Ecotypic differentiation of native rainbow trout (Oncorhynchus mykiss) populations from British Columbia

We sampled 34 native rainbow trout (Oncorhynchus mykiss) populations over a wide geographic area of British Columbia to determine whether variation in morphology is related to differences in habitat or fish community characteristics experienced by a population. After correcting for differences in body size, the most dramatic differences occurred in external characteristics between stream- and lake-dwelling populations. Rainbow trout from streams had more robust bodies with larger caudal peduncles and heads and longer paired fins than rainbow trout from lakes. Unlike other lake populations, piscivorous rainbow trout populations had relatively large heads and mouths that were more similar in size to those of stream-dwelling populations. We found fewer differences in the size of internal organs across ecotypes, producing little predictive ability of these characteristics to differentiate populations of rainbow trout. We also sampled 27 additional rainbow trout populations with unknown fish communities to determine how well large-scale patterns predict morphological differentiation over a smaller spatial scale. Of the five ecotypes that we detected at a smaller scale, the largest differences again occurred between stream and lake populations. Our data suggest that external morphological variation among populations of rainbow trout covaries with ecological conditions at a variety of spatial scales, particularly between lakes and rivers.