Availability, usage and expected contribution of potential nursery habitats for the California halibut

Coastal ecosystems have been identified as important nursery habitats for many of the worldʹs fishery species. Beyond this, there remain many questions about what exactly constitutes high-value, even critical, habitat for juvenile fish. A first step in investigating nursery habitat value should be to catalogue the spatial coverage (availability) of all potential nursery habitats as well as the distribution (usage) of juvenile fish within those habitats. We conducted two years of fall surveys in the nearshore areas of San Diego County, CA, examining the spatial distribution of 0-group California halibut, Paralichthys californicus. The database generated by 527 otter trawls and block-net seine collections was used to produce a series of models employing regression trees to study the abiotic factors (water column and bottom features) that affect juvenile distributions. Along the exposed coast, highest 0-group densities (0.002–0.008 individuals/m2 (indiv/m2)) occurred where temperatures exceeded 21.5 °C (2003), and at depths between 3.3 and 5.2 m (2004). Within protected embayments, densities were higher at depths less than 1.5 m (0.054–0.430 indiv/m2) and, in 2004, inside channeled marsh estuaries (0.156 indiv/m2). The spatial coverage of potential nursery habitats was calculated using a Geographic Information System (GIS) database, and the total number of resident 0-group halibut within each site was estimated (habitat area × juvenile halibut density) as a proxy for expected contribution of halibut advancing to the adult stock from each nursery. Although 85% of the potential nursery habitat area occurred along the exposed coastline, 69% (2003) to 58% (2004) of 0-group halibut resided in protected embayments. Embayment contribution is much greater in the southern half of the study region, largely due to Mission and San Diego bays. We conclude that all nursery habitat types demonstrate the potential to contribute significantly to stock fitness, and that in general, shallow and better-protected habitats are highly utilized nurseries for 0-group halibut and deserve special attention in management or conservation efforts.