ONTOGENIC PATTERNS OF RELATIVE GROWTH IN YOUNG ROACH RUTILUS-RUTILUS - WITHIN-RIVER BASIN COMPARISONS

To address the lack of information on the ecomorphology of young of th e year juveniles (0+) freshwater fishes, despite its importance to und erstanding geographical variation in life history, we compared the mor phology of 0+ juvenile roach Rutilus rutilus from eight sub-regions of the River Great Ouse basin (U.K.). We tested for variation due to wat er retention structures, which could act to isolate upstream stocks fr om the general population. Differences in mensural characters between the eight different regions appeared in ventral fin length and eye dia meter only, which both were strongly affected by variable relative gro wth during early juvenile development; thus the discrepancies between regions were not due to geographical location. Morphological developme nt in roach greater than or equal to 15 mm standard length (SL) consis ts of two different intervals: during the first (approximate to 15-40 mm SL), in contrast to adults, several mensural characters continue to develop allometrically, either positively or negatively, suggesting a continuance of the larval period. Whereas, in the second interval (> approximate to 40 mm SL), all the mensural characters except head leng th and eye diameter grew close to isometrically with body length, and the rate of variation in relative growth became as strongly limited as that previously reported for adults. In contrast to previous studies, which are based on few specimens, we suggest this second interval to be the true start of the juvenile period.