Early warnings of modern population collapse in black abalone Haliotis cracherodii, Leach, 1814 at the California Channel Islands

Abstract

Abundance and distributions of selected rocky intertidal organisms were monitored in fixed plots at 10 sites within Channel Islands National Park, California from 1985 to 1992. While abundances of barnacles (Balanus, Tetraclita, and Chthamalus), mussels (Mytilus californianus) algae (Pelvetia fastigiata, Hesperophycus harveyanus, and Endocladia muricata), and owl limpets (Lottia gigantea) remained relatively stable, black abalone populations declined precipitously, with less than 10% of the 1985 levels present in 1992. At the southeastern islands, in the warm waters of the California Province, 90% of the abalone died between 1985 and 1988, and the proportion of large individuals among survivors increased as the population declined to less than 1% of its 1985 level. In contrast, populations at the northwestern islands, in the cold water of the Oregonian Province, declined gradually, until after the southeastern islands' populations crashed and were closed to commercial harvest in 1991. The proportion of large individuals declined as abundance dropped at the northwestern islands, implicating harvest as a contributing factor in the decline there. In both Province, recruitment of juvenile abalone virtually ceased when adult populations dropped below 50% of their initial abundance. Withered and weak abalone were frequently observed, suggesting an infectious agent. No single cause for the mass mortalities has been found to date.