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

Marine invertebrates have long been consideredto be resistant to overfishing. However, agrowing number of exploited taxa have declinedsubstantially and even disappeared from partsof their former range. We consider the case ofthe white abalone (Haliotis sorenseni);the first marine invertebrate proposed for theUS endangered species list. This high-valuespecies was one of five abalones targeted inthe California and Mexico fisheries; it is nowrare and protected from fishing. The biologicalcharacteristics of this deep-living abaloneindicate that it was particularly vulnerable toover-exploitation; reduction of density orgroup size is now known to lead to declines infertilization success and recruitment failure.Warning signs of potential problems existedboth pre- and post-exploitation but were notrecognized. In particular, serial depletion wasnot detected because catch was not analyzedspatially, perhaps because total landings werereasonably stable for the short period ofexploitation. Recent submersible surveys led toestimates that white abalone now number lessthan 2,600 animals or 0.1% of the estimatedpre-exploitation population size. Densities andestimated population sizes are less than 100animals, at all but one location. Alternateexplanations for the decline in abundance wereconsidered and only exploitation-linkedfactors, such as sub-legal mortality andillegal fishing, were likely contributors.Episodic recruitment appears to be acharacteristic of broadcast-spawning,long-lived species and may make themparticularly vulnerable to over-exploitation.Management strategies based on size limits thatallow a few years of spawning prior to reachingminimum legal size are insufficient.Sustainable fisheries will require multipleprotected areas to preserve brood stockaggregations necessary for successfulfertilization.