The onset of spring and fall usually heralds some welcome harbingers, such as the arrival of a robin in spring or that first nippy morning of autumn. For cruisers, though, the most welcome harbinger of spring and fall is bargains.

If you're interested in cruise bargains and don't already know about one of cruising's open secrets, let me tell you about repositioning cruises.Twice a year, cruise ships switch from sailing in one part of the world to cruising grounds in another. During the seasonal migration, entire fleets of ships are repositioned. For instance, they move from the Mediterranean to the Caribbean for their winter season. Ships that spend the summer cruising Alaska make lengthy cruises via the Panama Canal to reach Caribbean home ports for winter itineraries, and those in southern cruising grounds during winter may cross the Atlantic to sail Mediterranean and European cruises in spring.

These in-between trips at the cusps of the seasons can deliver windfalls. To fill ships for these one-way-only repositioning voyages, cruise lines offer lower-than-normal per diems, resulting in opportunities to nab a cruise on a ship whose regular rates otherwise may be unaffordable. Repositioning discounts on Crystal Cruises' Panama Canal sailings, for example, can be as high as 50 percent off brochure rates.

Because of the distances covered, such cruises tend to be long, with some lasting more than two weeks. Cruises of this length are sought after by experienced cruisers who like the languor of lots of days at sea.

Princess, which normally may call at only one or two British Isles ports during a regular Europe season, has enhanced its repositioning itineraries to be more port-intensive than usual. The Aug. 29 repositioning of Royal Princess, for example, includes calls at four ports in the British Isles.

Because repositionings occur only twice a year, they are not heavily advertised or promoted. However, these cruises have become so popular that they tend to book up early and are frequently the first cruises to sell out.

Here are some repositioning bargains for fall (rates quoted are per person, double occupancy). For more information on these and other repositioning cruises, contact a travel agent.

- On Oct. 8, Celebrity Cruises' Horizon repositions from Alaska to the Caribbean via the Panama Canal. The voyage from Los Angeles to Ft. Lauderdale includes calls at Cabo San Lucas and Acapulco, Mexico; Puerto Caldera, Costa Rica; Cartagena, Colombia; Grand Cayman and Cozumel, with six days at sea. Cruise-only rates begin at $1,649.

- Among the more exotic repositioning cruises is Renaissance Cruises' Athens-to-Seychelles Islands voyage, a 22-day cruise/tour departing Nov. 8. Renaissance VIII transits the Suez Canal with stops in Cairo, Luxor and Sharm El Sheik, Egypt; Aqaba, Jordan; Aden, Yemen and three islands in the Seychelles.The full cruise tour includes the 17-day voyage, two-night hotel stay in Athens and round-trip air fare. Fares start at $3,999.

- On Oct. 25, Holland America Line's Maasdam crosses the Atlantic from Lisbon to Ft. Lauderdale on a 10-day voyage priced from $1,646.(This sailing also is a Big Band theme cruise.)

HAL also offers repositioning cruises on its five other ships - Statendam, Noordam, Nieuw Amsterdam, Ryndam and Veendam - between Vancouver, B.C., and Ft. Lauderdale through the Panama Canal. These range from 10 to 21 days with fares from $1,714.

- Royal Caribbean's newest ship, Enchantment of the Seas, which debuts in Europe this summer, repositions to the United States on Sept. 6. The 14-night voyage between Harwick, England, and Boston visits ports in Scotland, Denmark, Iceland, Newfoundland and New Brunswick. With advance purchase discounts, rates begin at $2,199; specially-priced air fare is $549 for passengers booking inside cabins, $749 for those in outside cabins.

- Orient Lines offers a two-for-one special on the Marco Polo's Dec. 7 "Southern Trans-Atlantic" voyage, a 15-day cruise/tour between Cape Town and Buenos Aires. Included in the rate are a three-night hotel stay in both Cape Town and Buenos Aires, sightseeing and a 10-day voyage. The cruise-only fare starts at $2,500.

- On Nov. 1, Norwegian Cruise Line's Dreamward moves from a season of New York/Bermuda cruises back to the Caribbean. The 15-day voyage from New York to Miami calls in Bermuda (two days), St. Thomas, Dominica, St. Vincent, Caracas, Aruba, Grand Cayman and Cozumel, with five days at sea. Rates begin at $1,599, including air fare to New York and from Miami.

- On Aug. 29, Princess' Royal Princess crosses the Atlantic from London to New York, calling at Cornwall; Cork and Dublin, Ireland; Glasgow, Scotland; and Reykjavik, Iceland. In Greenland, the ship will cruise Prince Christian Sound and visit the remote village of Nanortalik. Rates for the 13-day voyage start at $4,854. The sailing includes free round-trip air from most U.S. cities or a $100 air add-on from West Coast cities. Princess' LoveBoat Savers rates also can net you 50 percent off second-person fares.