<P> Although the reason for their migration to the pier is unclear, the refurbishing of the docks in September 1989 required the removal of all boats from that area, leaving large open spaces for the sea lions to move into . Once the project was completed, boat owners returned, but did their best to navigate around the sea lions; no efforts were made to encourage the new guests to leave . By the end of that year, less than a dozen sea lions frequented the docks at Pier 39 . By January 1990, their numbers had increased to 150 animals . Owners of the 11 boats docked there began to complain about having to avoid the animals who can weigh up to half a ton, and odor and noise complaints began to pour in . Press releases caught national attention, and the sea lions began to attract tourists . Advice from The Marine Mammal Center was to abandon the docks to the animals, and to relocate the boats elsewhere . </P> <P> Although fluctuations in the number of sea lions at Pier 39 are dramatic, as many as 1,701 (Thanksgiving Week, 2009) have been officially reported at one time, many of whom are recognizable to researchers and others, and some of whom have been unofficially named . Volunteers and staff at The Marine Mammal Store and Interpretive Center monitor the sea lion population each day, and educational information is provided to tourists who visit from around the world . Scientists continue to collect information there, adding to knowledge about sea lion health, dietary habits, and behavior . In November 2009 the more than 1,701 (Thanksgiving Week, 2009) sea lions that had lived at the pier began to leave, and by late December 2009 nearly all were gone; a similar flux in population occurs annually, with the animals returning in the spring . Although the reason for their seasonal appearance and departure is not known for certain, according to Jeff Boehm, executive director of the Marine Mammal Center in Sausalito, "Most likely, they left chasing a food source," anchovies and sardines . A handful of sea lions did return in February, and by late May several hundred could once again be seen on Pier 39 . It remains unknown exactly where they went and why . However, in December 2009, nearly 4,000 sea lions that were identified as members of the California sub-species were seen outside Oregon's Sea Lion Caves, suggesting that they were likely the sea lions from Pier 39 . </P>

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