California Sea Grant College Program

I examined habitat specific differences in the morphology, reproduction, and juvenile abundance of red sea urchins, Strongylocentrotus franciscanus. In addition, we conducted a tag-recapture study and examined early growth, to develop an alternative growth model for urchins in northern California. Urchins in shallow habitats encounter high wave energy and drift food such that principle component analysis identified shallow urchins as having large gonads, short spines, and thick tests. Shallow adult urchins also resided in "scars" where they provide spine canopy protection for juvenile urchins. These patterns suggest that not only are shallow urchins important spawners, but they may also act as living nurseries. Since their morphology is unique, shallow urchins could be set aside in harvest refugia offering an alternative urchin fishery management strategy.

The sea urchin fishery is California's largest fishery. Fishery managers are interested in growth models which can predict the time required for urchins to reach the minimum legal size (89 mm). We conducted a tag-rapture study with wild urchins (N=600) and laboratory cultured juveniles (N=240) in shallow (5m) and deep (15m) subtidal sites. Recapture rates were twice as high in the shallow site (38.5%) compared with the deep site (15%) for wild urchins as well as for outplanted juveniles (20.8% and 10.8%, respectively). We present an alternative growth function with three desirable features 1) infinite growth (lacks asymptotic size), 2) two well defined parameters, and 3) asymmetry about the function's maximum. We obtained parameter values for size at 1 year, a= 4.499 (± SE 0.207), and growth rate constant K = 0.375 (± SE 0.003). We estimate red urchins attain harvestable size (89 mm) in 9 years and caution model parameters must be calculated independently in 'northern and southern California.

Cultured red urchins experienced a lag in growth and jaw emergence immediately post-settlement. Newly settled juveniles (90%) lacked jaws and did not increase in test diameter from size at settlement (0.404 mm ± SD 0.038mm, N=10) for 50 days. This period was followed by an exponential growth phase after jaw emergence and the onset of exogenous feeding. Newly settled juveniles also appear to be highly susceptible to mortality in the pre-jaw stage. Therefore, jaw development may encompass a critical stage in urchin recruitment.