Porpoises maintain mental maps, a mental image of their surroundings, which also includes the topography of the ocean floor. They use that knowledge to find their way around without having to rely on their ability to echo-locate using sound.
Accidental by-catch in fishing nets is estimated to kill 300,000 whales, dolphins and porpoises every year. Porpoises are air-breathing mammals, so once they become stuck in a net, they only have minutes before they drown. Active fishing gear is just as dangerous as abandoned gear. “Ghost nets”, forgotten or abandoned at sea by fishermen, are…
The single most serious thread to the vaquita, and the cause for its rapid decline, is the use of gill-nets in the vaquita habitat.
A gill-net is a wall of netting that hangs in the water column. The mesh is designed so that fish can get their heads through, but not the rest of their bodies. As they struggle to free themselves, they get entangled with their gills. Gill-nets are very effective and used around the world, but often lead to large amounts of by-catch and pose a threat to other marine animals, such as sea turtles, seals and sea lions and cetaceans like the vaquita. If a vaquita gets entangled, it only has minutes to free itself. Most animals drown, and those that escape often do so with severe injuries.