166 



Fishery Bulletin 94|l). 1996 



tions along the counting axis, adding to the difficulty 

 of making reliable daily increment estimates. Total 

 minimum daily increment counts (many increments 

 were probably missed) from light microscope obser- 

 vations ranged from 190 to 353 increments, averag- 

 ing 250 increments from the core to the first trans- 

 lucent zone. 



a 



c 



0) 



cr 



CD 



Stage i 



FEB MAR APR MAY 



JUL AUG SEP OCT 



JAN FEB MAR APR MAY JUN JUL AUG SEP OCT 



JAN FEB MAR APR MAY 



JUL AUG SEP OCT 



Month 



Figure 2 



Results of marginal increment stage determination from otoliths 

 with 2-3 translucent zones. Stage 1 = a translucent zone along 

 the otolith growth margin; stage 2 = area of opaque growth be- 

 tween the otolith growth margin and adjacent translucent zone 

 that is nearly one-fourth the size of the previously deposited 

 opaque zone; stage 3 = area of opaque growth between the otolith 

 edge and adjacent translucent zone that is nearly one-half the 

 size of the previously deposited opaque zone; and stage 4 = area 

 of opaque growth between the otolith edge and adjacent translu- 

 cent zone that is nearly equivalent in size to the previously de- 

 posited opaque zone. 



Seasonal length data 



FOCI larval data FOCI data showed relatively small 

 increases in Atka mackerel larval lengths from Sep- 

 tember to April, averaging about 1.5 mm per month. 

 A more rapid increase in mean length appeared to 

 occur after April. However, FOCI data for May and 

 June were sparse; only two specimens were 

 caught in May and 10 in June (Table 1 ). Lengths 

 from Gorbunova (1962) also showed relatively 

 small monthly increases in size for larval Atka 

 mackerel and closely resembled lengths from 

 FOCI larval data (Fig. 4). 



NRIFSF offshore larval survey Average sum- 

 mer lengths (minimum and maximum lengths 

 and sample size are in parentheses) of Atka 

 mackerel caught from the NRIFSF offshore sur- 

 vey are 49 mm SL in June (39 mm SL-59 mm 

 SL; n-8), 54 mm SL in July (46 mm SL— 67 mm 

 SL; ra=15), and 101 mm SL in August (99 mm 

 SL-104mmSL;«=2). 



AFSC-RACE bottom trawl survey The average 

 summer length for one-translucent-zone fish 

 was 250 mm FL (210 mm FL-290 mm FL; 

 n=43), for two-translucent-zone fish it was 306 

 mm FL (240 mm FL-410 mm FL; n=286), and 

 for three-translucent-zone fish it was 358 mm 

 FL (310 mm FL-440 mm FL; n=38). 



Discussion 



Seasonality of translucent-zone 

 formation 



The highest frequencies of translucent-zone for- 

 mation (stage 1) occurred around April. The 

 period during which most otoliths exhibited the 

 stage-1 pattern spanned at least 6 months, 

 whereas the period exhibiting stage-2 and 

 stage-3 patterns was relatively short but in- 

 tense. In other words, the frequency of a rela- 

 tively small opaque growth beyond the trans- 

 lucent zone ( stage 2 ) peaked in July, and opaque 

 growth doubled in size (stage 3) by August. This 

 rapid otolith growth appeared to coincide with 

 the spurt in somatic growth observed in sum- 

 mer-caught NRIFSF Atka mackerel. The bimo- 

 dal appearance of the stage-4 pattern was prob- 

 ably due to the reader's confusion with the 

 pseudo-translucent nature of the otolith edge 

 (North, 19881 with the result that some stage-1 

 otoliths were classed as stage-4 otoliths. Nev- 



