Abstract
By common usage rather than by strict definition a parr is understood to be a young salmonid with vertical dark stripes, and a smolt is a slightly older and larger fish in which the dark stripes are obscured by a deposition of guanine in the scales and the skin, giving a silvery color. Although parr and smolt remain satisfactory terms for general usage, they are not sufficiently precise to describe the normal genetically based developmental variants in salmon, and certainly they cannot specify or describe the non-synchronous development of young salmon subjected to experimentally altered environments (e.g., special diets, hormonal manipulation, photoperiod, temperature, salinity). To aid in more detailed description of developmental variants in the parr-smolt transformation a study was made of externally measurable features that change significantly during this period. In addition to pigmentation and length, the following items proved to be useful indices of differentiation: emergence and growth of teeth on the maxilla, mandible and tongue; growth and changes in shape of integumentary folds adjacent to the cloacal opening; growth and change in shape of the auxillary appendage of the pelvic fin; and growth in the scales with respect to radius and number of circuli. These additional features appear to reflect chronological age more faithfully than does integumentary pigmentation. In further studies of experimentally altered parr and smolt, and in normal genetic variants, these features, when related (or not) to differentiating physiological and biochemical properties, should greatly amplify and improve the precision of descriptions of developing salmonids.
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