Cruise passengers are paying more in gratuities, as almost all big cruise lines seek to retain and attract good service personnel and remain competitive with each other.

Five lines have raised or announced increases so far this year, with one, Norwegian Cruise Line, raising its gratuities twice.

Princess Cruises was the latest to jump on the bandwagon, saying last week that it would raise gratuities 12.6% starting Jan. 1.

The moves raise the overall cost of a cruise without raising the fares on which travel agents earn commission. But several agents said clients are not noticing or pushing back against higher gratuities and that agents don’t go out of their way to discuss them, other than to say they are automatically billed at the end of a cruise.

Princess, which as late as 2007 was at $10 a day, raised the figure by 50 cents in 2008 and then waited until May 2011 to increase it to $11.50. Beginning with cruises departing Jan. 1, the suggested gratuity will jump  to $12.95 a day, with suite guests paying $13.95.

While most lines’ gratuities are now at $12.95, Carnival Cruise Line’s are at $12 after raising its rate to that level a year ago.

At the high end of the scale, Norwegian Cruise Line’s suggested gratuity charge is $13.50, having raised it in March from $12 to $12.95 and then again, in August, to the current rate.

Frank Del Rio, chairman of Norwegian’s parent company, Norwegian Cruise Line Holdings, has made it known that he considers better service one of the keys to improving customer retention at Norwegian.

It isn’t clear exactly how much of crew compensation is derived from tips, since most cruise lines closely guard that information. But O’Connor said that gratuities represent an “important” part of crew compensation.

Estimates of the share of crew salary that tips constitute vary from around 50% to nearly all of it in some positions. Cabin stewards earn from $1,400 to $2,400 a month in wages alone, according to Cruiseshipjob.net, a recruitment site. Waiters earn between $2,500 and $4,000 a month in combined wages and tips, according to another site, Cruiselinesjobs.com.

Historically, cruise lines have provided breakdowns of where tips go, and some still do. On its website, Carnival says of its $12 total gratuities that $6.10 is allocated to dining staff, $3.90 to stateroom attendants and $2 to kitchen, entertainment, guest services and other hotel staff members.

Increasingly, however, gratuities are being treated as lump-sum service charges added to the bill at the end of the cruise. Guests remain free to adjust the amounts, although Norwegian recently said charges can only be altered after guests depart the ship, not onboard.