Recent declines of black abalone Haliotis cracherodii on the mainland coast of central California

The black. abalone Haliotis cracherodii was once abundant in many intertidal habitats in southern California, USA. Beginning in 1985, however, black abalone in southern California suffered mass mortalities attributed to a condition termed withering syndrome. The mass mortalities and withering syndrome were confined to southern California except for Diablo Cove, 70 km north of Point Conception, where mortalities were limited to the immediate vicinity of a warm-water discharge. Beginning in 1992, we monitored 5 locations along the mainland coast of central California to determine whether mass mortalities of black. abalone would occur there. Abalone numbers decreased at the 3 southern sites but not at the 2 northern sites. The decline was greatest at the southernmost site, Government Point (near Point Conception), where there was a 97% reduction in numbers between 1992 and 1995. The 2 sites immediately upcoast of Government Point, Boathouse and Point Arguello, experienced a 39 to 46% decline, while the 2 northernmost sites (Stairs and Purisima) experienced a slight increase in abalone. All abalone size classes declined at Government Point, Boathouse and Point Arguello, indicating the decline was not caused by overharvesting. Symptoms of withering syndrome, assessed by pulling on individuals, were noted at Boathouse and Point Arguello. Withering syndrome was also detected in the final survey at Stairs, as was a slight decline in abalone abundance, suggesting that mass mortalities may be beginning there. Our observations demonstrate that widespread mass mortality associated with withering syndrome has reached central California and is progressing northward from Point Conception.