FISHERY BULLETIN: VOL. 84, NO. 1 



tion of subdaily increments in Oncorhynchus 

 tshawytscha. Both Tanaka et al. (1981) and Taubert 

 and Coble (1977) found that feeding time had no ef- 

 fect on increment deposition in larval mouthbrooders 

 (Tilapia nilotica and T. mossambica). 



Little agreement has been reached in these studies 

 concerning the effect of light, temperature, or 

 feeding on increment formation. The effects of 

 variability in temperature, food, salinity, and other 

 factors (extreme photoperiods would not be en- 

 countered) relate directly to the problems of ac- 

 curately aging larvae from the field. At the moment, 

 environmental effects appear to be species-specific. 

 Indeed, specific tests of the effect of suboptimal con- 

 ditions (which are likely to occur in the field) on in- 

 crement deposition have rarely appeared in the 

 literature Such analyses, conducted for more 

 species, might confirm the conventional wisdom that 



deviation from daily deposition rate is abnormal. 

 However, the questions raised by the studies re- 

 viewed here (Table 4) remain to be fully addressed 

 or dispelled. 



APPLICATION IN THE FIELD 



Current Applications 



The ability to age larval fish precisely provides 

 more accurate estimates of growth, mortality, and 

 the ability to discern the effects of environmental 

 variables on the first year of life Rapid growth in 

 the first months of life has commonly been thought 

 to be critical to survival. Evidence in support of this 

 hypothesis (Brothers et al. 1983) and contrary to it 

 (Methot 1983) exists. 



The otolith increment aging technique has been 



Table 5. — Application of the otolith increment aging technique in field grown larvae. 



Species 



Source 



Based on prior 



validations 



(validations in 



Table 1) 



Validation 

 source 



Sample 

 size 



Application 



Back-calculated growth. 



Determine hatching dates and de- 

 lineate cohorts which are followed 

 through time. 



Determine hatching dates and as- 

 sess growth rates of larval cohorts. 

 Noted cessation of growth in winter. 



Use age to delineate growth. Fit 

 Gompertz function of length-at-age 

 data. 



Determination of within-season 

 growth differences based on uncer- 

 tainty in otolith aging. 



Fit Gompertz function to length-at- 

 age data to obtain growth rates. Also 

 mention that starvation slowed incre- 

 ment deposition. 



Compare length-frequency histo- 

 grams with increment-frequency his- 

 tograms. Show relationship between 

 hatching and lunar cycle. 

 Regression of age estimated from 

 morphologic development versus in- 

 crement counts. 



Back-calculated hatch date from in- 

 crements. Compare these to field 

 observations of spawning time. 



Correspondence between otolith 

 microstructure and events in the life 

 history. Derive "otolith" growth 

 rates. 



Determine daily deposition of incre- 

 ments and use to determine settling 

 pattern. 



Allometric relationship between oto- 

 lith length and fish length tested for 

 2 lakes. 



98 



