Abstract-Otolith thermal marking 

 is an I'llii'it'nt method for mass mark- 

 ing hatehcry-rcared salmon and can 

 be used to estimate the proportion of 

 hatchery fish captured in a mixed-stock 

 fishery. Accuracy of the thermal pattern 

 classification depends on the promi- 

 nence of the pattern, the methods used 

 to prepare and view the patterns, and 

 the training and experience of the per- 

 sonnel who determine the presence or 

 absence of a particular pattern. Esti- 

 mating accuracy rates is problematic 

 when no secondary marking is avail- 

 able and no error-free standards exist. 

 Agreement measures, such as kappa 

 I K). provide a relative measure of the 

 reliability of the determinations when 

 independent readings by two readers 

 are available, but the magnitude of k 

 can be influenced by the proportion of 

 marked fish. If a third reader is used 

 or if two or more groups of paired read- 

 ings are examined, latent class models 

 can provide estimates of the error rates 

 of each reader. Applications of K and 

 latent class models are illustrated by 

 a program providing contribution esti- 

 mates of hatchery-reared chum and 

 sockeye salmon in Southeast Alaska. 



The use of agreement measures and 

 latent class models to assess the 

 reliability of classifying 

 thermally marked otoliths* 



D. James Blick 



Peter T. Hagen 



Alaska Department of Fish and Game 



Division of Commercial Fisheries 



10107 Bentwood Place 



Juneau, Alaska 99802-5526 



E mail address ((or P T Hagen, contact author) peter hagenmifishgame state ak us 



Manuscript accepted 16 April 2001. 

 Fish. Bull. 100:1-10(2002). 



The ability to induce patterns in salmon 

 otoliths by manipulating water temper- 

 atures has proved to be an efficient 

 means for marking large numbers of 

 salmon (Volket al., 1990). Wlien salmon 

 embryos or alevins are exposed to 

 a rapid drop in temperature, otolith 

 growth is temporarily disrupted, and 

 this results in a discontinuity in the 

 otolith "s microstructure. When viewed 

 under transmitted light microscopy, 

 this discontinuity appears as a dark 

 ring. By controlling the number of tem- 

 perature drops and the timing between 

 drops, a coded pattern of dark rings 

 can be recorded on the otolith and this 

 pattern can be recovered from otoliths 

 of older fish by removing the overlay- 

 ing material and exposing the otolith 

 core. For hatcheries that release a large 

 number of fish, this type of marking 

 method has shown to be particularly 

 cost effective for marking 100% of the 

 releases (Munk et al. 1993). 



Several fisheries management pro- 

 grams in Alaska use thermal marking 

 to estimate hatchery contributions to 

 commercial fisheries (Hagen et al., 

 1995). Typically, several hundred salm- 

 on otoliths are systematically collected 

 during each two- or three-day com- 

 mercial opening during the fishing sea- 

 son. The otoliths and sampling data 

 are shipped to a processing laboratory 

 where a subsample of otoliths (generally 

 50 to 100) are processed immediately 

 to meet in-season management needs; 

 a portion of the remaining otoliths are 

 processed later to provide an overall es- 

 timate of hatchery contribution to the 

 fisheries. 



The process by which a reader de- 

 termines the presence or absence of a 

 thermal mark in an otolith can be char- 

 acterized as one of pattern recognition 

 and image matching. Prior to examin- 

 ing otoliths of unknown origin, the read- 

 ers gain familiarity with the patterns 

 likely to be encountered by carefully 

 examining fry otoliths that were ob- 

 tained after thermal marking but prior 

 to their release into the wild. Because 

 there can be wide variation in the ap- 

 pearance of the thermal marks within 

 a mark group (due in part to differenc- 

 es in developmental stages at marking), 

 a single mark group may be represent- 

 ed by a variety of patterns. As a result, 

 secondary characteristics and measure- 

 ments of the patterns are sometimes 

 necessary to identify an otolith to a 

 mark group. The examination is also 

 used to confirm that all the hatchery 

 fish have been successfully marked. 



The process of making a determina- 

 tion on otoliths from returning adult 

 salmon can become problematic be- 

 cause wild salmon may also contain 

 otolith patterns that can mimic the fea- 

 tures imposed through thermal mark- 

 ing. Referred to as "noisy patterns," 

 their presence can increase the rate of 

 false positives. Conversely, if the hatch- 

 ery employs poor temperature control 

 or unintended disruptions occur around 

 the period of marking, it may be diffi- 

 cult to identify the otolith as that of a 



* Contribution PP-184 of the Alaska De- 

 partment of Fish and Game, Commercial 

 Fisheries Division, Juneau, Alaska 99802- 

 5526. 



