Evidence for energy reallocation, not oxygen limitation, driving the deceleration in growth of adult fish.

The lifetime growth of almost all fishes follows a biphasic relationship - juvenile growth is rapid and adult growth subsequently decelerates. For a trend that is so ubiquitous, there is no general agreement as to the underlying mechanisms causing adult growth to decelerate. Ongoing theories argue that adult growth slows because either the gills fail to supply the body with surplus oxygen needed for continued somatic gain (i.e. oxygen limited), or sexual maturation induces a switch in energy allocation towards reproduction and away from growth (i.e. energy limited). Here, we empirically tested these notions by tracking the individual growth trajectories of ∼100 female Galaxias maculatus, ranging in size, during their first 3 months of adulthood. At a summer temperature of 20°C, we provided subsets of fish with additional energy (fed once versus twice a day), supplementary oxygen (normoxia versus hyperoxia), or a combination of the two, to assess whether we could change the trajectory of adult growth. We found that growth improved marginally with additional energy, yet remained unaffected by supplementary oxygen, thereby providing evidence for a role for energy reallocation in the deceleration of adult growth. Interestingly, additional dietary energy had a disproportionately larger effect on the growth of fish that matured at a greater size, revealing size-dependent variance in energy acquisition and/or allocation budgets at summer temperatures. Overall, these findings contribute towards understanding the mechanisms driving widespread declines in the body size of fish with climate warming.