Uneven resources, uneven fish: zooplankton patchiness drives variability in fish growth and metabolism

Fine-scale prey patchiness is underexplored, yet theory predicts effects on trait variance. We tested these predictions in juvenile Scardinius erythrophthalmus reared for 46 days under homogeneous versus patchy distributions of live zooplankton prey, with prey inputs matched at session start to isolate spatial configuration from initial prey delivery. Under patchy supply, fish rapidly aggregated into prey-rich patches, creating strong foraging-rate (FR) contrasts between prey-rich and prey-poor locations; patchiness altered how FR scaled with prey density without a consistent increase in mean FR. Despite pronounced behavioural redistribution, cohort mean dry mass declined slightly under a near-maintenance ration, and endpoint central tendencies in body length and wet mass were similar between treatments. In contrast, standard metabolic rate (SMR), measured post-absorptively under standardized respirometry conditions, was higher in absolute terms in fish from the patchy treatment, while body-mass-corrected SMR overlapped strongly between treatments but showed increased inter-individual dispersion under patchiness. Patchiness also amplified dispersion in endpoint body-size traits, indicating greater physiological and developmental heterogeneity under spatially structured resource access. Together, these results show that prey patchiness can decouple foraging dynamics, baseline metabolic costs and body-size outcomes, with the dominant signature being increased within-cohort trait variance even when initial prey delivery is controlled.