Beginning in 1989, we conducted a series of studies of red sea urchins Strongylocentrotus franciscanus aimed at gaining an understanding of population dynamics of this widely distributed species. Red sea urchins are of interest for several reasons. They are a relatively long-lived species with sedentary adults and a long planktonic larval stage (a minimum of one month to perhaps three months) that span a considerable latitudinal range (about 32° degees of latitude). In addition, they are the basis of valuable nearshore dive fisheries along much of the west coast of north America.

We studied the dynamics of 18 sub-populations ranging from southern California to Alaska, spanning about 70% of the geographic range, using internal tags chemical tags (the antibiotic tetracycline hydrochloride) that allowed us to measure yearly growth rates of individuals at each site. We combined these estimates with samples of density and size distributions and were able to estimate the growth and mortality rates of red urchins over a significant portion of their geographical range. This work was done in collaboration with resource biologists in Alaska, Washington, Oregon, and California, and many of its results are summarized in a recent paper in MEPS, co-authored with our colleagues.

This work not only provided valuable information for management of the red sea urchin populations, it also allowed us to examine changes in life-history parameters across a large geographical gradient. Significant findings included: 1) Although growth differed significantly among sites, contrary to our initial expectations of slower growth in the north and faster growth in the south, we found no evidence of there was no latitudinal cline in growth rates. 2) We found that growth rates were related to the size of the "jaws" (a term we use for components of the urchin’s mouth structure known as the Aristotle’s lantern) of red sea urchins relative to their test diameters, a measure of total body size. Slower growing urchins have larger jaws (relative to their test diameters than do faster growing ones. Other studies have shown that growth rates and relative jaw sizes are strongly related to food availability.
In contrast to our findings on growth rates, we did find a significant latitudinal cline in survival rates. Mean annual survival probability was 0.93 yr-1 from northern California to Alaska and 0.77 yr-1 in southern California. Likely causes for changes in survival rate with latitude are disease and temperature related stress.

These estimates of growth and mortality yielded some useful and intriguing results. Growth functions derived from our mark and recapture studies estimate the age of 1" urchins at about 1 year, 3 _ to 4 _ " (the size ranges harvested by divers) at between urchins at 7 to 10 years, respectively. This adjusted upward by several years previous estimates of the time required to recruit to the fishery. A more surprising result was the estimated age of recaptured animals that had test diameters between 5" to 6" at the time of tagging. Urchins that reach this size continue to grow, but very slowly, and also have extremely high rates of survival. Based on these parameter estimates, we have concluded that red sea urchins larger than about 6" in test diameter are about 100 years old, and animals with test diameters of about 6 _ " (the largest we tagged) have an average estimated age of more than 120 years (see Fig. 1). The low growth and high survival rates on which we base these extreme age estimates are based on a more that 1800 total recaptures, and about 350 recaptures of large, old individuals.

We have retained the "jaws" of most individuals and are currently pursuing research to cross-check these age estimates.

Note: Steve Schroeter and John Dixon are Associate Research Biologists in the Marine Science Institute as well as Adjunct Professors in the Biology Department at SDSU , Tom Ebert is an emeritus Professor of Biology at SDSU. This research is part of an ongoing collaboration that began in 1988.