Over 400 California sea lions (Zalophus californianus) died and many others displayed signs of neurological dysfunction along the central California coast during May and June 1998. A bloom of Pseudo-nitzschia australis (diatom) was observed in the Monterey Bay region during the same period. This bloom was associated with production of domoic acid (DA), a neurotoxin that was also detected in planktivorous fish, including the northern anchovy (Engraulis mordax), and in sea lion body fluids. These and other concurrent observations demonstrate the trophic transfer of DA resulting in marine mammal mortality. In contrast to fish, blue mussels (Mytilus edulus) collected during the DA outbreak contained no DA or only trace amounts. Such findings reveal that monitoring of mussel toxicity alone does not necessarily provide adequate warning of DA entering the food web at levels sufficient to harm marine wildlife and perhaps humans.

Blue whales (Balaenoptera musculus) are distributed worldwide, and although severely depleted by commercial whaling, their abundance off the California coast now appears to be increasing. Little is known about natural causes of mortality of blue whales, but human-related mortality continues despite legal protection. Ship strikes are a significant mortality factor for other species of baleen whale, and changes in shipping traffic have been advocated to minimize further deaths. Between 1988 and 2007, 21 blue whale deaths were reported along the California coast, typically one or two cases annually. Three pulses in strandings were observed, with three carcasses observed in fall 1988, three in 2002, and four in fall 2007. Two of the four animals in 2007 were first observed dead in the Santa Barbara Channel and had wounds typical of a ship strike. Blue whale strandings were spatially associated with locations of shipping lanes, especially those associated with the Ports of Los Angeles and Long Beach, and were most common in the fall months.

In 2007, the apparent increase in the number of harbor porpoises (Phocoena phocoena) stranding along the central California coast compared to the number of strandings the previous year resulted in the declaration of an Unusual Mortality Event by the National Marine Fisheries Service. A statistically significant increase in strandings occurred in 2008 and 2009, with more than twice the mean annual number of strandings documented per year in the previous decade occurring each year, but then strandings decreased in 2010. No single cause of mortality explained all the strandings, and there were no significant changes in age class or sex of strandings in 2008 and 2009. Trauma, including interspecific aggression and fisheries interactions, was the most common cause of death, and blunt force trauma increased significantly in August through October of 2008 and 2009. Domoic acid toxicosis was documented for the first time in this species. Although the cause of death for many strandings was unidentified, the increase in strandings in 2008-2009 reflects an increase in blunt trauma rather than an epizootic of disease.