256 



Abstract.— Sablefish, Anoploponia 

 fimbria, are especially difficult to age. 

 Ages are typically determined by 

 counting growth zones that are as- 

 sumed to be annuli on the burnt cross 

 section of an otolith. We evaluated the 

 accuracy of ageing methods by com- 

 paring the ages obtained from two ex- 

 perienced age readers with those ages 

 that were known. A mark-recapture ex- 

 periment on sablefish provided a 

 relatively large sample (n=49) of 2-9 

 year old sablefish. This sample of 

 known-age fish provided a unique op- 

 portunity to evaluate the accuracy of 

 ageing methods for young sablefish. 

 Our study generally confirmed the 

 criteria used to age young sablefish. 

 According to the assignment of ages 

 from two experienced age readers, 

 about two-thirds of the fish were 

 misaged. Of these, most fish (81% and 

 71% for the two readers) were misaged 

 by only one year After reexamination 

 of the otoliths, most of the discrepan- 

 cies between reader age and known age 

 could be resolved. Ageing-error matrices 

 that define the probability of assigning 

 an age to a fish of a given true age were 

 estimated from among-reader varia- 

 bility and from comparison of known 

 ages to reader ages. Estimates of ageing 

 errors, based on comparison of known 

 ages to reader ages, were considerably 

 higher than estimates obtained from 

 among-reader variability. We recom- 

 mend that ageing-error corrections ap- 

 plied in stock assessment models be 

 based on the ageing-error matrix de- 

 rived from known and reader ages. 



Age validation and analysis of ageing error 

 from marked and recaptured sablefish, 

 Anoplopoma fimbria 



Jonathan Heifetz 



Auke Bay Laboratory, Alaska Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 11305 Glacier Highway, Juneau, Alaska 99801 8626 

 E-mail address |on tieifetz a noaa gov 



Delsa Anderl 



Resource Ecology and Fisheries Management, Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA, 



7600 Sand Point Way NE, Seattle, Washington 98115-0070 



Nancy E. Maloney 



Thomas L. Rutecki 



Auke Bay Laboratory, Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



11305 Glacier Highway, Juneau, Alaska 99801-8626 



Manuscript accepted .5 May 1998. 

 Fish. Bull. 97:2.'56-263 ( 1998 1. 



Errors in fish ageing may result in 

 biases in stock assessments and 

 possible mismanagement of fish- 

 eries. Estimates of natural mor- 

 tality, age composition, growth pa- 

 rameters, and maturity sched- 

 ules of a fish population all depend 

 on accurate ageing for their relia- 

 bility. Thus, valid ageing is a key to 

 understanding the biology and dy- 

 namics offish populations. 



Ageing error in stock assessment 

 models can lead to errors in mod- 

 eling results (Kimura, 1990). Age- 

 ing errors tend to smooth differ- 

 ences in year-class strengths and 

 may confound attempts to estimate 

 stock recruitment relationships and 

 to relate year-class strength to 

 environmental factors (Fournier 

 and Archibald, 1982; Richards et al., 

 1992). Estimation of ageing errors 

 enables such errors to be accounted 

 for by allowing observed ages to be 

 converted back to correct ages 

 (Richards et al., 1992). If mis- 

 specified or unaccounted for, ageing 

 error can lead to erroneous vield 



projections and overfishing ( Lai and 

 Gunderson, 1987;Tyler etal., 1989). 

 Sablefish, aged as old as 63 years 

 in Alaska (Sigler et al., 1997), is an 

 especially difficult species to age 

 (Kimura and Lyons, 1991). Beamish 

 and Chilton (1982) first suggested 

 that sablefish was a much longer- 

 lived species than previously thought 

 and proposed an ageing method 

 using broken and burnt otoliths 

 that would result in more accurate 

 ages. Preliminary data from their 

 tagging studies, otoliths marked 

 with oxytetracycline (OTC), and a 

 small sample (;;=6) of known-age 

 fish appeared to validate this new 

 method. Results from subsequent 

 studies with recaptured OTC- 

 marked fish (Beamish et al., 1983; 

 McFarlane and Beamish, 1995), a 

 small sample of pen-reared fish 

 (Lai, 1985), and radiometric ageing 

 (Kastelle et al., 1994) have further 

 substantiated the reliability of this 

 method. This break-and-bum method 

 is now widely accepted by sablefish 

 age readers. Sablefish is still a very 



