Ferry operator Stena Line is cutting sailings between Anglesey and Ireland with the loss of up to 30 jobs.
The HSS Stena Explorer currently sails three times a day
Stena Line will gradually reduce fast ferry services between Holyhead and Dun Laoghaire, near Dublin, from May.
The Swedish-owned firm blamed increased fuel costs, a decline in tourists and competition from budget airlines.
Stena, which employs 250 in Holyhead, said jobs would be lost through natural wastage and a vacancy freeze. Union leaders expressed their "deep concern".
A Stena spokesman said it did not expect to make any compulsory redundancies.
From 2 May, fast ferry sailings will be cut from three round trips per day to two on Mondays to Thursdays.
There will be three sailings on Fridays, Saturdays, and Sundays until 25 September, when they will also be cut to two.
Stena route director Vic Goodwin said: "Our central corridor services have developed well over the last few years, but we have some significant challenges in front of us which we must address sooner rather than later.
"These include a decline in tourist volumes as a consequence of the competition from low-cost airlines and other ferry operators, and more importantly, very high fuel costs, which have doubled in the last 18 months."
Bob Crow, general secretary of the Rail Maritime and Transport (RMT) union which represents ferry workers, criticised the cuts and warned that they could lead to industrial action.
He said: "Stena have assured us that they have no intention of attacking the terms and conditions of our members and that they hope to avoid redundancies by redeploying staff.
"Those assurances are, of course, welcome and we hope we can secure our members' jobs through constructive dialogue.
"However, we should also make it clear that any suggestions of compulsory redundancies will be resisted, with industrial action if necessary."
The company said its service between Fishguard in Pembrokeshire and Rosslare would not be affected.
Gareth Winston Roberts, deputy leader of Anglesey council, said the news was not unexpected.
He said: "It's time people understand how gloomy the situation is, but I am happy that the company has said it is not making staff redundant and that they are trying to keep the workers - but who knows what will happen in six months."