Spatial variability in growth and mortality of the red sea urchin, Strongylocentrotus franciscanus, in northern California

Natural and fishing mortality rates of the red sea urchin, Strongylocentrotus franciscanus, in northern California were estimated from growth increment and size distribution data under the assumption of a constant recruitment rate. Mean asymptotic test diameter, standard deviation of asymptotic test diameter, growth rate coefficient, and natural mortality rate were first estimated for three nominally unharvested sites, Bodega Marine Reserve, Caspar Closure, and Salt Point. These estimated growth and mortality parameters differed among sites, leading to substantially different yield-per-recruit surfaces. Estimates of fishing mortality rate from size distributions collected at 11 harvested sites were then calculated based on the growth and natural mortality estimates obtained from the Caspar Closure and Bodega Marine Reserve sites. Estimates of fishing mortality rate ranged from 0.11 to 1.87·year-1. The alongshore pattern of fishing mortality rate was moderately correlated with landings and effort, but the spatial pattern of rare, strong recruitment events also appeared to influence values of fishing mortality rate. The positive bias in estimates of fishing mortality rate due to recruitment variability indicated that our observed pattern in estimated values for fishing mortality rate could have been caused by the historical spatial pattern of interannual variability in recruitment.