Differences in diet-induced flexibility in morphology and growth in a partially migratory species

Partial migration, in which some individuals of a population migrate while other individuals remain resident, is generally associated with ontogenetic shifts to better feeding or as a response to adversity, but its underlying mechanisms remain relatively unknown. Brown trout (Salmo trutta) exhibit partial migration, with some individuals remaining in fresh water (freshwater-resident) while others undertake an anadromous migration, gain most of their adult size at sea, and then return to fresh water to spawn. The option adopted by an individual trout is thought to be partly determined by its growth performance in early life, which in the stochastic and dynamic environment of freshwater streams may be dependent on its flexibility. To examine potential effects of parent type on phenotypic flexibility, we measured the metabolism, growth, and morphology of full-sibling groups of offspring from freshwater-resident and anadromous parents both before and after a switch in diet. We found that fry had a higher growth rate and a more rounded head and body shape when reared on chironomid larvae compared with when they were reared on Daphnia, but diet had no effect on standard metabolic rate. Interestingly, offspring of anadromous parents were less able to maintain their growth rate when fed on Daphnia than were those of freshwater-residents and showed a correspondingly greater increase in growth following a switch from Daphnia to chironomid larvae. Offspring of anadromous parents also showed less morphological flexibility in response to diet than did the offspring of freshwater-residents. We discuss how the migration history of the parents might interact with phenotypic flexibility in early life to influence the migration probability of the offspring