Spatial and Temporal Variation in Habitat Use and Feeding Ecology Across Ontogeny of an Invasive Fish

The early life stage of animals shapes later survival and fitness, yet this period remains poorly understood for many fish species. We examined how early juvenile round gobies Neogobius melanostomus, a highly invasive benthic fish, use habitat and food resources during development. We quantified spatial and temporal distribution patterns of early juveniles (<25 mm standard length, SL) through field sampling and then analysed diet composition and feeding-related morphology across size classes in the laboratory. We documented a developmental progression involving shifts in habitat use, foraging behaviour, and functional morphology with smaller juveniles (10.79 ± 0.32 mm SL) using pelagic zones and surface waters at night and consuming a broad array of zooplankton prey. Larger juveniles (23.62 ± 0.23 mm SL) increasingly moved to benthic habitats and a more specialized diet comprised in part by benthic macroinvertebrates. Morphological shifts in mouth and eye position aligned with this transition toward benthic foraging. Together, these results reveal integrated behavioural, morphological, and trophic changes during early development that likely reduce intraspecific competition and enhance establishment success in novel environments.