Staff numbers have also reduced across all departments, according to Shetland Recreational Trust (SRT) acting chief executive Robert Geddes.
The Scalloway and South Mainland pools, for example, will now be shut on Fridays after the school summer holidays.
The North Mainland Leisure Centre will no longer be open on Sundays after the holidays, whilst the leisure centres in Aith and Whalsay will be closed on Saturdays.
Yell will remain closed on Saturdays, although Unst will be staying with the same opening hours at the moment.
In some instances, pool opening hours will be different, but some of the building hours remain the same.
The recreational trust said it has sought to limit the impact on customers by moving activities around timetables.
Earlier this year, the SRT said it would have to explore ways of reducing its spending due to rising costs.
This included looking at voluntary redundancies and potentially compulsory lay-offs.
Speaking this week, Geddes told Shetland News:
“We are working through a process of reviewing all our staffing levels and opening hours to try and balance budgets.
“The levels of saving required have meant we’ve had to consider the number of operating days at sites, and this will be actioned with the effect of the new school term starting next week.
“We’ve used data we have available to us to make the decisions on which days are most suitable for closure, but we also hope to move any activity/service that is affected on the proposed closure day to another day in the week so with a view to limiting impact on service to customers.”
He said it will continue to be a “work in progress whilst we try to find a sustainable position”.
Geddes confirmed that the Clickimin Leisure Complex in Lerwick will remain open seven days a week.
The trust, a charity mainly funded by Shetland Charitable Trust, operates eight centres across the Isles, but the cost of maintaining them is increasing as they age.
In April, it was announced that Steven Laidlaw was stepping down as the chief executive of the trust.
Chair of the Board of Trustees David Thomson said at the time that the SRT “faces serious trials this year”.
