JONES: DETERMINING AGE OF LARVAL FISH 



deposited daily has been reported in 17 out of 20 

 species (Table 1) grown under optimal laboratory 

 conditions. However, evidence exists that daily 

 deposition can be interrupted under suboptimal and 

 abnormal conditions, or can be dependent on growth 

 rate (Table 6). When the effect of photoperiod is ig- 

 nored (changes in photoperiod are very gradual in 

 the field), more than 50% of the tests under subop- 

 timal and extreme conditions have shown nondaily 

 increment deposition rates. For other species, tests 

 under suboptimal conditions were not conducted and 

 the effect of these conditions on increment deposi- 

 tion rate is undetermined. The effect of varying con- 

 ditions on the age at initial increment deposition has 

 also not been addressed. To apply the otolith aging 

 technique to fish from the natural environment, the 

 scientist must either assume that larvae sampled 

 grew under optimal conditions (those exposed to 

 suboptimal conditions died) or verify that the species 

 almost always deposit daily increments under field 

 encountered conditions, or establish the error 

 bounds for the relationship between age and incre- 

 ment count. 



Attempts to clarify the natural phenomena that 

 drive daily increment formation have given con- 

 flicting results. Photoperiod, feeding periodicity, and 

 temperature fluctuations have all been cited as 

 causing daily increment formation. When these fac- 

 tors are within normal ranges, it is likely, for most 

 larvae, that deposition is daily. However, for larvae 

 experiencing conditions outside tolerable ranges or 

 abnormal conditions, the period of formation is likely 

 to deviate from daily deposition. It is important to 

 determine whether the minimum exposure to subop- 

 timal conditions which result in nondaily deposition 

 is actually experienced by larvae in the field. These 

 hypotheses are amenable to further testing. More 

 basic research on the causation of increment deposi- 

 tion or more extensive testing under a variety of con- 

 ditions for a given species will yield more informa- 

 tion. In situ testing with known-age larvae in 

 enclosures which closely mimic field conditions could 

 yield valuable results. The Bergen otolith workshop 

 report (Anonymous 1982) has recommended that in- 

 crement deposition be verified for each new species, 

 under a variety of test conditions. 



Two issues, cost effectiveness and accuracy, are im- 

 portant in determining whether the otolith incre- 

 ment technique is preferable to length-frequency 

 analysis. Recommendations made in the report from 

 the Bergen otolith workshop (Anonymous 1982) are 

 that "the precision of an age determination ... be 

 tested against other available methods ... by a cost 

 benefit analysis (i.e is enough precision gained by 



Table 6— Incidence of nondaily increment deposition for 

 species reared under suboptimal and extreme conditions. 

 Stars (*) indicate nondaily deposition caused by exposure to 

 suboptimal conditions; triangles (A) indicate nondaily deposi- 

 tion caused by exposure to extreme conditions; circles (O) in- 

 dicates no interruption of daily deposition. 



using this method to pay the costs and effort in 

 preparation)". A good example would be the results 

 shown in Laroche et al. (1982) when the otolith 

 method was compared with modal progression of 

 length frequencies, estimated growth rates differed 

 by a factor of 2-3. Benefits should also include non- 

 monetary considerations, such as decrease in error 

 which will propagate through estimates based on age 

 determinations (i.e, growth and mortality). Sensi- 

 tivity analyses can be used to show situations where 

 more accurate estimates are necessary. 



Specific recommendations for improving reliability 

 and replicability are discussed in the Bergen otolith 

 workshop report (Anonymous 1982). In addition to 

 these, Brothers 3 has suggested that other otoliths, ( 

 such as the lapillus, be used in analysis. 



Aging by the otolith increment technique is a 

 powerful tool. Not only can population estimates of 

 growth and mortality be refined, but growth of in- 

 dividuals can be obtained. Issues such as the impor- 

 tance of environmental factors to survival, the pro- 

 portion of fast growing larvae to recruitment, and 

 demonstration of compensation in field larvae may 

 become easier to address with the availability of this 

 technique However, it is equally important to make 

 sure that the technique is based on good scientific 

 technique 



ACKNOWLEDGMENTS 



I thank David Bengtson, John Forney, Saul Saila, 

 Ann Durbin, and Bernard Skud for their thoughtful 

 review of this manuscript and Walter Berry for his 

 many helpful suggestions and discussions. 



3 Edward Brothers, 3 Sunset West, Ithaca, NY 14850, pers. com- 

 mun. September 1983. 



101 



