UC San Diego

To determine the linkages between nursery habitat utilization and the population dynamics of a coastal finfish of southern California, the California halibut, Paralichthys californicus, the following questions were asked: (1) What are the habitat associations (distribution patterns) of juvenile halibut among nearshore ecosystems (2) What are the nursery habitat origins for the individual fish that successfully recruit to the adult population, and can the contributions from individual nursery types be predicted from distribution data? (3) Over what spatial scale are nursery contributions realized? (4) What role do nurseries play in regulating population size? and (5) What are the demographic consequences (population growth) related to utilization of nursery habitat alternatives? Potential nurseries in San Diego County, California, were grouped using a novel classification scheme that delineated exposed, bay, lagoon and estuarine environments. Retroactive assignment of nursery origin for individual fish via elemental fingerprinting indicated that exposed coasted, bays, lagoons and estuaries contributed 42%, 45%, 11% and 2% of advancing juvenile California halibut during 2003 and 2004, respectively. These results were remarkably similar to the expected contribution from nursery habitats based on field surveys, indicating that in this system juvenile distributions are a good indicator of unit-area productivity of nurseries. Over smaller scales, unique chemical fingerprints were observed along the main-axis of coastal embayments (Punta Banda Estuary, Baja California, Mexico and Mission Bay, California) and were used to document the contribution of individual embayment zones. Results from the Punta Banda Estuary indicated that the majority of embayment contribution is derived from the lower two-thirds of the embayment system, and that individual zones within the embayment are utilized differently throughout the year by juvenile halibut. Elemental fingerprinting also revealed that individuals egressing from bays did not migrate far from their nursery origin (<10 km), and this resulted in reduced connectivity between the northern and southern halves of the San Diego County coastline over the timescale of̃ ̃ 1 generation. Because of this, coastal sites far from large embayments appeared to have smaller sub-adult population sizes due to nursery habitat limitation. Juvenile cohort analyses and nursery contribution results indicate that density- dependent juvenile mortality does not limit nursery contribution for this species. However, mortality rates were considerably higher in exposed habitats, and because of this at least 38% of recruits to the adult stage must originate from embayment nurseries to produce stable or positive population growth based on population projection matrix model simulations that included four years (1987- 1988, 2002-2003) of nursery habitat-specific vital rate data. Together, these results generate a number of valuable metrics for evaluating nursery habitat value, including: (1) unit-area contribution of recruits to replenish adult populations, (2) raw contribution of recruits to replenish adult populations, and (3) impacts on population growth rate. These data should be of considerable value for management and conservation of coastal ecosystems by expanding our understanding of what defines the nursery-role of coastal habitats