An evaluation of the nursery role of estuaries for flatfish populations in central California

The purpose of this research was to determine if estuaries in central California are higher quality juvenile habitats than coastal sandy habitats and, thus, function as nurseries by contributing more individuals to the adult populations than an equivalent area of coastal habitat. I evaluated the nursery role of estuaries for two species of flatfish—the English sole (Pleuronectes vetulus) and the speckled sanddab ( Citharichthys stigmaeus).; I assessed relative habitat quality by comparing growth rates of juveniles in estuarine and coastal habitats using two methods: (1) a caging experiment in which juveniles of each species were held for 28 days in August 2000 Elkhorn Slough and Monterey Bay; and (2) a comparison of the width of daily increments in otoliths from juvenile speckled sanddab collected from estuaries and coastal areas in four regions in 1999 and 2000. Results from both the caging experiment and the comparison of daily increments indicated that juvenile flatfish grow faster in estuaries. Thus, based on comparisons of growth rates, estuaries were determined to be the higher quality juvenile habitat.; I directly assessed the nursery role of estuaries by determining the proportion of the adult population that recruited from estuaries. The chemical composition of otoliths (Sr/Ca and Li/Ca) was used to differentiate between fish that had resided as juveniles in either estuaries or sandy coastal habitats. Classification models based on juveniles collected in both habitat types were used to assign juvenile and adult fish to either the estuarine or coastal habitat group. Juveniles were assigned to the habitat type where they were captured with approximately 80% accuracy. The proportion of adult fish that were assigned to the estuarine habitat group was estimated to range between 45% and 57%. This is a much higher level of contribution than would be expected based on the relative area of estuarine and sandy coastal habitats in central California. These results indicate that estuarine habitats are an important source of new individuals for adult flatfish populations and conservation of estuaries may help maintain high levels of recruitment to harvested populations in central California.