Hmm interesting. Back in October, I reported a dead sea lion outside my house (I live on the Bay) to the department of Ornithology and Mammalogy at the California Academy of Sciences. They came out and removed the animal's carcass, and told me it had died of kidney failure due to a sexually transmitted disease (Bacterial Vaginosis). They explained that half of the Pier 39 sea lions were victims of the STD.

More information can be found at http://dx.doi.org/10.1016/j.vetmic.2005.11.045

I doubt it, but I wonder if this STD had any impact on their leave...