Validation of the Otolith Increment Aging Technique for Striped Bass, Morone Saxatilis, Larvae Reared Under Suboptimal Feeding Conditions

Striped bass, MOTOIl£ suxatili.s, 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 survival. To apply this aging technique to larval striped bass, Morone saxatilis. daily deposition of increments 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 fieldcaptured striped bass (Brothers et al. 1976). However, "tests of depositional rate under suboptimal laboratory 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. 2EFS Consultants, 3 Sunset West, Ithaca, NY 14850. Manuscript accepted December 1986. FISHERY BULLETIN: VOL. 85, NO.2, 1987. assumptions upon which the age and growth inferences are based or considered the potential for environmental 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 increment counts and thus, result in overestimation of the growth and mortality rates, and underestimation of variance in growth, all of which have important 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 temperature and feeding regimes to determine growth under laboratory conditions. Larvae grew well at