Rogers-Bennett et al,: Modeling growth of Strongylocentrotus fianciscanus 



615 



for management, there is a need to evaluate a number of 

 growth models with a single data set that encompasses a 

 large range of urchin sizes. 



In our study we report the results from six individual 

 growth models applied to data from a one-year tag and 

 recapture study of red sea urchins {Strongylocentrotus 

 franciscanus) in northern California. We supplemented 

 the number of juveniles in the field by stocking tagged ju- 

 veniles. Estimates of the number of years required for ur- 

 chins to gi-ow to minimum legal size in northern California 

 are generated by the models. We examine the robustness of 

 these results to changes in the parameters and the impact 

 of a limited data set from a small range of urchin sizes on 

 our results. We determine if there are spatial differences 

 in growth between shallow and deep sites. Finally, we rank 

 the models according to quality of fit, present a generic 

 growth curve that combines the six models, and discuss the 

 implications of our results for fishery management. 



Materials and methods 



Study sites 



Growth rates were determined for red sea urchins in 

 the Salt Point (38°33'06"N, 123n9'45"W) and Caspar 

 (39°21'49"N, 123°49'47"W) urchin harvest reserves in 

 northern California. Commercial urchin harvesting is pro- 

 hibited in these reserves. We examined spatial variation 

 within Salt Point by tagging red sea urchins at one shal- 

 low site (5 m) south of the southern border of the Gerstle 

 Cove Reserve and at one deep site (17 m) on the leward 

 side of a large wash-rock. In addition, laboratory-reared 

 juvenile red sea urchins were stocked at the two sites in 

 Salt Point. Both of these sites are relatively isolated, sur- 

 rounded by sand and seasonally dense kelp (Nereocystis). 

 At the Caspar Reserve, sea urchins were tagged outside a 

 small cove with seasonally dense kelp (Nereocystis) at a 

 single depth (7 m). 



Tagging 



Sea urchins at the study sites were tagged internally and 

 recaptured after one year. At Salt Point, wild sea urchins 

 were tagged with tetracycline injections in situ by using 

 0.5-1.2 mL of 1 g tetracycline/100 mL of seawater (cf 

 Ebert, 1982; Ebert and Russell, 1992). Six hundred and 

 nine red urchins were measured with vernier calipers 

 (±1-2 mm) and tagged at Salt Point on 19 August 1992. 

 Urchins were recaptured from the Salt Point sites on 18 

 September 1993 (;!=374 shallow; n=S52 deep). This data 

 set was normalized to one year by using the factor 12/13. 

 Our study was not a longitudinal study examining growth 

 over many years, but rather for one year only. 



Juvenile urchins reared in the laboratory for one year 

 (mean test diameter=17.6 mm) were tagged and stocked 

 into the shallow and deep Salt Point sites. Juveniles were 

 tagged by immersion for 24 hours in a calcein solution 125 

 mg/L seawater, pH adjusted to 8.0 (Wilson et al., 1987). 

 After tagging, juveniles were transported to the Salt Point 



sites and released. Juveniles were stocked (120 at each 

 of the two depths) on 31 August 1992 and harvested on 

 18 September 1993 with the adults (see Rogers-Bennett, 

 2001). 



Urchins at the Caspar Reserve were tagged internally 

 with personal individual transponder (PIT) tags on 28 

 August 1996 and recovered 20 August 1997 (Kalvass'). 

 PIT tags are glass coated mini-transponders with unique 

 individual codes that can be read noninvasively by using 

 a Destron(S) tag reader. Tags were implanted into the body 

 cavity of the sea urchins through the peristomial mem- 

 brane. PIT tags are too large for tagging small urchins 

 (<40 mm). 



Estimates of urchin density were made within a circle 

 (12 m in radius) at each of the two Salt Point sites at the 

 time of harvest. Drift algae collections were made along a 

 2 X 10 m transect (20 m^) at each site. Gut contents were 

 collected from a subsample of 20 urchins from each site. 

 Gut contents were fixed in alcohol, sorted on a petri dish, 

 and the most abundant items were recorded from 5 out 

 of 25 lO-mm'^ grids (Harrold and Reed, 1985). We used a 

 conservative definition of optimal foods, defining them as 

 fleshy red or brown algae (Harrold and Reed, 1985). Sub- 

 optimal foods included green algae, upright and encrust- 

 ing coralline algae, detritus (animal, plant, and inorganic), 

 plants (Phyllospadix), mud, and sand. 



Growth measurements 



Sea urchins can not be reliably aged by using rings on 

 test ossicles (Pearse and Pearse, 1975; Ebert 1988; Gage, 

 1992), therefore growth increments after one year must be 

 measured directly. For the urchins tagged with fluorescent 

 dyes (tetracycline and calcein), growth was measured as 

 the change in urchin jaw length (AJ =J,^j-J,) after one 

 year (Ebert and Russell, 1993). Urchin jaws were dissected 

 from Aristotle's lantern, excess tissue was removed with 

 10% sodium hypochlorite, and the jaws were measured to 

 the nearest 0. 1 mm. Growth was measured by determining 

 the width of the calcium deposit one year after tagging. 

 Tags on jaws are more accurate than tags on test ossicles 

 because ossicles move toward the oral surface during 

 growth (Duetler, 1926), requiring matching ossicles at the 

 time of tagging with ambitus ossicles at the time of collec- 

 tion (Ebert, 1988). 



Fluorescence tagged urchins were identified when ex- 

 posed to an ultraviolet epi-illuminator (Lite-Mite) on a 

 dissecting scope. Growth increments were determined by 

 using the Confocal Microscope (BioRad MRC-600, BioRad 

 Industries, Hercules, CA) with a BHS fluorescence filter 

 (blue wavelength) and the COMOS software package 

 (BioRad Industries, Hercules, CA). Growth was measured 

 from the fluorescent band (indicating size at tagging) to 

 the esophageal edge of the jaw (final size). Growth was 

 also recorded from a second growth zone at the labial tip of 

 the jaw, represented by a glowing arc when present. Initial 

 jaw size (J,) equals jaw size after one year (J,^i) minus the 



' Kalvass, P. 1997. Personal commun. Calif. Dep. Fish and 

 Game, 19160 S. Harbor Dr., Fort Bragg, CA. 95437. 



