Use of a Bar Code Symbology to Produce Multiple Thermally Induced Otolith Marks

Abstract

From 1987 to 1993, the Washington Department of Fisheries mass-marked over 35 million juvenile salmonids by inducing unique banding patterns into their otoliths by means of short-term water temperature changes during incubation. All five species of Pacific salmon Oncorhynchus spp. were successfully marked, and several broods of returning adults have demonstrated the persistence of these marks for at least 5 years. Using systematic rules developed in the bar code industry and a marking pattern of six bands and five spaces, we created a method for producing up to 10 different bar codes for marking the otoliths of incubating salmonids. Multiple groups (N) of six bands can produce 10N different codes. Ten unique patterns were decoded, both visually and with the aid of a computerized image-analysis system combined with a decoding algorithm. Repeated blind tests showed that pattern recognition was virtually error free with both methods. Because the symbology is based upon the relative spacing of bands, the pattern can be discerned along any clear otolith axis; a consistent otolith measurement axis or section plane is not required. As a result, relatively gross, rapid otolith preparation procedures were adequate for accurate pattern recognition. Although the decoding algorithm may result in a failure to decode, the likelihood of misclassifications is small due to restrictive rules in the bar code symbology. Many research and management applications of thermal mass-marking require the use of a number of unique patterns. This bar code method provides a simple means of organizing information so that multiple unique patterns are easily distinguished. Although demonstrated on chinook salmon Oncorhynchus tshawytscha, this method is applicable to any species for which thermal marking of otoliths is appropriate.