Climate variability, kelp forests, and the southern California red abalone fishery

Declines in landings of Southern California abalone fisheries and the eventual collapse of many stocks over the last two decades coincided with a period of greatly increased environmental variability. This included massive storms, an increase in the frequency of warm-water El Nino events after 1976, and an interdecadal-scale increase in sea surface temperatures. Kelp populations may be decimated by severe storms or warm water. Because of the strong inverse relationship between nitrate availability and water temperature, temperature is a good indicator of nitrate availability or stress SINCE kelp growth ceases in warm nutrient-depleted water, tissue decays, and standing stocks may be greatly reduced. Abalones are affected by the availability of the drift kelp on which they feed. Anomalously warm temperatures may affect reproduction, and altered current patterns may affect larval dispersal. Because water temperature varies with location in Southern California and each of the five exploited abalone species has its own thermal preferences, we chose to evaluate the role of environmental variability on populations of red abalone (Haliotis rufescens) on three northern Channel Islands spanning a temperature gradient. We compared water temperature regimes and anomalies, monthly aerial surveys of canopies of giant kelp (Macrocystis pyrifera), and field evidence of poor abalone growth and reproduction during El Nino events. The severity of El Nino disturbances and long-term changes in kelp standing stocks both correlated with the temperature gradient. Declines of red abalone total landings and area-specific landings on the warmer Santa Cruz and Santa Rosa Island began a decade after the large 1957-1959 El Nino. The subsequent collapse of many populations appears related to warm anomalies after the 1976-1977 regime shift, kelp declines, and poor reproduction coupled with fishing-induced declines in adult abalone density. Red abalone populations have persisted on cooler San Miguel Island where thermal anomalies had less effect and kelp canopy biomass has been more stable. Southern California abalones evolved in this disturbance regime, but the combination of extended periods of increased environmental variability with intense fishing pressure may have led to the loss of local populations, especially in warmer areas.