Onset of Melanophore Patterns in the Head Region of Chinook Salmon: A Natural Marker for the Reidentification of Individual Fish

We used pattern recognition algorithms and image processing to identify individual Chinook salmon Oncorhynchus tshawytscha . Using melanophore spot patterns located on the dorsal head region, algorithms ranked all database images against each other. We coupled this technology with a graphical user interface to visually confirm or reject top-ranked algorithm results and tested this process on 295 juvenile Chinook salmon in seven photo sessions over a 251-d period. Juveniles began developing spots, identifiable in photo images, between 167 and 197 d after conception (52.7-mm fork length [FL]). Unique spot patterns appeared 197–232 d from conception, beginning at approximately 104-mm FL. Of 254 fish surviving the experimental period, 106 (42%) demonstrated identifiable patterns, 102 (40%) developed spots but patterns were insufficient for identification, and 46 (18%) exhibited a complete lack of spots. Spot patterns continued developing on individual fish by study end. On average, fish that developed recognizable spot patterns did so at approximately 140-mm FL. Once they did, reidentification was 100% correct in up to four subsequent trials. Patterns remained identifiable even after a 25–32% size increase over a 55-d period and as juveniles went through smoltification. Although patterns occurred at sizes typically larger than salmon observed at some California Central Valley monitoring locations, this technique provides a potentially valuable, noninvasive method of identifying individual salmon during emigration. Improved image collection techniques and use of body areas exhibiting identifiable patterns at earlier developmental stages may increase fish available for pattern identification. These results demonstrate the indexing of a large database using pattern recognition algorithms for Chinook salmon. The utility of such an approach may be valuable for addressing specific biological questions associated with mass-produced (wild and hatchery), migratory salmonids, especially as individuals develop, grow, and move through the various habitats available to them.