EARLY WARNINGS OF MODERN POPULATION COLLAPSE IN BLACK ABALONE HALIOTIS-CRACHERODII, LEACH, 1814 AT THE CALIFORNIA CHANNEL-ISLANDS

Abundance and distributions of selected rocky intertidal organisms wer e monitored in fixed plots at 10 sites within Channel Islands National Park, California from 1985 to 1992. While abundances of barnacles (Ba lanus, Tetraclita, and Chthamalus), mussels (Mytilus californianus) al gae (Pelvetia fastigiata, Hesperophycus harveyanus, and Endocladia mur icata), and owl limpets (Lottia gigantea) remained relatively stable, black abalone populations declined precipitously, with less than 10% o f the 1985 levels present in 1992. At the southeastern islands, in the warm waters of the Californian Province, 90% of the abalone died betw een 1985 and 1988, and the proportion of large individuals among survi vors increased as the population declined to less than 1% of its 1985 level. In contrast, populations at the northwestern islands, in the co ld water of the Oregonian Province, declined gradually, until after th e southeastern islands' populations crashed and were closed to commerc ial harvest in 1991. The proportion of large individuals declined as a bundance dropped at the northwestern islands, implicating harvest as a contributing factor in the decline there. In both Provinces, recruitm ent of juvenile abalone virtually ceased when adult populations droppe d 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.