Movement Ecology of Early Juvenile Round Goby

Abstract

Movement plays a critical role in shaping survival, dispersal, and population dynamics of aquatic species. For invasive species, movement traits can determine both the likelihood of establishment and the pace of range expansion. The round goby (Neogobius melanostomus), one of the most successful aquatic invaders in the Great Lakes and European waters, has spread rapidly since its introduction in the early 1990s. Yet, despite increasing research attention, the traits that influence dispersal in its early life stages remain poorly understood. Here, I investigated how ontogeny, diel activity, and environmental cues influence movement, feeding ecology, and dispersal potential of early juvenile round goby (<25 mm SL). First, I quantified swimming capacity and swimming behaviour in very small juveniles, showing that despite their small body size, early juveniles exhibit considerable swimming capacity that may facilitate dispersal. Second, I examined diel vertical migration (DVM) in laboratory and field settings, providing the first evidence of round goby DVM in Lake Ontario. Early juveniles were observed at the water surface but only at night and similarly showed more vertical movement upwards in the dark in laboratory experiments. This upwards swimming behaviour likely increases their exposure to currents and human-mediated transport vectors, suggesting a mechanism for long-distance dispersal. Third, I characterized ontogenetic shifts in habitat use, diet, and morphology. Smaller juveniles found at the surface consumed mainly zooplankton, including pelagic taxa. In contrast, larger juveniles found on the substrate were primarily fed on a combination of zooplankton and benthic macroinvertebrates. Morphological changes, such as downward orientation of the mouth and eye positioning rotating dorsally, mirrored this pelagic-to-benthic transition. Together, the results of this thesis show that early juvenile round goby display complex movement and behavioural traits that may facilitate dispersal and survival. These findings provide a mechanistic framework for understanding how early life stages can contribute to population establishment and range expansion.