Growth Decline in Subyearling Brook Trout (Salvelinus fontinalis) after Emergence of Rainbow Trout (Salmo gairdneri)

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Abstract
In a tributary of Lake Superior, brook trout (Salvelinus fontinalis) subyearlings, after emergence in April, initially gained length at 0.51 mm/d and weight at a specific growth rate per day (G) of 0.058. After the emergence of larval rainbow trout (Salmo gairdneri) in June, however, length increments of brook trout declined to 0.05 mm/d and G was reduced to 0.005. During August, brook trout growth rebounded somewhat (0.12 mm/d; G = 0.008). Rainbow trout subyearlings, by contrast, grew at relatively constant rates of 0.42 mm/d and G of 0.040, from emergence until September. By late August, mean weights of brook and rainbow trout, 2026 and 2282 mg, respectively, did not differ significantly. Stream positions occupied by brook trout, characterized by flows of less than 20 cm/s and depths of less than 40 cm, did not change over the first summer. During June and July, stream positions of rainbow trout subyearlings coincided with those of brook trout. During August, however, rainbow trout moved to faster waters (>20 cm/s). The taxonomic composition of the diets of the two species did not differ. Mean width of food items consumed was linearly correlated with the total length of the fish (r = 0.97). The range of sizes of items eaten by the two species overlapped considerably by mid-June. For brook trout, mean food size increased, and amount consumed decreased, after the emergence of rainbow trout larvae. Growth reduction during the first summer, an outcome of interspecific competition for food and space, may result in increased overwintering mortality of fish at high latitudes, and be a mechanism by which brook trout are excluded by rainbow trout.

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