The effects of light intensity and water clarity on growth rates of age-0 walleye (Sander vitreus)

Growth rates in fish are a function of temperature and may also be dependent on successful foraging periods. Successful foraging of visually mediated piscivores is often contingent on available light. In north-temperate lakes, tannin concentrations can absorb downwelling light leading to lower light intensities which may influence fish foraging success and growth. We tested tannin and light intensity effects on wild age-0 walleye Sander vitreus growth, which may have implications for survivorship and recruitment. Age-0 walleye (127–182 mm TL) were held under three light intensities (3.16 × 10¹⁴, 3.16 × 10¹⁵, and 2.88 × 10¹⁷ photons m⁻² s⁻¹) and tannin concentrations (0 mg/L, 10 mg/L, and 20 mg/L). Walleye were grown at 18 °C for 16 days and TL (mm) and weight (g) were measured at days 0, 8, and 16. The number of fathead minnow (Pimephales promelas; 31–42 mm total length) consumed was recorded each day. Age-0 walleye growth was positive, ranging from 0.0092 g g⁻¹ day⁻¹ at 9.44 × 10¹⁵ photons m⁻² s⁻¹ (∼750 lx surface intensity) in clear water to 0.0223 g g⁻¹ day⁻¹ at 1.58 × 10¹⁵ photons m⁻² s⁻¹ (∼10 lx surface intensity) in heavily stained water. Growth was greatest in low light conditions and heavily stained waters. Our results suggest that heavily stained lakes and rivers with good optical refuge where light intensity is reduced at depth may be best for juvenile walleye growth.