VALIDATION OF THE OTOLITH INCREMENT 



AGING TECHNIQUE FOR STRIPED BASS, MORONE SAXATILIS, LARVAE 



REARED UNDER SUBOPTIMAL FEEDING CONDITIONS 



Cynthia Jonesi and Edward B. Brothers^ 



ABSTRACT 



Striped bass, Morone saxatilis. larvae were reared in the laboratory for 97 days to validate the otolith 

 increment aging technique for this species. Otolith-increment deposition rates were determined under 

 optimal laboratory conditions for growth and under three conditions of restricted feeding and using both 

 light and scanning electron microscopy (SEM). Under optimal laboratory conditions, increments were 

 deposited daily from the fourth day after hatching through the first 2 months of life and were discernible 

 with the light microscope. For larvae reared under restricted feeding regimes and readings done with 

 the light microscope, counts did not reflect true age. Counts obtained from these same otoliths using 

 SEM, however, more closely reflected true daily age. Results indicate that the use of light microscopy 

 alone can result in inaccurate estimation of age for larvae that have experienced starvation episodes. 



When otolith increments in larval fish are deposited 

 daily, with a known time of onset, precise age of 

 each individual can be determined and the growth 

 curves for the individuals may be generated. The 

 ability to follow changes in growth of individuals and 

 populations on as fine a scale as, say, a week may 

 provide a means to improved understanding of the 

 effects which environmental factors have on sur- 

 vival. 



To apply this aging technique to larval striped 

 bass, Morone saxatilis, daily deposition of incre- 

 ments and the age at first increment deposition had 

 to be confirmed in the laboratory with known-age 

 larvae. Although daily depositional rates of otolith 

 increments in known-age larval striped bass have 

 not been previously reported, daily deposition has 

 been noted for larvae and juveniles of 17 other 

 species of fish reared in the laboratory (see Jones 

 1985 for review). Nonvalidated data exist to support 

 the concept of daily increment deposition for field- 

 captured striped bass (Brothers et al. 1976). How- 

 ever, tests of depositional rate under suboptimal lab- 

 oratory conditions, using light microscopy, have 

 shown that depositional rates can be affected by the 

 specific growth rate (Geffen 1982), by photoperiod 

 (Radtke 1978), by food supply (Geffen 1982; Neilson 

 and Geen 1982), and by temperature (Brothers 1978; 

 Geffen 1983). Campana and Neilson (1985) stated 

 that "few workers have critically assessed the 



'Old Dominion University, Department of Oceanography, Nor- 

 folk, VA 23508. 

 ^EFS Consultants, 3 Sunset West, Ithaca, NY 14850. 



assumptions upon which the age and growth infer- 

 ences are based or considered the potential for envi- 

 ronmental modification of microstructural features." 



Of particular importance is the potential for count- 

 ing fewer otolith increments when otolith growth 

 rate is slowed to the extent that increments being 

 deposited are too narrow to resolve with a light 

 microscope. Inadequate resolution with the light 

 microscope could lead to systematically low incre- 

 ment counts and thus, result in overestimation of 

 the growth and mortality rates, and underestima- 

 tion of variance in growth, all of which have impor- 

 tant biological implications. Hence, to demonstrate 

 that striped bass larvae from the field could be aged 

 accurately by the otolith increment technique, we 

 found it necessary to determine the regularity and 

 readability of otolith-increment deposition under 

 simulated laboratory suboptimal field conditions. 



Lack of scanning electron microscopy (SEM) 

 validation hinders the resolution of an important 

 issue: Is daily formation of increments a robust 

 biological rhythm common to most teleosts which 

 requires serious and prolonged starvation to disrupt, 

 or is it a more volatile physiological connection in 

 which daily formation occurs only under optimal 

 food concentrations as certain laboratory studies 

 indicate? 



Factors which affect growth and survival of 

 striped bass larvae have been studied extensively 

 (see Westin and Rogers 1978 for review). Rogers 

 (1978) raised larval striped bass under various tem- 

 perature and feeding regimes to determine growth 

 under laboratory conditions. Larvae grew well at 



Manuscript accepted December 1986. 

 FISHERY BULLETIN: VOL. 85, NO. 2, 1987. 



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