JONES and BROTHERS: AGING TECHNIQUE FOR STRIPED BASS LARVAE 



Figure 4.— SEM photomicrograph of an otolith of an intermittently starved larva. Note 

 the pattern of narrow bands, indicated by the bracket, typical of starvation episodes. 

 Increment width during starvation is approximately 5 ^<m. Legends in the micrographs 

 indicate 1) length of scale bars in jum, 2) accelerating voltage KV, 3) mm working 

 distance, 4) coded photo number. 



years that have poor conditions, hence poor recruit- 

 ment, that good age-based growth and mortahty 

 estimation would be the most useful. During such 

 years, more of the young larvae could have their true 

 age underestimated. This would result in overesti- 

 mation of the abundance of younger larvae and 

 therefore a steepened mortality curve. The best ap- 

 proach for routine field work may be to incorporate 

 a design in which a small subsample of otoliths are 

 analyzed by SEM to test for bias using the light 

 microscope. 



Bias in light microscope counts may account for 

 the less-than-daily otolith increment deposition that 

 has been demonstrated in the laboratory (Geffen 

 1982). For field-captured fish, there is no way of 

 knowing whether light microscope counts are biased 

 without the use of SEM. Additionally, this poten- 

 tial bias affects the variance of the estimate of size- 

 at-age. When larval age is underestimated, under 

 conditions which have resulted from poor growth, 

 the variance is improperly decreased for young fish. 

 Hence, lower variances may be a product of both 

 high mortality and bias in age estimation. 



Finally, the light microscope biases are more im- 

 portant for young larvae. By simple arithmetic, a 

 bias of 3 days in a 7-d-old fish will result in a far 

 more inflated growth rate than will a bias of 3 days 



(or for that matter 10 days) in a 60-d-old fish. The 

 growth rate of the younger fish will be inflated 1.75 

 times compared with only 1.05 times (or 1.2 times 

 using the 10-d bias) for the older fish. 



ACKNOWLEDGMENTS 



We wish to thank Deborah Westin, who lent her 

 expertise in rearing larvae, and Ann and Ted Dur- 

 bin, who made their laboratory equipment available. 

 This work was partially supported by a Grant-in- Aid 

 to Research from Sigma Xi and by a grant from the 

 Graduate School of Oceanography, University of 

 Rhode Island, alumni fund. 



LITERATURE CITED 



Bayless, J. D. 



1972. Artificial propagation and hybridization of striped bass, 

 Morone saxatilis (Walbaum). S.C. Wildl. Mar. Resour. 

 Dep., Columbia, SC, 135 p. 

 Brothers, E. B. 



1978. Exogenous factors and the formation of daily and sub- 

 daily growth increments in fish otoliths. [Abstr.]. Am. 

 Zool. 18:631. 

 Brothers, E. B., C. P. Mathews, and R. Lasker. 



1976. Daily grovrth increments in otoliths from larval and 

 adult fishes. Fish. Bull., U.S. 74:1-8. 



177 



