NOTE Anderl et a\ Is the first annulus missing on the otolith of Pleurogrammus monopterygius? 



165 



NRIFSF offshore larval survey Twenty-five Atka 

 mackerel, collected from 15-minute surface tows with 

 larval nets ( 1.3-m diameter and 0.333-mm mesh size) 

 in the central Bering Sea from 23 June to 20 August 

 1991 were provided by the National Research Insti- 

 tute of Far Seas Fisheries (NRIFSF) of Japan. Stan- 

 dard lengths were measured. These fish were stored 

 in formalin; therefore all otoliths from this sample 

 were partially dissolved and unusable. 



AFSC-RACE bottom trawl survey Average lengths 

 of Atka mackerel observed with 1, 2, and 3 otolith 

 translucent zones were calculated from data compiled 

 by the AFSC's Resource Assessment and Conserva- 

 tion Engineering (RACE) division and taken from 

 samples collected during the 1991 Aleutian Islands 

 summer bottom trawl survey at depths ranging be- 

 tween 100 and 200 meters. 



Results 



Seasonality of translucent-zone formation 



Translucent-zone formation along the otolith growth 

 margin (stage 1) was the dominant edge-type ob- 

 served on otoliths collected from January to June, 

 reaching a 75% frequency peak in April (Fig. 2). Most 

 otoliths exhibited the stage-2 pattern in July (90%) 

 and the stage-3 pattern in August (65%). Stage-4 

 otoliths were most common in February (50%) and 

 May (40%). 



Larval otoliths 



The April collection of larval Atka mackerel ranged 

 in lengths from 10 mm SL to 23 mm SL and aver- 

 aged 17 mm SL. Average otolith diameter was 240 pm. 



No evidence of accessory primordia was observed in 

 any larval otoliths. Daily increments surrounding the 

 otolith core were narrowly spaced and many daily 

 increments could easily have been missed. In some 

 areas, distinction between increments was not clear 

 (owing to poor contrast) so that estimating the num- 

 ber of increments in these areas was necessary. The 

 ring intervals widened as the daily increments ap- 

 proached the otolith edge. Total increment counts 

 from the core to the edge of the larval fish otoliths 

 ranged from 60 to 136 and averaged 90 increments. 



Adult otoliths 



As with the larval otoliths, enumerating daily incre- 

 ments surrounding the core of adult otoliths proved 

 difficult because of the dense arrangements of incre- 

 ments. SEM micrographs revealed increments nar- 

 rowing and becoming indistinguishable around 90 

 /jm distal to the core along the frontal plane (Fig. 

 3A). At about 380 pm, the increments gradually wid- 

 ened so that about 200 increments were clearly ob- 

 served with a regular periodicity. The distance be- 

 tween increments reached a maximum width of 3 to 

 4 pm in the first opaque zone (Fig. 3B) before nar- 

 rowing again at about 1,000 /im. The increments 

 became obscure even at high magnifications and were 

 observed to merge into a deeply etched groove (about 

 1,400 fum from the nucleus), presumably the site of a 

 translucent zone viewed under light microscopy (Fig. 

 3C). Toward the next opaque zone the increments 

 widened and became easy to identify again. A simi- 

 lar sequential pattern was observed through the light 

 microscope in both transverse and sagittal sections 

 though distances varied owing to the different view- 

 ing axes. 



Accessory primordia were clearly evident in the 

 adult fish otoliths. These structures created disrup- 



