[2] On December 22, 2017, the Federal Bureau of Investigation arrested Everitt Aaron Jameson, a 25-year-old former Marine, on suspicion of planning a terrorist attack in the Pier 39 area over Christmas.

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.

[4] Although fluctuations in the number of sea lions at Pier 39 are dramatic, as many as 1,700 have been officially reported at one time (Thanksgiving week in 2009),[6] 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.

[4] In November 2009, the more than 1,700 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.