Britannica Sources used in this answer

How does mid-water trawling catch pelagic fish without dragging a net along the bottom?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A worker in dark clothing inspects large frozen tuna lined up on the floor at Tsukiji fish market in Tokyo, Japan, using a tool to check the fish. — commercial fishing Worker in the Tsukiji fish market, Tokyo.

Mid-water trawling catches pelagic fish by operating in the water column between the ocean floor and the surface, rather than dragging gear along the seabed.1

To control the depth of the net, fishermen adjust the length of the towing warps and the speed of the vessel:
* Depth adjustment: Increasing the length of the warps or decreasing the towing speed causes the trawl to sink, while shortening the warps or increasing speed causes it to rise.1
* Monitoring: The depth is tracked using a netsonde, a transducer mounted on the trawl that transmits real-time data to the vessel, showing the net's position relative to the seafloor and schools of fish.1

This method allows for targeted fishing of pelagic species without the environmental damage associated with bottom trawling, which involves heavy gear plowing the seafloor. In some cases, a semipelagic trawl may be used, where the otter boards remain in contact with the bottom while the net itself floats above it to target fish concentrated just off the seabed.12