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

Marine invertebrates have long been considered to be resistant to overfishi ng. However, a growing number of exploited taxa have declined substantially and even disappeared from parts of their former range. We consider the cas e of the white abalone (Haliotis sorenseni); the first marine invertebrate proposed for the US endangered species list. This high-value species was on e of five abalones targeted in the California and Mexico fisheries; it is n ow rare and protected from fishing. The biological characteristics of this deep-living abalone indicate that it was particularly vulnerable to over-ex ploitation; reduction of density or group size is now known to lead to decl ines in fertilization success and recruitment failure. Warning signs of pot ential problems existed both pre- and post-exploitation but were not recogn ized. In particular, serial depletion was not detected because catch was no t analyzed spatially, perhaps because total landings were reasonably stable for the short period of exploitation. Recent submersible surveys led to es timates that white abalone now number less than 2,600 animals or 0.1% of th e estimated pre-exploitation population size. Densities and estimated popul ation sizes are less than 100 animals, at all but one location. Alternate e xplanations for the decline in abundance were considered and only exploitat ion-linked factors, such as sub-legal mortality and illegal fishing, were l ikely contributors. Episodic recruitment appears to be a characteristic of broadcast-spawning, long-lived species and may make them particularly vulne rable to over-exploitation. Management strategies based on size limits that allow a few years of spawning prior to reaching minimum legal size are ins ufficient. Sustainable fisheries will require multiple protected areas to p reserve brood stock aggregations necessary for successful fertilization.