One of San Francisco's most famous tourist attractions -- a colony of sea lions -- has disappeared overnight from Pier 39.

The mammals have reportedly migrated south to mate but are expected back after undergoing a similar migration around four and a half years ago.

In December 2009, the sea lions suddenly headed south to breed, leaving behind hordes of disappointed sighteers. At the time, San Francisco's Marine Mammal Centre explained that sea lions were migratory animals.

A disappointed tourist, Thomas Crombie from Pittsburgh, Pennsylvania, told CBS Local: "Came to see the sea lions and we hear they went south to go mate, so there's none here. I saw one earlier today. A little disappointed but nature has a way of interrupting things sometimes."

According to SF Gate, tourists have speculated as to the whereabouts of the sea lions, including an earthquake or the threat of great white sharks.

Marine experts have suggested several possible reasons for their departure. Aside from annual breeding pattern, a hunt for food or a disturbance such as a construction project may be the answer.

The mammals arrived on the wooden decks of Pier 39 after the Loma Prieta earthquake hit San Francisco in 1989, measuring 6.9 on the Richter magnitude scale and killing 63 people throughout Northern California.

By January 1990, the barking pinnipeds started to arrive in droves and completely took over K-Dock, much to the exasperation of Pier 39′s Marina tenants.

Staff on the marina turned to the Marine Mammal Center, an organisation devoted to the rescue and rehabilitation of marine mammals, for advice about their new slippery tenants. Eventually, it was decided that the sea lions should stay in their new home.

With a plentiful supply of food from the Bay and an environment protected from predators, Pier 39 proved to be an ideal living situation for the animals. Within a few short months, the number of sea lions grew to more than 300.

By November 2009, the colony had reached an all-time high of 1,701, but one month later only 10 remained.

According to Pier 39's sea lion website, most sea lion pups are born on the Channel Islands off Southern California in June. They also migrate across the Pacific Coast, from Vancouver to the southern tip of Baja California.