METHOT: SURVIVAL OF LARVAL ENCRAULIS MORUAX 



alongshore distribution of juveniles in 1979 (Methot 

 1981). The regional breakdown was necessary 

 because the overlap in size between juveniles and 

 older fish varied latitudinally (Table 1). Although 

 data in Table 1 are entirely from 1979, they were 

 used to calculate juvenile size distributions from 

 overall size distributions in both 1978 and 1979. In 

 1978 few adults were collected and no comparable 

 samples were obtained from south of San Diego. 2 



Otolith Preparation 



Thawed specimens were measured to the nearest 

 1.0 mm standard length. Sagittae (largest otoliths) 

 were removed, cleaned in distilled water, dried, and 

 mounted on a microscope slide with a clear 

 methacrylate-based mounting medium. Otoliths of 

 northern anchovy larger than about 40 mm are too 

 thick to transmit sufficient light for viewing the 

 increments. Material was removed from the otolith's 

 medial surface by applying 5-10% HC1 to selected 

 regions for about 10 s at a time. Immersion oil, pe- 

 troleum jelly, or mounting medium were used to mask 

 the outer edge of the otolith and regions already suf- 

 ficiently thin. The selectively etched surface 

 develops high relief but a thin layer of immersion oil 

 renders this relief nonrefractory and permits 

 examination of the otolith. After most increments 

 became visible, the mounting medium was softened 

 with 809c ethanol, and the otolith was turned over 

 and remounted. Etching of the lateral surface con- 

 tinued until all increments were visible within, but 

 not necessarily at the surface of, the remaining 

 material. 



The otoliths in 1978 were prepared by embedding 

 in polyester casting resin and grinding sagittal sec- 

 tions on 400 and 600 grit wet sandpaper. Selective 

 etching was faster and more successful than 

 grinding. 



Age Determination 



Specimens used for this study also were used to 

 back calculate juvenile growth and a direct count of 

 all increments in an otolith was rarely made. Instead, 

 age was determined from numerical integration of 

 otolith growth (increment width). Increments were 

 measured with a video camera mounted on a com- 

 pound microscope, an electronic device which 



; Two samples from the Ensenada commercial fishery were pro- 

 vided by G. Broadhead (Living Marine Resources, San Diego). The 

 fish hada similar size/birth-date relation to fish from the bait fishery 

 in the U.S. coastal waters. Because the commercial fishery is biased 

 against small fish, the size distribution of juveniles in Mexico could 

 not be estimated. 



positioned a cursor in the video image, and a mi- 

 crocomputer interfaced to the device. All measure- 

 ments were made along the longest radius of the 

 otolith (towards the posterior margin). The observer 

 positioned the cursor at the outer edge of an incre- 

 ment and keyed in the number of increments be- 

 tween that point and the previous point while the 

 computer recorded the radius to that increment. 

 Increment width usually did not change rapidly so 2- 

 10 increments of similar size were entered together. 

 Data from different regions along the longest radius 

 were recorded at various stages of the etching 

 process. 



Data from both otoliths and several replicate tran- 

 sects per otolith were combined in the calculation of 

 age. Mean increment width was calculated at all 

 points along the longest radius. Etching errors 

 occasionally produced a region in which increments 

 could not be seen. When this occurred, increment 

 width was interpolated from mean increment width 

 in adjacent intervals by a linear interpolation of 

 increment width on radius. Age was calculated from 

 numerical evaluation of the following expression: 



!= 1 



r — r, 



G(r,) 



where the following definitions and boundary con- 

 ditions apply: 



G{r.) = 



G{r Y ) 

 G(r„) 



set of all radial distances where increment 

 width changed perceptibly 

 average increment width between r, , 

 and r, 



otolith radius at onset of increment forma- 

 tion (6.5 jLtm) 

 maximum otolith radius 

 typical initial increment width (0.8 fxm 

 per increment) 

 G(r n _,) if G{r n ) not measurable. 



The result converges exactly to the count of 

 increments if each individual increment is measured 

 once. The age estimate was accepted if <20% of the 

 age was from interpolated increments. About 3% of 

 the fish were rej ected by this criterion. The mean per- 

 centage of interpolated increments for the accepted 

 fish was 4.7% and the median was 2.5%. Usually an 

 independent age estimate could be made from each 

 otolith. When fish were stratified into 50- d age inter- 

 vals, the coefficient of variation of age between 

 otoliths within fish averaged 4.6% in 1978 and 3.5 % 

 in 1979. Thus, the 95% confidence interval for a 250- 

 d-old fish was ±14 d. 



743 



