DETERMINING AGE OF 

 LARVAL FISH WITH THE OTOLITH INCREMENT TECHNIQUE 



Cynthia Jones 1 



ABSTRACT 



Aging of larval fish from otoliths rests on the assumption that increments are formed daily. Indeed, proper 

 validation of the relationship between increment deposition and age is fundamental to accurate age deter- 

 mination of field-captured fish, lb evaluate the universality of daily deposition of otolith increments, the 

 literature was reviewed and exceptions discussed. 



Laboratory studies under optimal conditions generally (17 species out of 20) show that larvae deposit 

 daily increments. However, in studies that examined increment deposition under suboptimal or extreme 

 conditions, deposition was not daily in over half of the species. Nondaily deposition caused by extreme 

 conditions (eg, total starvation, abnormal photoperiod) may not invalidate the otolith increment tech- 

 nique if those conditions do not occur in the field. Nondaily deposition under suboptimal conditions (eg., 

 low temperature, intermittent starvation) that larvae may face in nature cause concern about this tech- 

 nique for aging field-captured larvae Deposition in many species has not been examined under suboptimal 

 conditions, nor has the effect of suboptimal conditions been shown on the age at first increment forma- 

 tion. The literature shows that the technique should be validated under both optimal conditions and those 

 that mimic nature 



Otoliths have been used to age fish since Reibisch 

 (1899) first observed annular ring formation in 

 Pleuronectes platessa (as reported in Ricker 1975). 

 Assessing age by counting annular rings works well 

 in adults of temperate species where pronounced 

 seasonal changes in growth result in bands (formed 

 from tightly spaced growth increments deposited in 

 the winter) in the otolith which correspond to each 

 year of life Discovery of fine increments, analogous 

 to annual rings, but instead formed daily, has per- 

 mitted the age of larval fish to be determined. 



While studying temperate water species, Pannella 

 (1971) observed that about 360 fine increments oc- 

 curred between annular rings and suggested that 

 these were deposited daily. He used this knowledge 

 when reading the otoliths of adult tropical fish 

 (whose otoliths also had fine increments) to show pat- 

 terns of growth that were grouped into 14- and 28-d 

 cycles (Pannella 1974). 



The initial application of the otolith aging tech- 

 nique to larval fish was done by Brothers et al. (1976). 

 Daily increment deposition was verified for northern 

 anchovy, Engraulus mordax, and California grunion, 

 Leuresthes tenuis, which were reared from eggs in 

 the laboratory. Since this initial application, the 

 otolith increment technique has been used widely to 



Graduate School of Oceanography, University of Rhode Island, 

 Kingston, RI 01882-1197; present address: Department of Natural 

 Resources, Cornell University, Ithaca, NY 14853. 



estimate age in at least 29 species of larval fish. It 

 has been used in freshwater and marine species, and 

 applied to field-captured species, at times without 

 adequate validation. 



The ultimate purpose in developing the otolith 

 aging technique for application to young fish is the 

 ability to accurately age field larvae and juveniles. 

 If the technique is to be applied directly to the field, 

 based on conclusions drawn from rearing larvae in 

 the laboratory, then the deposition of increments 

 must be daily under conditions experienced in the 

 field during these early life stages. The applicability 

 of this technique relies on the assumption that 1) 

 either surviving larvae (or sampled larvae) are those 

 that grew under moderately good conditions (few lar- 

 vae under suboptimal conditions survive) or 2) lar- 

 vae can encounter suboptimal conditions, a propor- 

 tion of these larvae will survive, and increment 

 deposition is not affected by these suboptimal con- 

 ditions. The first assumption is difficult to evaluate 

 without using the hypothesis that increments are 

 daily. The second assumption has been tested and 

 the results can be summarized. The second assump- 

 tion is based on increment deposition being triggered 

 by a zeitgeber, an external factor that entrains a diel 

 cycle within the larvae 



Validation of daily increment deposition under con- 

 ditions within the natural range of experience of the 

 larvae is fundamental to accurate estimation of age 

 in field-captured fish. When the estimation technique 



Manuscript accepted March 1985. 



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