine birds in the process.  Experimental permits were issued to fishermen to investigate alternative ways of fishing without gill nets, and funding was allocated to help compensate fishermen for lost revenue.

However, by far the largest kill of dolphins in the world occurred due to the deliberate targeting, chasing and netting of spinner, spotted and common dolphin pods in the Eastern Tropical Pacific (ETP) Tuna fishery, using purse seine nets.  More than 7 million dolphins have been killed by this method of fishing.  During the late 1980’s, the dolphin kill was estimated at 80,000 to 100,000 dolphins annually.  

In 1990, IMMP reached agreement with the largest tuna companies in the world to end the practice of targeting dolphins, establishing the Dolphin Safe tuna label, which means no dolphins were deliberately chased, netted or killed or injured (both deliberately and accidentally) during the entire fishing trip in which the tuna was captured.  Official estimates are that around 2,000 dolphins are year are killed in the tuna fishery today in the ETP, largely by fleets from Mexico, Colombia and Venezuela that persist in targeting and netting dolphins.  Scientists believe these numbers of dolphins dying in tuna nets are undercounted due to a variety of circumstances.

Unfortunately, closures of areas to the use of gill and other entangling nets have not worked as well in Mexico and New Zealand.  Activists believe New Zealand closures are too limited to fully protect the Maui and Hector’s dolphins, while illegal gill nets proliferate in closure areas in the Gulf of California.  The fishing industries in some nations are extremely powerful politically, as they harvest vast amounts of sea life in order to meet burgeoning demand in places such as the United States.

The reform of fisheries must take place country by country, in order to safeguard dolphins and whales and protect other marine life as well.

Photo credit the Florida Fish and Wildlife Conservation Commission.