It sounds shocking. Dragging electrified nets across the seabed to jolt flatfish and shrimp off the bottom and catch them in vast numbers. Members of the European Parliament agree. This week they voted for a proposed ban on so-called pulse trawling in EU waters.
The vote is not the final word on this, but campaigners opposed to such fishing, who liken it to Tasering the seabed, have welcomed it. Dutch fishers, who increasingly use the method, are disappointed.
Is it the right outcome? Pulse trawling is being explored as an alternative to beam trawling, which targets flatfish and invertebrates like shrimp that burrow into the seabed. To encourage these species from the sediment, beam trawlers drag heavy 鈥渢ickler chains鈥� that plough up the sea floor, flushing out quarry into a net.
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. As boats have gained power, they have added more and more chains to increase ground contact and improve the chance of stirring up fish. But as fuel prices have risen, so too has the price of dragging this heavy gear. Beam trawling is costly in euros, as well as in terms of damage to the seabed, and carbon emissions.
Pulse trawling, proposed as an alternative, sets up an electrical current at the seabed via wires. When a fish enters the electric field, its spine becomes a conductor. For valuable Dover sole, the pulse sends this flatfish into a U-shape, making it easier to net. What is the evidence on this new technique?
Methods must adapt
First, pulse trawling uses less fuel. The boats don鈥檛 drag heavy chains and they move more slowly. And by scooping up less of the seabed and abrasive debris, it also produces catch of higher quality.
But this lighter gear allows vessels to exploit sea floor areas that are too rough to beam trawl 鈥� an economic boon that鈥檚 an ecological bane. This means its environmental footprint may get bigger, not smaller, unless the sea floor is carefully zoned to avoid overfishing sensitive areas. China provides a cautionary note. Pulse trawling was banned there after its use led to greater catch success, and overfishing. When fishing gets more efficient, say scientists, management must adapt by reducing vessel numbers, or restricting fishing to fewer days.
What鈥檚 the impact of shocking fish and invertebrates? Some of the science is clear. Pulse trawling, according to lab experiments, does not compromise short-term (two week) survival of many species. In fact, the challenge was finding thresholds for survival even as pulse rate and voltage were progressively increased.
One notable exception is cod. Dutch fisheries scientist Adriaan Rijnsdorp and colleagues , the cramp response to electrical jolting can break their neck. Why cod and not other species? We don鈥檛 yet know. 鈥淲e want to have a scientific basis and understand the mechanisms behind these fractures,鈥� says Rijnsdorp. His team is conducting a formal impact assessment comparing pulse with traditional beam trawl fishery, for release in 2019.
Proceed with caution
But cod injury is largely an ethical rather than biological issue, argues Michel Kaiser of Bangor University, UK, chair of the International Scientific Advisory Committee for the Dutch pulse trawling project. That鈥檚 because cod has poor survival when landed as by-catch by beam trawlers too, due to decompression of its swim bladder.
Maarten Soetaert, a Belgian researcher who has studied by-catch in pulse versus beam trawling, says an important unanswered question is what happens to smaller zapped cod that escape the net. By-catch quantities may be a poor measure of impact without understanding affected species鈥� long-term survival and reproduction.
One thing scientists and environmentalists agree on is that testing pulse trawling does not require as many boats as currently use it. Exploiting a loophole in the law that makes electric fishing illegal except for research, the Dutch government has passed off a commercial decision as a scientific one. No more than a handful of pulse trawlers, and not the 84 currently licensed there, are needed to further assess the method鈥檚 efficacy and impacts.
As Kaiser says, any fishing activity has an impact on the marine environment. Pulse trawling is no different. It has pros, cons, and many unknowns. That suggests proceeding in a limited capacity with caution, not a whole fleet.
