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Fishery Bulletin 103(1) 



cod (Thompson and Methot 2 ). In the otoliths of young 

 Pacific cod (under 6 years), there is a tendency for sub- 

 annual marks (also known as "checks") to be very dark 

 and evenly spaced, making them difficult to distinguish 

 from annuli. This confusion makes it difficult to age the 

 species to an exact age. 



From 1990 through 1992, the AFSC noticed that the 

 average length at a specific age was smaller than it 

 had been in previous years. The decrease was noticed 

 in ages 1-6 but was especially dramatic in 1-, 2-, and 

 3-year-olds. It is generally theorized that the shift was 

 the result of one of two scenarios: either the fish popu- 

 lation experienced an actual decrease in length-at-age 

 or the age readers were over-aging fish by counting 

 marks other than annuli. Unable to pinpoint the reason 

 for the shift and given the inherent difficulty of aging 

 cod, production (large-scale) aging of Pacific cod was 

 indefinitely suspended at the AFSC. 



Pacific cod stock assessments in Alaska have since 

 depended largely on length-frequency data alone to 

 model population age structure because of the difficul- 

 ties in obtaining age estimates (Thompson and Dorn 1 ). 

 However, the use of length-frequency data as proxies for 

 age data can be problematic. If external factors such as 

 ocean conditions affect somatic growth to such a degree 

 that length-at-age within the population is highly vari- 

 able, such as appears to be the case for Pacific cod, then 

 the population becomes difficult to model. Otoliths, on 

 the other hand, are permanent records of growth that 

 are more independent of external factors. 



Consequently, the Age and Growth Program initiated 

 a new study in 1998 to re-examine the otolith aging 

 structure for Pacific cod. This study used otoliths from 

 tagged Alaska Pacific cod to validate aging criteria for 

 otoliths. 



Methods 



Otoliths and length data were collected during a tag- 

 ging study conducted by the AFSC. Between 1982 and 

 1990, 12,396 Pacific cod were tagged and released in the 

 eastern Bering Sea during summer bottom-trawl surveys 

 (See Shimada and Kimura, 1994). Fish were measured 

 to the nearest 0.5 cm fork length, tagged with uniquely 

 marked spaghetti tags, and set free. Over a period of 

 13 years, commercial fishing vessels recaptured 375 

 (3%) of the tagged fish and returned otoliths from 112 

 fish (106 of which were usable) (Table 1). More details 

 on the tagging methods can be found in Shimada and 

 Kimura (1994). 



2 Thompson, G. G., and R. D. Methot. 1993. Pacific cod. 

 In Stock assessment and fishery evaluation report for the 

 groundfish resources for the Bering Sea/Aleutian Islands 

 regions as projected for 1994 (plan team for groundfish fish- 

 eries of the Bering Sea/Aleutian Islands), p. 2-28. North 

 Pacific Fishery Management Council, 605 W. 4 lh Avenue 

 Suite 306, Anchorage, AK 99501. 



Otolith preparation 



One sagittal otolith from each recaptured fish was 

 selected for our study. We did not discriminate between 

 left and right otoliths based on the results of Sakurai 

 and Hukuda (1984) who were unable to detect any con- 

 sistent differences between the weight and length of 

 right and left Pacific cod otoliths. 



Each otolith was cleaned and preserved in 95% etha- 

 nol. After having been preserved for approximately one 

 month, a line was penciled across the otolith center 

 from the dorsal apex to the ventral apex to ensure that 

 the otoliths would later be sectioned at the core. 



The otolith was then placed in a polyester mold and 

 set in black resin (Technovit 3040, Energy Beam Sci- 

 ences, Agawam, MA), forming a block of resin. A slow- 

 speed saw was used to cut the blocks in half. This pro- 

 duced two smaller blocks, each with an exposed view 

 of the otolith in the transverse plane and cut through 

 the center. One of the two blocks was selected and 

 glued (otolith side down) to a glass slide. The glass 

 slide was mounted to a Hillquist thin section machine 

 (Hillquist Inc., Fall City, WA) and the section was 

 ground down to a thickness of 0.25 mm. A coverslip 

 was permanently glued on the top of the section with 

 marine-grade epoxy. 



Sections were placed on a piece of black velvet (which 

 added contrast) on the stage of a 50x dissecting micro- 

 scope, and reflected light was used for illumination. 

 The sections were viewed on a computer monitor by 

 using a Cohu 6500 monochrome video camera, Integral 

 Flashpoint 128 frame grabber and Optimas 6.5 imaging 

 software (Media Cybernetics, Silver Spring, MD). 



Age-reading criteria 



Traditional qualitative aging criteria were used to distin- 

 guish annuli from checks. The criterion for identification 

 as an annulus was a continuous translucent band that 

 could be seen along the entire structure or as a ridge or 

 groove on the structure (Secor et al., 1995). Checks (i.e., 

 subannual marks) are translucent zones that appear 

 very similar to annuli. They were determined primarily 

 by the incompleteness of the zone around the entire sec- 

 tion, by zone darkness, and by spacing between zones. 

 When translucent zones could be classified as either 

 annuli or additional subannular marks, they were clas- 

 sified as checks. Annuli, checks, and edges (the space 

 between the last annulus and the edge of the otolith) 

 were traced by using the Optimas 6.5 software package 

 and measurements of their areas and major axis lengths 

 were collected (Fig. 1). All otoliths were read blind; that 

 is, information about fish length and date of capture 

 (Table 1) was withheld from the reader to prevent bias. 

 When all the otoliths had been aged and measured, 

 the age reader returned to each otolith section to es- 

 timate the area and length of the otolith when the 

 fish was tagged. This was accomplished by following a 

 two-step process. The first step was to approximate the 

 location of the otolith cross-section that corresponded to 



