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Why are purse seines especially effective for dense schools of tuna, herring, and sardines?

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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.

Purse seines are highly effective for catching species like tuna, herring, and sardines because these fish naturally congregate in large, dense shoals, which is a prerequisite for the successful use of this gear.1

The effectiveness of the purse seine lies in its ability to enclose a school of fish completely. Once the vessel surrounds the shoal with a wall of netting, the bottom of the net is closed by drawing a line through rings attached to the lead line. This action effectively "purses" the net shut, preventing the fish from escaping by diving downward or swimming out from the sides.12

Because these species exhibit schooling behaviour—often reacting to threats or environmental cues by forming tight, compact balls—they are particularly vulnerable to being captured in a single, concentrated mass. Once the net is hauled in, the fish are easily removed by a brail or pumped directly onto the fishing vessel.3415