Distinguishing Between Natural and Hatchery Snake River Fall Chinook Salmon Subyearlings in the Field Using Body Morphology

We used body morphology to distinguish between natural-origin and hatchery-origin subyearling fall Chinook salmon Oncorhynchus tshawytscha in rearing areas of the Snake River and at a downstream dam during seaward migration.   Field personnel, using subjective eye and body shape characteristics, correctly classified 89.9-100% of natural subyearlings (N = 626) and 90.0-100% of hatchery subyearlings (N = 867) in rearing areas from 2001 to 2008.  The morphological characteristics used by field personnel proved to have a quantitative basis as shown by digital photography and principal-components analysis.  Natural subyearlings had smaller eyes and pupils, smaller heads, deeper bodies, and shorter peduncles than their hatchery counterparts during rearing and at the dam.  A discriminant function fitted from this set of morphological characteristics classified origin of fish during rearing and at the dam with over 97% accuracy.  We hypothesize these morphological differences were primarily due to environmental influences during incubation and rearing because it is highly probable that a large portion of the natural juveniles we studied were the offspring of hatchery × hatchery mating in the wild.  Findings in this paper might provide guidance to others seeking to differentiate between natural and hatchery fishes.