In one of the most powerful scenes of Sir David Attenborough’s new film Ocean, the audience sees industrial fishing from a fish’s perspective.

Confronting a bottom trawl net as it thunders across the seabed, terrified fish scatter in desperate but futile attempts to escape the vast net swallowing them. The heavy chain that holds the trawl down sweeps away sponges, corals, seagrass and other seabed life, leaving behind utter devastation.

Attenborough’s latest nature documentary is a visually magnificent and highly personal meditation on the relationship humans have with the sea. It is the most important part of our world, he says. But we have taken it for granted.

A century of intensifying and destructive fishing has culminated in bottom trawl nets, some as big as cathedrals and weighing many tonnes, being towed along the seabed to catch fish. To allow them to fish more effectively in areas of rough seabed, which is where most marine life is found, fishers in the 1920s invented “rock-hopper” gear: rollers placed along the foot rope that touches the bottom, allowing the net to bounce over obstacles.

This innovation followed the trajectory of many fishing methods, which was to become more destructive over time to sustain the size of catches in the face of declining fish stocks.

Shellfish dredging, another fishing method that destroys as it catches, is shown in a second horrifying scene. To catch scallops, steel dredges armed with spikes (imagine the harrows farmers use to break up soil on ploughed fields) drag along the seabed, smashing and pummelling everything. In minutes, seabed life of astonishing diversity and beauty is erased.

Together, Attenborough explains, bottom trawling and dredging wreak their havoc across an area of seabed larger than the Amazon........

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