FISHERY BULLETIN: VOL. 84, NO. 1 



used to age larvae is inaccurate, estimates of growth 

 and mortality, which rely on knowledge of age, will 

 also be inaccurate 



The purpose of this paper is to discuss the use of 

 the otolith increment technique to age larval fish. 

 The published literature is used to evaluate the 

 hypothesis, H : Larval age is equal to otolith incre- 

 ment count (plus age at first increment deposition) 

 under conditions that are encountered in the field. 

 An additional idea can be evaluated: That time of 

 initial increment deposition is influenced by incuba- 

 tion time 



The paper will discuss the factors which affect 

 deposition of increments, validation studies that have 

 been performed, and application of the technique in 

 the field. Factors which are likely to affect increment 

 deposition in the field must be assessed by the valida- 

 tion procedure In addition, the adequacy of valida- 

 tion that has been performed is evaluated, and 

 ramifications in field applications are discussed. 



FACTORS AFFECTING 

 DEPOSITION RATES 



Mechanisms that have been postulated as initiators 

 of differentiation of otolith microstructure are photo- 

 period, feeding, and temperature Increment deposi- 

 tion has been tested in the literature under two 

 conditions: 1) tests within the natural range of 

 experience of the fish which could be optimal (non- 

 stressful) and suboptimal (stressful), and 2) abnor- 

 mal conditions that are wholly outside of their 

 experience 



Taubert and Coble (1977) stated that photoperiod 

 entrained a diel clock that resulted in daily forma- 

 tion of otolith increments. Tanaka et al. (1981) stud- 

 ied the formation of increments in Tilapia nilotica 

 using scanning electron microscopy and found that 

 the fast growth (incremental) zone started a few 

 hours after light stimulus and that the slow growth 

 (discontinuous) zone was formed immediately after 

 light stimulus. Neither change in photoperiod length 

 nor feeding time affected increment initiation. 

 Brothers and McFarland (1981), however, reported 

 that the discontinuous zone began near midnight. 

 These results are contradictory, and without further 

 investigations force the conclusion that the temporal 

 formation of increments is species-specific 



Abnormal photoperiods have been shown to dis- 

 rupt daily increment formation in Fundulus hetero- 

 clitus (Radtke and Dean 1982) and in Tilapia 

 mossambica (Taubert and Coble 1977). Constant 

 light, however, did not disrupt daily increment forma- 

 tion in Oncorhynchus tshawytscha (Neilson and Geen 



1982) or in Scophthalmus maximus (Geffen 1982). 



Unlike photoperiod changes, which are regular and 

 gradual in nature, feeding times can occur at irre- 

 gular intervals and might cause deviations in daily 

 increment deposition. Two studies have tested the 

 effects of feeding within the normal range experi- 

 enced by fish larvae Neilson and Geen (1982) found 

 that subdaily increments could be induced through 

 frequent discrete feedings: feeding four times a day 

 resulted in formation of more than one increment 

 in Oncorhynchus tshawytscha. Daily and subdaily in- 

 crements were not distinguished in counts. Tanaka 

 et al. (1981) found conversely that feeding time had 

 no effect on the initiation of increment formation 

 in Tilapia nilotica. Larvae were fed once a day, but 

 the times of feeding were changed. Perhaps multi- 

 ple feeding during the day results in the subdaily in- 

 crements that sometimes appear in otoliths. The ef- 

 fect of starvation (an extreme circumstance in the 

 field) on increment deposition has been tested in only 

 three species: Scophthalmus maximus (Geffen 1982), 

 Morone saxatilis (Jones 1984), and Oncorhynchus 

 nerka (Marshall and Parker 1982). Geffen raised the 

 turbot larvae on rotifers and Artemia until they were 

 10 d old. Larvae were then starved for 23 d. Jones 

 did not supply exogenous food from hatch onward. 

 Both Geffen and Jones found that starvation 

 disrupted increment formation. Marshall and Parker 

 fed their sockeye salmon larvae for the first 3 wk 

 of life, and then starved them for 2 wk. Marshall and 

 Parker found that starvation over 2 wk had no ef- 

 fect on increment deposition. It is possible that the 

 difference might reflect different age-specific sen- 

 sitivity to starvation, rather than species-specific 

 responses. 



Brothers (1978) has linked temperature as a prime 

 factor in increment deposition. Working with tem- 

 perate stream populations, he has found that diel 

 temperature changes result in daily increment for- 

 mation. Brothers (1978) stated that "six or more in- 

 crements per day may be formed as the result of 

 short term, . . .relatively minor. . . temperature fluc- 

 tuations." Other investigators (Radtke and Dean 

 1982; Geffen 1982) found that small temperature 

 changes had no effect on the rate of increment 

 deposition. Apparently, temperature response is also 

 species-specific. 



LABORATORY STUDIES OF 

 INCREMENT DEPOSITION 



Initial Ring Deposition 



When fish are raised in the laboratory from eggs 



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