The most likely culprit has since been identified as Otarine herpesvirus 1 [1] with DDT- and PCB-related pollutants playing a large contributing role (which the California coast has had an ongoing problem with since WWII dumping and the Palos Verdes shelf).

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC7918579/

Bioaccumulation in action

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I wonder if it could be related to dumping vast quantities of toxic chemicals into the ocean.