The three-month suspension runs until May 15 and covers seas off Prachuap Khiri Khan, Chumphon and Surat Thani.
A survey of mackerel last year found that the ban offered effective protection for spawning fish in the gulf. The average length of spawning mackerel is 18.5 centimetres, making them vulnerable to net fishing. The area targeted by the three-month closure is also rich in other economically important species.
The ban will allow the larvae of mackerel and other species to grow undisturbed in prime spawning stretches such as Khao Mong Lai peninsula to Hua Hin in Prachuap Khiri Khan.