Navigating turbulence: the effects of eddy size on the swimming performance of walleye (Sander vitreus) larvae

Walleye (Sander vitreus) populations experience substantial interannual fluctuations driven largely by high rates of larval mortality. To investigate the potential mechanisms underlying recruitment in walleye larvae, we assessed the effect of turbulence on larval swimming performance in a recirculating flow chamber. We measured the critical swimming speeds (Ucrit) of larvae throughout their first five weeks of development in response to increasing levels of turbulence and varying eddy sizes, generated through controlled water flow and the use of grid turbulence. As early as the first week post hatch, larvae exhibited a rheotactic response, demonstrating the ability to resist and swim against turbulent flows to some extent. Measured Ucrit increased with larval total length (LT; or time), and was lower in the grid-turbulence treatment in which both the turbulence and the size of eddies was constrained by the grid spacing. Conversely, the relative critical swimming speeds based on body length (Ucrit-rel) declined with LT; swimming performance declined significantly when the eddy diameter approached approximately two-thirds of the larvae's total length. This ratio declined with age in the No-Grid treatment, but was relatively constant in the Grid treatment. Our results suggest that the scale of turbulence, rather than the magnitude of turbulent energy, has a greater influence on swimming performance. These findings highlight the importance of considering eddy length scale when assessing the swimming performance of fish larvae. Additionally, the swimming parameters established in this study can inform more realistic larval dispersal models for walleye and inform fisheries management decisions.