SHIFTS IN MORPHOMETRICS AND THEIR RELATION TO HYDRODYNAMIC POTENTIAL AND HABITAT USE DURING GRAYLING ONTOGENY

The theory of saltatory ontogeny predicts sudden morphological shifts during fish development which often occur simultaneously with physiolo gical, anatomical or behavioural shifts. Therefore, our objectives wen to identify potential sudden morphological shifts during grayling Thy mallus thymallus ontogenesis and to check if these shifts affected the hydrodynamic potential of grayling in periods of known shifts in habi tat use. In the 144 grayling studied (total length: 14.9-142.7 mm), th ree of 28 morphological variables considered revealed saltatory events during ontogenesis: position of maximal body height, position of. max imal body width, and length to pelvic fin insertion. Using all variabl es(except total and standard length), Eve morphological groups were se parated by multivariate analysis and each group had a particular hydro dynamic potential related to the drag of flow. Between these five diff erent morphological groups, differences in the physical habitat use we re observed. Basically, the saltatory pattern during grayling ontogene sis corresponded to a sequential habitat use that increased flow expos ure and, simultaneously to a stepwise improvement (at each habitat shi ft) of the hydrodynamic potential until the body had a shape that mini mized drag of flow. The study confirms the theory of saltatory ontogen y for grayling in a context of low physics and hydrodynamic adaptation s. (C) 1997 The Fisheries Society of the British Isles.