163 



Is the first annulus on the otolith 

 of the Atka mackerel, 

 Pleurogrammus monopterygius, 

 missing? 



Delsa M. Anderl 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way NE, Seattle, Washington 98 1 I 5 



Akira Nishimura 



National Research Institute of Far Seas Fisheries 

 5-7-1, Ondo, Shimizu, 424, Japan 



Sandra A. Lowe 



Alaska Fisheries Science Center 



National Marine Fisheries Service. NOAA 



7600 Sand Point Way NE. Seattle. Washington 98 1 I 5 



Atka mackerel, Pleurogrammus 

 monopterygius, is a semidemersal, 

 shallow-water species distributed 

 along the Northern Pacific Ocean 

 from the Kamchatka Peninsula to 

 Southeast Alaska. Resource sur- 

 veys show that the largest concen- 

 tration of this species is along the 

 Aleutian Islands chain where it 

 sustains an important and growing 

 commercial fishery. 



Few studies (Gorbunova, 1962; 

 Lee, 1985) have described the life 

 history of Atka mackerel. Adults mi- 

 grate to coastal areas from the open 

 ocean before the July to October 

 spawning season (McDermott and 

 Lowe. 1 ). Hatching occurs about 40- 

 45 days after spawning. Gorbunova 

 (1962) reported that newly hatched 

 larvae are about 8-10 mm long 

 (whether measurements were stan- 

 dard or fork lengths was not indi- 

 cated); whereas fish 60-70 mm and 

 170-190 mm long are 1 and 2 year 

 olds, respectively. However, the 

 smallest Atka mackerel observed 

 with a single translucent zone on 

 their otoliths were May-captured 



180-mm-FL fish caught during bot- 

 tom trawls in the Northwestern Pa- 

 cific (Fig. 1). Because there are no 

 other translucent zones visible on the 

 otoliths, convention dictates that the 

 one formed along the otolith margin 

 represents the first-year annulus and 

 that the fish is assumed to be one 

 year old. The contr ist in lengths at 

 age between otolith-determined ages 

 and Gorbunova's data has long vexed 

 fisheries scientists. 



The purpose of this study is to 

 determine whether Atka mackerel 

 form an annulus during the first 

 year of life. Annuli are also referred 

 to as translucent zones (or dark 

 zones), observed on the otolith 

 when viewed with reflected light. 

 To accomplish this, we 1) identified 

 the season of translucent-zone for- 

 mation for Atka mackerel; 2) com- 

 pared characteristics between 

 otoliths from spring-captured Atka 

 mackerel larvae and otoliths ob- 

 served with at least one translucent 

 zone; and 3) examined average sea- 

 sonal lengths of Atka mackerel dur- 

 ing the first four years of life. 



Methods 



Seasonality of translucent- 

 zone formation 



A monthly series of Atka mackerel 

 otoliths was selected from a collec- 

 tion maintained by the Age and 

 Growth Unit at the Alaska Fisher- 

 ies Science Center (AFSC). Young 

 fish otoliths (observed with 2 to 3 

 translucent zones) were chosen to 

 minimize affects of age-related fac- 

 tors on the marginal increment for- 

 mation. A lack of fall and winter 

 collections limited our samples to 

 19 or 20 otoliths per month. Oto- 

 liths meeting our criteria of less 

 than four translucent zones were 

 available from all months except 

 November and December. 



The sample was assigned to an 

 age reader to determine the mar- 

 ginal increment stage of each 

 otolith. Otoliths were prepared by 

 snapping each one along the dor- 

 sal-ventral plane and by passing 

 the broken surface over a flame. 

 The burnt surface was examined 

 with a dissecting microscope and 

 illuminated by reflected light. Mar- 

 ginal increment determination was 

 made along the ventral proximal 

 edge of the broken surface at 50x 

 magnification. 



The stages of marginal increment 

 development were defined as fol- 

 lows: stage 1 = a translucent zone 

 is forming along the otolith growth 

 margin; stage 2 = area of opaque 

 growth between the otolith growth 



McDermott, S. F., and S. A. Lowe. 

 1995. The reproductive cycle and sexual 

 maturity of Atka mackerel (Pleuro- 

 grammus monopterygius) in Alaskan 

 waters. Alaska Fisheries Science Center, 

 Resource Ecology and Fisheries Manage- 

 ment, 7600 Sand Point Way NE, Seattle, 

 WA 98115. Unpubl. manuscr., 30 p. 



Manuscript accepted 8 September 1995. 

 Fishery Bulletin 94:163-169(1996). 



