Recognition of cohorts and growth of larval and juvenile roach Rutilus rutilus (L.), using size-class ordination of developmental steps

Roach larvae and 0+ juveniles were sampled weekly in three different floodplain channels (stagnant, semi-lotic, lotic) of the Upper Rhône River, France, from spring to autumn 1985 using Point Abundance Sampling by electrofishing. Length-frequency analysis by size-class ordination of developmental steps successfully distinguished both overlapping and non-overlapping histograms within the data matrix and revealed the temporal and spatial segregation of a number of cohorts, individuals born at a common site on the same day, which in some cases presented significant differences in standard length within a given developmental step. Despite these initial differences, the various cohorts converged to form one relatively homogeneous cohort around the moment of transition to the juvenile period. Significant differences in standard length were found between populations in the open lotic channel (relatively cooler) and those in the closed, stagnant (relatively warmer) former channel, suggesting that fish growth in isolated ecosystems is influenced by higher or periodically elevated temperatures and by greater levels of resource competition and predation. A similar pattern of differences in length was also observed in 0+ juvenile roach collected from 18 other floodplain channels of the Upper Rhône.