Over-exploitation of a broadcast spawning marine invertebrate: Decline of the white abalone

Abstract

Marine invertebrates have long been considered to be resistant to overfishing. However, a growing number of exploited taxa have declined substantially and even disappeared from parts of their former range. We consider the case of the white abalone (Haliotis sorenseni); the first marine invertebrate proposed for the US endangered species list. This high-value species was one of five abalones targeted in the California and Mexico fisheries; it is now rare and protected from fishing. The biological characteristics of this deep-living abalone indicate that it was particularly vulnerable to over-exploitation; reduction of density or group size is now known to lead to declines in fertilization success and recruitment failure. Warning signs of potential problems existed both pre- and post-exploitation but were not recognized. In particular, serial depletion was not detected because catch was not analyzed spatially, perhaps because total landings were reasonably stable for the short period of exploitation. Recent submersible surveys led to estimates that white abalone now number less than 2,600 animals or 0.1% of the estimated pre-exploitation population size. Densities and estimated population sizes are less than 100 animals, at all but one location. Alternate explanations for the decline in abundance were considered and only exploitation-linked factors, such as sub-legal mortality and illegal fishing, were likely contributors. Episodic recruitment appears to be a characteristic of broadcast-spawning, long-lived species and may make them particularly vulnerable to over-exploitation. Management strategies based on size limits that allow a few years of spawning prior to reaching minimum legal size are insufficient. Sustainable fisheries will require multiple protected areas to preserve brood stock aggregations necessary for successful fertilization.