FISHERY BULLETIN: VOL. 81, NO. 3 



ing to estimate change in fish length and, ultimately, 

 somatic growth rate. Similarly, Brothers et al. (1976) 

 and Methot and Kramer (1979) numerically in- 

 tegrated increment thickness data to estimate age. 

 Although of some significance, no formal or detailed 

 treatment of the subject was presented. More recent- 

 ly Methot (1981) has measured the widths of the 

 outer three daily increments in the otoliths of larval 

 fishes, using them as explicit measures of recent daily 

 growth rate. Brothers (1981) discussed this kind of 

 application and cautions against short-term un- 

 couplings of otolith growth with changes in fish 

 length. 



In this paper, we intend to analytically formalize the 

 concept that increment width can be used as a 

 measure of otolith growth rate. This eliminates the 

 constraint of having to count all increments in an 

 otolith, providing a framework for studying age and 

 growth in large, slow- growing species offish in which 

 increment microstructure frequently becomes am- 

 biguous with age (Brothers 1979; Pannella 1980). 

 Rather than attempting a numerical solution to this 

 problem, we have analytically integrated increment 

 growth -rate data obtained from the otoliths of 

 Hawaiian snapper, or opakapaka, Pristipomoides 

 filamentosus (Lutjanidae). This is a commercially im- 

 portant species of bottomfish harvested in the 

 Hawaiian deep-sea handline fishery (Ralston 1981; 

 Ralston and Polovina 1982). While admittedly sac- 

 rificing some of the extreme precision theoretically 

 possible with daily increments, the intent here is to 

 provide reasonably reliable age estimates, eco- 

 nomically obtained, when age is measured on a scale 

 of years. Preliminary results of this research have 

 been reported elsewhere (Ralston and Miyamoto 

 1981). 



METHODS 



length)), captured by hook and line, were acclimated 

 to a 1,135 1 flowthrough aquarium. The fish were ex- 

 posed to a natural photoperiod through an overhead 

 skylight and were fed to satiation twice daily (mid- 

 morning and late afternoon). All six fish were initially 

 in good condition, and after 12 d they appeared well 

 adjusted to the tank, having resumed what seemed to 

 be normal feeding behavior. 



Three 1 kg specimens were injected intraperi- 

 toneally with 30 mg of oxytetracycline (dosage from A. 

 Wild 4 ). Following injection, all six fish were exposed 

 for 18 h to 125 ppm acetazolamide in seawater 

 (dosage from J. Dean 5 ). This compound has been 

 shown to affect otolith calcification by inhibition of 

 carbonic anhydrase (Mugiya 1977), providing a sim- 

 ple means of creating a checkmark on otoliths. The 

 fish resumed feeding 2 d after treatment. One fish 

 jumped out of the tank 27 d later, and the experiment 

 ended prematurely when the remaining five speci- 

 mens died unaccountably 38-39 d after treatment. 



Preparation of Otoliths 



All otoliths in this study were prepared for viewing 

 as outlined in Ralston and Miyamoto (1981). Thin 

 sections (0.5 mm) were made through the focus along 

 a frontal plane to the most distal portion of the post- 

 rostrum (Figs. 1, 2). Preliminary observations showed 

 that increment counts made in a transverse plane 

 were less than those made from frontal sections, pre- 

 sumably due to pinching and coalescing of rings along 

 the shorter transverse axis (Pannella 1974; Taubert 

 and Coble 1977; Dunkelberger et al. 1980). Tet- 

 racycline marks were obliterated by the clearing 

 agent (Euparal 6 ) and were located prior to etching by 

 viewing with an ultraviolet fluorescence microscope. 

 In addition, several otoliths were prepared for scan- 

 ning electron microscope (SEM) examination. 



Marking Experiment 



A marking experiment was done to validate the exis- 

 tence of daily growth increments in opakapaka. 

 Otoliths are composed primarily of the aragonitic 

 crystalline form of calcium carbonate in association 

 with small amounts of otolin protein (Hickling 1931; 

 Irie 1960; Degens et al. 1969). The presence of 

 calcium carbonate allows otoliths to be successfully 

 marked in vivo with the antibiotic tetracycline (Black- 

 er 1974; Wild and Foreman 1980; Campana and 

 Neilson 1982). A dated, visible time-mark in otoliths 

 provides direct validation of the periodicity of pre- 

 sumed daily increments. 



Six juvenile opakapaka (30-34 cm FL (fork 



Otolith Growth Rate 

 and Specimen Age 



After otoliths had cleared, they were read with a 

 compound binocular microscope using transmitted 

 light at a magnification of 400X. The distance in mi- 

 crons (ju.m) between the focus and the postrostral 

 margin, representing the total length (L) of the otolith 

 along the postrostral radius, was measured with a 



4 A. Wild, Inter- American Tropical Tuna Commission, LaJolla, CA 

 92037, pers. commun. May 1980. 



5 J. Dean, Professor, Belle Baruch Institute, University of South 

 Carolina, Columbia, SC 29208, pers. commua May 1980. 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



524 



