AGE DETERMINATION OF PACIFIC COD, GADUS MACROCEPHALUS , 



USING FIVE AGEING METHODS^ 



Han-Lin Lai,^ Donald R. Gunderson,' and Loh Lee Low" 



ABSTRACT 



A comparative study of age determination methods for Pacific cod, Gadus macrocephalus , was carried 

 out using dorsal and pectoral fin rays, scales, otoliths, and coracoids. A preliminary validation using 

 the modal length of a strong year class confirmed that sections of dorsal fin rays were the most reliable 

 ageing method. A Monte Carlo method was developed for converting scale ages to dorsal fin-ray ages. 

 An analysis by log-linear model was developed for testing the effects of ageing method and age class 

 on repeatability of age reading. 



Scales have been widely used for ageing Pacific 

 cod, Gadus macrocephalus , in the North Pacific 

 since Kennedy (1970) developed the method for 

 fish in Hecate Strait, British Columbia. However, 

 Bakkala (1981)^ found that the scale method may 

 not be an appropriate ageing method since the 

 estimated ages from scales do not appropriately 

 reflect the progress of known year classes in the 

 eastern Bering Sea. Beamish et al. (1978) also 

 found that Kennedy's criteria might not be satis- 

 factory for scales from juvenile Pacific cod in 

 Canadian waters. 



Beamish (1981) reported that thin sections of 

 fin rays can be reliably aged and might be more 

 accurate than scale ages when ageing older fish. 

 However, Westrheim and Shaw (1982) encoun- 

 tered difficulties with fin-ray cross sections and 

 reported that fin-ray ages were younger than 

 scale ages. They also discovered false checks on 

 the scales during the first year of life, which fitted 

 the annulus criteria of Kennedy (1970), and vali- 

 dated the scale ageing method for age-1 and -2 

 Pacific cod. Chilton and Beamish (1982) noted 

 problems associated with scales and fin rays, and 



^Contribution No. 726, School of Fisheries, University of 

 Washington, Seattle, WA. 



^School of Fisheries, WH-10, University of Washington, Seat- 

 tle, WA 98195; present address: Center for Quantitative Sci- 

 ences, HR-20, University of Washington, Seattle, WA 98195. 



^School of Fisheries, WH-10, University of Washington, Seat- 

 tle, WA 98195. 



''Northwest and Alaska Fisheries Center, National Marine 

 Fisheries Service, NOAA, 7600 Sand Point Way N.E., BIN 

 C15700, Seattle, WA 98115. 



^Bakkala, R. G. 1981. Pacific cod of the eastern Bering 

 Sea. Document submitted to the annual meeting of the Inter- 

 national North Pacific Fisheries Commission, Vancouver, BC, 

 Canada, October 1981, 32 p. Available at the Northwest and 

 Alaska Fisheries Center, NMFS, NOAA, Seattle, WA 98115. 



Manuscript accepted August 1987. 

 FISHERY BULLETIN: VOL. 85, NO. 4, 1987. 



recommended a judicious mixture of scale ages, 

 fin-ray ages and length-frequency analysis for 

 ageing Pacific cod in Canadian waters. 



In earlier studies, Mosher (1954), Moiseev 

 (1953), and Ketchen (1970) reported that the 

 otolith surface ageing method was not satis- 

 factory for Pacific cod. Ketchen (1970) also had 

 no success with ageing of vertebrae, or opercula. 

 Lai (1985) reported that age determination from 

 the bony tissues in the gillcover, scapula, and 

 cleithrum was not feasible. 



This paper reports the results of a comparative 

 study and preliminary validation of age determi- 

 nation methods for Pacific cod in the eastern 

 Bering Sea using dorsal and pectoral fin rays, 

 scales, otoliths and coracoids. In addition, we de- 

 velop a method to convert scale ages to dorsal 

 fin-ray ages. This age conversion method makes 

 it possible to use existing ages estimated from 

 scales. 



MATERIALS AND METHODS 



Dorsal and pectoral fin rays, otoliths, and 

 scales were taken from 230 Pacific cod collected 

 from foreign fishing vessels operating in the east- 

 ern Bering Sea during September 1983 to March 

 1984. Among these samples, coracoids were also 

 taken from 101 fish. 



Dorsal and pectoral fin-ray sections were pre- 

 pared using an Isomet low-speed saw (Lai 1985), 

 and the annuli were identified by the criteria of 

 Beamish (1981) and Chilton and Beamish (1982). 

 Scale images were made by acetate plate (Koo 

 1962) and then read by a microfisch reader. An- 

 nuli on scales were identified by the methods de- 



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