Crabtree et al. Age and growth of Albula vulpes 



443 



growth of bonefish from the waters off the Florida 

 Keys and estimate mortality rates from a sample of 

 fish whose ages were determined from an examina- 

 tion of sectioned otoliths. 



Methods 



We examined 528 bonefish collected from South 

 Florida waters from February 1989 to April 1995. 

 Most of these bonefish were caught with hook-and- 

 line gear either by biologists or by a single profes- 

 sional bonefish guide and his anglers from waters 

 off the Florida Keys and in Florida and Biscayne 

 Bays. Five bonefish caught with hook-and-line gear 

 were obtained from taxidermists in Fort Lauderdale 

 and five others from tournaments in waters off the 

 Keys. Supplemental collections of small bonefish 

 (<425 mm) were made with various-size seines and 

 gill nets in waters off the Keys. We collected young- 

 of-the-year (YOY) bonefish using 21.4-m seines con- 

 structed with 6.35-mm mesh. These collections were 

 made from shallow (<1 m) sand- and grass-bottom 

 areas on the ocean (Florida Straits) side of Key West, 

 Bahia Honda Key, and Lower Matecumbe Key and 

 from the Indian River Lagoon on Florida's Atlantic 

 coast. 



Standard length (SL), fork length (FL), and total 

 length (TL) were measured to the nearest millime- 

 ter (mm) and fish were weighed to the nearest gram. 

 All lengths reported are fork lengths. Otoliths 

 (sagittae) were removed, cleaned with bleach (5.25% 

 sodium hypochlorite), and rinsed first in water and 

 then in 95% ethanol. Otoliths were stored dry until 

 sectioned. Sex of all fish, except that of YOY (age 0), 

 was recorded and confirmed histologically. 



The left sagitta was usually used for age estima- 

 tion; however, in cases where the left otolith was bro- 

 ken, lost, or damaged during processing, the right 

 otolith was substituted. We prepared most otoliths 

 by cutting three or four thin sections approximately 

 0.5 mm thick, one of which was through the otolith 

 core, using a Buehler Isomet low-speed saw with a 

 diamond blade. Sections were then mounted on a 

 microscope slide with Histomount. Initially, we pre- 

 pared some otoliths for age estimation by embedding 

 them in Spurr, a high-density plastic medium (Secor 

 et al., 1992). A 1- to 2-mm thick transverse section 

 containing the otolith core was cut. The section was 

 mounted on a microscope slide with thermoplastic 

 glue (CrystalBond 509 adhesive) and polished with 

 wet/dry sandpaper (grit sizes ranging from 220 to 

 2,000 grits per sheet) until annuli were visible. Sec- 

 tions were then polished on a Buehler polishing cloth 

 with 0.05-u gamma alumina powder to remove 



scratches. There was no consistent difference in the 

 quality of either preparation technique. Mounting the 

 sections in Histomount took less time than embed- 

 ding them in Spurr; therefore, this technique was 

 adopted as our standard protocol. 



Annuli were counted three times by each of two 

 independent readers who used a dissection micro- 

 scope equipped with reflected light at magnifications 

 of 8-25x. After the readers completed counting an- 

 nuli on all the otoliths, we re-examined the otoliths 

 for which different counts had been given. Most dis- 

 agreements in counts concerned the presence or ab- 

 sence of an annulus on the otolith's margin. We were 

 usually able to reconcile these disagreements and to 

 assign an age to the otolith. A few otoliths had ir- 

 regular and poorly defined annuli (n=20,4.2%); these 

 were discarded from the analysis. 



Measurements for marginal-increment analysis 

 were made with a digital image-processing system 

 along an axis extending from the otolith's core to the 

 ventral proximal margin of the section (Fig. 1). We 

 expressed the distance from the final annulus to the 

 otolith's edge (marginal increment) as a percentage 

 of the distance between the last two annuli formed 

 on the otolith. For all bonefish, the distance between 

 the otolith core and the first annulus (rj) was typi- 

 cally much greater than the distance between the 

 first and second annuli (r 9 -rj). For this reason, we 

 divided the distance between the first and second 

 annuli by the distance between the otolith's core and 

 the first annulus for each otolith measured and then 

 calculated the mean of this number for the entire 

 sample, 



X(^2-n> /'l), 



= 0.406 



(SE = 0.0081). 



We then estimated the expected distance between 

 the first and second annulus for each age-1 bonefish 

 otolith as a function of the distance between the 

 otolith's core and the first annulus. The percent mar- 

 ginal increment for age-1 fish was then calculated 

 as (M/A0.406 x r ; ))100, where MI = the marginal 

 increment. We then plotted the median percent mar- 

 ginal increment as a function of capture month for 

 the 36-month period during which we made regular 

 monthly bonefish collections. We also plotted mar- 

 ginal increments for individual age classes 4-9 for 

 all study years pooled. Our monthly sample sizes for 

 other individual age classes were insufficient for 

 marginal-increment analysis. 



Bonefish were captured with hook-and-line gear 

 from the waters off the Florida Keys for age-valida- 



