Variability in Growth, Mortality, Recruitment, and Larval Dispersal Paths in California Populations of the Sand crab, Emerita analoga
A complete understanding of marine population dynamics requires information on spatial and temporal variability of growth, mortality, and recruitment rates as well as knowledge of larval dispersal paths. I sampled seventeen populations of the sand crab, Emerita analoga (Stimpson), ranging over 800 km of California for two years. Size distributions and recruit density were collected bimonthly to estimate vital rates and to investigate temporal effects of oceanographic conditions on larval dispersal paths. Genetic spatial subdivision was determined with female juveniles from seven sites.
I developed a model to describe how growth and mortality rates of discretely growing organisms, such as crabs, affect the shape of size distributions. Growth and mortality were estimated from seven Emerita populations. Growth rates revealed more variation than had been previously been estimated, ranging from 0.001 to 0.12 mm/day for crabs between 4 and 31.6 mm. Although both spatial and temporal variability was present, a few sites exhibited consistent mortality and growth for all cohorts in both years.
In marine species, adult populations are spatially separated on the benthos, but interconnected by pelagic larvae. Although larval dispersal paths are virtually unknown, they affect recruitment and population dynamics. Previous studies of the influence of flow on larval dispersal in California suggest that larvae being exported offshore and southward during upwelling are retained in eddies associated with headlands and redistributed to sites north of the retention zones during relaxation. In 1998, recruitment magnitude declined with distance north of four major headlands in California: Point Conception, Monterey Peninsula, Point Reyes, and Point Arena. This pattern was not present in 1999. The annual differences in spatial patterns of recruitment corresponded with interannual variability in oceanographic conditions. Both the Bakun upwelling index and windstress levels were constant across latitude in 1998 and 1999, but values were higher in 1999. This suggests that the upwelling-relaxation mechanism operated effectively in 1998 when oceanographic conditions were at "normal" levels but did not operate in 1999 when conditions were anomalously high. Preliminary genetic data using microsatellite DNA markers suggest that gene flow, and therefore dispersal, is restricted across headlands, consistent with the retention mechanism.