Scale Characteristics of Yearling Coastal Cutthroat and Steelhead Trout

Abstract

Some scale characteristics of 88 yearling coastal cutthroat trout (Salmo clarki clarki) and 96 yearling steelhead (Salmo gairdneri) were compared in some detail. Although the number of diagonal scale rows above the lateral line was significantly higher in cutthroat, the counts of the two species overlap. The correlation of scale circuli counts with scale diameter was 0.94 for steelhead and 0.90 for cutthroat. At any given scale size cutthroat had 8 per cent more circuli than steelhead scales. Measurements of lateral and longitudinal diameters and anterior radii of scales show that the anterior portions of cutthroat scales tend to be longer and narrower than those of steelhead.Regression analyses of each scale measurement on fork length indicated that, for any given fork length, cutthroat scales are smaller than those of steelhead; the greatest difference being apparent when anterior radii are compared. Growth of scales in a dorsoventral plane approximates isauxesis in relation to fork length but scale growth in a longitudinal plane tends toward positive heterauxesis which is particularly pronounced in the anterior portions of scales.Multiple regression analyses of scale diameter, scale row count and fork length indicated that the smaller size of cutthroat scales is not balanced by proportionate increase in their number. When scale diameter is adjusted for scale number and fork length, cutthroat scales are 15.6 per cent smaller than those of steelhead, indicating that young cutthroat are covered by their scales with less overlap.