FISHERY BULLETIN: VOL. 74, NO. 1 



METHODS 



Some material was examined with a Stereo- 

 scan^ S4 scanning electron microscope (Cam- 

 bridge Scientific Instruments Ltd.). These otoliths 

 were prepared for viewing by embedding them in 

 polyester resin, grinding and polishing them 

 to the vertical mid-sagittal plane with a graded 

 series of silicon carbide or aluminum oxide com- 

 pounds (400, 600, and 900 grit), and finishing 

 with 1-yum diamond paste. The polished surface 

 was then etched with 0.1 N HCl before being 

 rotary coated in a vacuum evaporator with 150 A 

 of gold-palladium alloy. 



Both this technique and that of Pannella (1974) 

 involve the use of equipment and materials that 

 may be inaccessible in many countries. This is 

 particularly true for those countries in which 

 daily growth increments might prove to be espe- 

 cially helpful in stock assessment of commercial 

 fish, so that an alternative practical method with 

 minimal equipment was also used here and found 

 to be successful. 



Otoliths of adult fish were ground by hand on a 

 glass plate covered with a water-silicon carbide 

 powder mixture (400-600 grit). The final polish 

 may be administered with diamond paste, but this 

 step is not essential. The ground otolith was then 

 examined in immersion oil. The grinding was 

 done in the same plane as described by Pannella 

 (1974). It is possible that storage in oil over a long 

 period of time may reduce the resolution obtained 

 when an otolith is examined. This appears to be 

 particularly true for larval otoliths. The above 

 technique is simple and requires only a good com- 

 pound microscope. Magnifications used in this 

 work ranged to 1,800 x; at least 600 x is required 

 for general viewing. 



Otoliths from larvae were removed by teasing 

 them from fresh specimens. Oven-dried material 

 needed only to be moistened with a drop of water 

 before otolith removal. The otoliths were manipu- 

 lated and transferred to clean slides by picking 

 them up on the end of a fine dissecting needle 

 wetted with immersion oil. No additional prep- 

 aration was necessary, and the otoliths were 

 examined in immersion oil or after being perma- 

 nently mounted under a cover slip in a quick- 

 drying, neutral mounting medium. Ground sec- 

 tions from juveniles and adults may be similarly 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



mounted with no apparent loss in clarity. Larval 

 otoliths are thin enough that only optical section- 

 ing (i.e., carefully focusing to the plane of 

 maximum clarity) is necessary to make total 

 increment counts. 



Material from a variety of species was ex- 

 amined and larval material of known age was 

 obtained by rearing eggs that had been fertilized 

 in the laboratory (Lasker et al. 1970; Leong 1971). 

 The chronological age from these fish was known 

 and could be compared with the number of growth 

 increments observed in their otoliths. Larvae of 

 northern anchovy, Engraulis mordax, were kindly 

 made available to us by John R. Hunter of the 

 Southwest Fisheries Center, National Marine 

 Fisheries Service, NOAA, at La Jolla, Calif 



RESULTS 



Otoliths of 15 E. mordax, aged 6 days, were 

 examined. The mean total length of the fish was 

 4.5 mm. The yolk-sac had been absorbed by the 

 fifth day after hatching. Figure la shows the ap- 

 pearance of the otolith of one of these fish. 



Only one or two daily increments were present, 

 suggesting that daily growth increments ap- 

 peared in the otoliths of E. mordax only after 

 completion of yolk-sac absorption. In the labora- 

 tory, anchovy larvae were maintained in 14 h of 

 light when feeding took place and 10 h of darkness 

 when no feeding occurred (Lasker et al. 1970). 



Table 1 shows the relation between chronologi- 

 cal age and number of apparently daily incre- 

 ments for larvae of E. mordax aged 6 to 100 days. 

 It is clear that there is an extremely close corre- 

 spondence between the chronological age in days 

 and the number of increments. Figure lb is a 

 micrograph showing the daily increments in an 

 anchovy otolith from a larva 18 days old. 



Additional data presently being collected on 

 laboratory and wild-caught larvae indicates that 

 there is some interaction between the rate of 

 larval growth and the rate of increment formation 

 which may complicate the interpretation of oto- 

 lith age estimates. 



Figure 2 shows the structure of adult anchovy 

 otoliths with successively greater magnification 

 of the scanning electron microscope. The darker 

 areas in the photographs represent areas of the 

 otolith that were more heavily etched because 

 they contained a higher proportion of CaCOs, 

 while the lighter areas have relatively more 

 organic material, probably otolin (see Degens et 

 al. 1969). It is seen from Figure 2 that the smallest 



