King scallop catch limits will be imposed in Manx waters when the annual season begins next week.
The Manx government said they are needed to "protect vulnerable stocks" amid sustainability concerns.
Environment Minister Geoffrey Boot said the quotas follow recommendations from marine scientists at Bangor University.
The island's scallop management board (SMB) said declining stocks mean a precautionary approach is needed to ensure "long-term viability".
The king scallop season, worth more than £12m to the Manx economy each year, runs between November and May.
The SMB recommended a daily catch limit of 700kg (1,543lb) - down from 1,050kg in 2017-18.
A total catch limit for the season of 2,562 tonnes - down from 3,203 - will also be imposed.
Mr Boot said his department "fully supports supports the recommendations" and that fisheries will be "monitored closely".
"The economic effect of the declining king scallop stock has been hidden by the higher prices being charged, but there is no mistaking the downward trend in the availability of this valuable resource," he added.
"Reducing the total catch will ensure that this continues to be a sustainable and economically worthwhile business for all those who fish for king scallops in Manx waters."