The integrated reproductive response: the scope for seasonal phenotypic flexibility shapes reproductive outcomes across fish populations

Across north‐temperate environments, rapid environmental change has disrupted the seasonal life histories of resident organisms, decreasing the reliability of environmental cues. Flexible responses capable of buffering the effects of unpredictable environmental variation to defend performance and reproductive outcomes may thus be key to species‐ and population‐level resilience. Yet, our existing understanding of how seasonally flexible responses shape ecological outcomes across populations remains limited. Here, we regularly sampled wild fish populations with putatively different life history strategies from early winter to late summer to quantify how seasonal foraging and phenotypic flexibility across a range of somatic tissue types (gastrointestinal, mesenteric, hepatic) contributed differentially to reproductive outcomes. In all populations, reproductive preparation was coordinated through flexible increases in foraging, mesenteric lipid, gastrointestinal mass, and liver mass and quality, which occurred relatively simultaneously as an ‘integrated reproductive response.' Then, the seasonal scope for phenotypic flexibility, directly or indirectly, mediated reproductive allocation across populations. Individuals from populations that exhibited a greater scope for phenotypic flexibility, especially in the gastrointestinal tract, allocated more to reproduction and tended to develop higher quality reproductive tissue. Our results suggest that the scope for phenotypic flexibility, especially in the digestive tract, modulates energy allocation and conservation in response to environmental variability.