15 - USA

Fisheries collapses highlight need for management of spawning aggregations in Southern California 

The barred sand bass Paralabrax nebulifer and the kelp bass P. clathratusthe most numerically abundant seabasses in coastal waters of southern California. Both species form spawning aggregations during the summer months, although the spatial and temporal dynamics of spawning differ between the two species. Sand bass form small groups over inshore habitats for most of the year but migrate to form massive spawning aggregations over offshore sand flats during July and August at six main areas.Conversely, kelp bass aggregate on kelp beds and rocky reefs year-round and form localized spawning aggregations of several hundred fish from May to September.

Both species have been excluded from commercial harvest in California since 1953,but collectively have ranked as the two most important recreational marine fisheries in southern California since 1980. Between 1963 and 2008 approximately 80% of annual harvest of sand bass and kelp bass occurred during their spawning months, and their spawning aggregations represented the staple of the Commercial Passenger Fishing Vessel (“party boat”) fleets during that period.

Analyses of long-term population trends from fisheries-dependent and fisheries independent data indicate that regional stocks of both species have collapsed due to overfishing of spawning aggregations and changes in oceanographic and climatic conditions within Southern California. In barred sand bass, biomass declined by 90% since 1980 whereas catch and catch rates have decreased by 97% and 70% respectively since 2000. In kelp bass, biomass has decreased by 90% since 1980, adult densities have declined 80%, and catch and catch rates have dropped by more than 90% each since 1965.

Even more alarming than the declines is the fact that monitoring of fisheries trends provided little warning of imminent collapse for either species. Comparisons of fisheries and population data indicate that both fisheries exhibit hyperstability, in which catch rates remain high even as population sizes declined precipitously. As a result, harvest rates of both species appeared to be sustainable and no changes in management were implemented. The non-linear relationship between catch rates and true population size is a result of the aggregating behaviour of fish and the nonrandom targeting of aggregations by fishers (Sadovy and Domeier, 2005; Erisman et al., 2011).

The hyperstable relationship between catch rate and stock abundance suggests the urgent need to incorporate fisheries -independent monitoring within the management programmes of both species. However, while fisheries monitoring is a key part of management, such data alone are insufficient for an accurate assessment of stock condition. In fact, for barred sand bass, catch rates show no relationship at all to changes in population size

Both fisheries have been managed by a 10-fish daily bag limit (collective daily limit for any of the three Paralabrax species that occur in the region) and a 27 cm minimum size limit since 1959. Given the large-scale declines in both fisheries, it is time to review the management plan and devise new strategies to bolster recovery and maintain more sustainable harvest levels. Such an effort would also benefit areas well beyond southern California, as both barred sand bass and kelp bass have emerged as two of the most productive artisanal fisheries along the coast of Baja California (Mexico), yet no regulations exist there to manage either fishery.

Brad Erisman
Scripps Institution of Oceanography & Society for the Conservation of Reef Fish Aggregations
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Reference

Erisman, B.E., Allen, L. G., Claisse, J. T., Pondella II, D. J., Miller E.F. and Murray, J. H. (2011) The illusion of plenty: hyperstability masks collapses in two recreational fisheries that target fish spawning aggregations. Canadian Journal of Aquatic Sciences 68:1705-1716.

Sadovy, Y., and Domeier, M. (2005) Are aggregation fisheries sustainable: reef fish fisheries as a case study. Coral Reefs 24:254 – 262.