Growth, metabolism, anatomy, behaviour, invertebrate drift

We hypothesized that i) selection for larger size in piscivorous adults would select for higher juvenile growth at the cost of lower active metabolism; ii) elevated growth of piscivores would be supported by a greater productivity of their natal stream and more proactive foraging behaviours; and iii) general patterns of multivariate trait associations would match the predictions of the Pace-Of-Life Syndrome. Relative to insectivores, piscivorous fry showed a pattern of higher growth (+63%), maximum food intake (+33%), growth efficiency (+41%), and standard metabolic rate (SMR; +47%), but lower active metabolic capacity (maximum metabolic rate (MMR; -17%), aerobic scope (AS; -48%)), suggesting that faster piscivore growth is supported by greater food intake and digestive capacity but is traded-off against lower scope for active metabolism.

A similar trade-off appeared among organ systems, with piscivorous fry exhibiting an 83% greater investment in average mass of organs associated with food consumption and processing (i.e. stomach and intestine), but an apparently smaller relative investment in organs involved in cardio-vascular or cognitive activities (heart and brain, respectively). Higher invertebrate drift in their natal rearing habitat, quicker behavioural transition to a novel food source and lower anxiety after a frightening event in piscivorous fry suggest that faster growth requires both proactive foraging behaviours and higher prey availability in the environment.

Finally, the sampling of replicate insectivore populations confirmed their lower juvenile growth (-73% on average) and reduced environmental productivity of their natal streams (-45% lower drift abundance) relative to the piscivore ecotype. Our results suggest that selection for large adult body size influences selection on high juvenile growth, high basal metabolism and proactive behaviours, and that the intense phenotypic divergence between piscivorous and insectivorous rainbow trout may be constrained by environmental productivity.