Crabtree et al.: Age and growth of Albula vulpes 



447 



E 50 



• 



JMMJ SNJMMJS NJMMJ SN 



1992 



1993 

 Month 



1994 



Figure 3 



Mean monthly water temperatures (°C) calculated from 

 daily temperature measurements at the water intake pump 

 in Florida Bay adjacent to the Keys Marine Laboratory, 

 and monthly median percent marginal increment and 

 interquartile range for bonefish, Albula vulpes, otoliths. 



P=0.0 12 ). Most bonefish ( 86% ) in our sample that were 

 caught with hook-and-line gear were from 3 to 10 years 

 old (Fig. 6). On the basis of visual inspection of the age- 

 frequency distributions, bonefish were not fully re- 

 cruited to the fishery until an age of about 5 years. 



The instantaneous rate of total mortality (Z) esti- 

 mated by our catch-curve analysis was 0.25 (95% 

 confidence interval 0.098-0.406, r 2 =0.805, n=7, 

 P<0.001) for males and 0.21 (95% confidence inter- 

 val 0. 167-0.303, r 2 =0.922, n =9, P<0.001 ) for females. 

 The instantaneous rate of natural mortality (M) es- 

 timated with Pauly's formula was 0.29 for males and 

 0.31 for females. The 20% reduction of this estimate 

 recommended by Pauly ( 1983) for schooling species 

 suggests a range of 0.2-0.3 for natural mortality. 



Otolith weight was significantly related to age ( Fig. 

 7). The slopes U-test, df=417, f=1.34, P=0.09) and 

 intercepts «-test, df=418, ^=0.49, P=0.31) of the 

 otolith weight-age equations for males and females 

 were not significantly different (Table 2). 



Two of the bonefish that we aged were exception- 

 ally small for their estimated ages and the weights 

 of their otoliths were exceptionally light. One was a 

 351-mm female that was 7 years old and the other 



was a 458-mm female that was 18 years old. These 

 two fish were statistically significant outliers, and 

 we excluded them from growth models, age-frequency 

 distributions, and otolith weight-age regressions. 

 Both fish were caught with hook-and-line gear on 

 the ocean (Florida Straits) side of North Key Largo, 

 and they were the smallest females examined with 

 active vitellogenic ovaries. 



Discussion 



We obtained bonefish from a variety of fishery-de- 

 pendent and fishery-independent sources, but all fish 

 larger than 405 mm that we examined were caught 

 with hook-and-line gear. The size-frequency distri- 

 bution of the hook-and-line-caught fish in our sample 

 is probably similar to that of the fish caught in the 

 fishery. Most of the hook-and-line-caught fish we 

 examined were killed without regard to size, but a 

 few fish included in our analysis were received from 

 private anglers, tournaments, and taxidermists, 

 sources that were presumably biased towards larger 

 fish. Bonefish were not fully recruited to the hook- 

 and-line gear until they reached a length of 550-600 

 mm and an age of about 5 years. We assume that our 

 sample was representative of the Keys bonefish popu- 

 lation beyond this size and age, but it is possible that 

 hook-and-line gear introduced unknown biases that 

 affected our mortality rates and growth-parameter 

 estimates. We have no basis to evaluate the extent 

 of these potential biases. 



Age-validation experiments with OTC-marked 

 otoliths supported our hypothesis that bonefish 

 otoliths formed annual marks, and marginal-incre- 

 ment analysis suggested that annuli formed during 

 March-June. Although sample sizes were insufficient 

 to reveal trends for some individual age classes, age 

 classes 4-9 showed marginal-increment patterns 

 consistent with our hypothesis that annuli formed 

 during March-June. Annulus formation appears to 

 take place shortly after the season of minimal water 

 temperatures in the Keys (Fig. 3). Monthly mean 

 temperatures in Florida Bay at the Keys Marine 

 Laboratory's water intake pump varied seasonally 

 from a low of about 21°C during mid-December-Feb- 

 ruary to a high of about 31°C during June-Septem- 

 ber (Fig. 3). These temperatures were probably simi- 

 lar to those of typical Keys bonefish habitat, and in- 

 deed on several occasions we collected bonefish on 

 the flats adjacent to the pump. It is possible that 

 holding fish under captive conditions and an artifi- 

 cial feeding regime affected the periodicity of annu- 

 lus formation. However, we saw no change in the 

 appearance or widths of the alternating translucent 



