Continued declines of black abalone along the coast of California: are mass mortalities related to El Niño events?

Abstract

The intertidal black abalone Haliotis cracherodii has experienced mass mortalities along the coast of California, USA, since the mid-1980s. Mortality is due to infection by a pathogen that leads to a fatal wasting disease called 'withering syndrome', where the foot of the abalone atrophies until it can no longer adhere to the substratum. Massive die-offs due to withering syndrome were first noted on the Channel Islands in 1986, and by 1992 withering syndrome was observed near Point Conception on the mainland and was suspected to be spreading northward up the coast of California. The timing of the initial mass mortalities following the strong 1982 to 1983 El Nio and an isolated outbreak of withering syndrome in 1988 at Diablo Cove, north of Point Conception, following warm water discharge from a power plant, led to the hypothesis that the onset of mass mortalities due to withering syndrome may be triggered by elevated seawater temperatures. We surveyed black abalone populations at 7 sites along the mainland coast of California (including 3 where withering syndrome was already present) from 1992 to 2001, a period spanning 2 El Nio events, to determine whether (1) withering syndrome and associated declines of black abalone were spreading northward up the coast; and