Focal Scale Damage Among Chum Salmon (Oncorhynchus keta) of Hokkaido, Japan

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Abstract
Fifty percent of chum salmon (Oncorhynchus keta) sampled from seven hatcheries in Hokkaido, Japan, were found to have either damage to the osseous scale layer where a hole resulted or intrusion of the osseous layer into the underlying fibrillary plate. Damage resulting in a hole is not related to age, body size, sex, or hatchery stock size. Intrusion of the osseous layer is similarly not related to body size, sex, or stock size; however, this phenomenon may be related to age. All damage was confined to the first, and rarely extended into the second, annual growth zone, suggesting occurrence during that period and retention of the mark throughout the life cycle. No evidence was found to support the hypothesis that either type of focal scale damage is a result of reduced diet. Examination of historical records from one hatchery revealed that the phenomenon of a hole has occurred for at least the past 10 yr at that location. It is not presently possible to identify hatchery of origin using these traits; however, such permanent, uniformly occurring traits may be of use in the identification of chum salmon originating from Hokkaido.

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