153 



Abstract — Two examples of indirect 

 validation are described for age-read- 

 ing methods of Pacific cod [Gadus 

 macrocephalus). Aging criteria that 

 exclude faint translucent zones 

 (checks) in counts of annuli and cri- 

 teria that include faint zones were 

 both tested. Otoliths from marked 

 and recaptured fish were used to 

 back-calculate the length of each 

 fish at the time of its release by 

 using measurements of the area of 

 annuli. Estimated fish size at time 

 of release and actual observed fish 

 size were similar, supporting the 

 assumption that translucent zones 

 are laid down on an annual basis. A 

 second method for validating read- 

 ing criteria used otolith age and von 

 Bertalanffy parameters, estimated 

 from the tagging data, to predict 

 how much each fish grew in length 

 after tagging. We found that otolith 

 aging criteria applied to otoliths from 

 tagged and recovered Pacific cod pre- 

 dicted quite accurately the growth 

 increments that we observed in these 

 specimens. These results provide fur- 

 ther evidence that the current aging 

 criteria are not underestimating the 

 age of the fish and support our cur- 

 rent interpretation of checks (i.e., as 

 subannual marks). We expect these 

 indirect validations to advance age 

 determination for Pacific cod, which 

 in turn would enhance development 

 of stock assessment methods based 

 on age structure for this species in 

 the eastern Bering Sea. 



Indirect validation of the age-reading method 



for Pacific cod (Gadus macrocephalus) 



using otoliths from marked and recaptured fish 



Nancy E. Roberson 



Daniel K. Kimura 



National Marine Fisheries Service, NOAA 



Alaska Fisheries Science Center 



7600 Sand Point Way, NE 



Seattle, Washington 98115 



E-mail address (for N E Roberson): Nancy Roberson (a 1 noaa gov 



Donald R. Gunderson 



University ol Washington 



School of Aquatic and Fishery Sciences 



1122 NE. Boat Street 



Seattle, Washington 98105 



Allen M. Shimada 



National Marine Fisheries Service, NOAA 



Office of Research 



1315 East- West Hwy 



Silver Spring, Maryland 20910 3282 



Manuscript submitted 7 November 2002 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



20 September 2004 by the Scientific Editor. 



Fish. Bull. 103:153-160 (2005). 



Pacific cod (Gadus macrocephalus 

 Tilesius, 1810) is an important spe- 

 cies in eastern Bering Sea commercial 

 fisheries and is second only to walleye 

 pollock (Theragra chalcogramma) in 

 landings (Thompson and Dorn 1 ). It is 

 also considered to be one of the most 

 difficult of all commercially impor- 

 tant Alaska groundfish species to age. 

 Scientists from both Canada and the 

 United States have experienced simi- 

 lar difficulties in finding an appro- 

 priate aging structure, which can 

 be consistently interpreted to track 

 large year classes of cod through time. 

 Historically, scales and otoliths have 

 been the two most common struc- 

 tures used for determining the ages 

 of fish species. Unfortunately, age- 

 readers employing these structures 

 have experienced limited success in 

 the case of Pacific cod (Kimura and 

 Lyons, 1990). 



The Pacific Biological Station in 

 Canada stopped aging Pacific cod 

 in 1978, after age estimates derived 

 from scale readings began yielding 



year classes that were inconsistent 

 with length-frequency time series 

 from field surveys (Westrheim and 

 Shaw, 1982). The Alaska Fisheries 

 Science Center's (AFSC) Resource 

 Ecology and Fisheries Management 

 (REFM) Division is responsible for 

 stock assessment of Pacific cod in the 

 Gulf of Alaska and eastern Bering 

 Sea. The REFM Division's Age and 

 Growth Program used scales for de- 

 termining the age of Pacific cod from 

 1976 to the early 1980s. Thereafter, 

 the program used the break-and-burn 

 method with otoliths to age Pacific 



Thompson, G. G., and M. W. Dorn. 

 1999. Pacific cod. In Stock assessment 

 and fishery evaluation report for the 

 groundfish resources for the Bering Sea/ 

 Aleutian Islands regions (plan team for 

 groundfish fisheries of the Bering Sea/ 

 Aleutian Islands), p. 151-205. North 

 Pacific Fishery Management Council, 

 605 W. 4 th Avenue Suite 306, Anchor- 

 age, AK 99501. 



