Protecting larval fish at water intakes: hydraulic and biological evidence for the effectiveness of modern fish-protection screens.

Water intakes entrain large numbers of fish larvae in waterways where drift coincides with large-scale extraction. While modern fish-protection screens can reduce these losses, many are not designed for larvae and were developed or evaluated primarily for juveniles and adults. This study evaluated the effectiveness of Australia's fish screen design criteria (which specify a maximum approach velocity of 0.1 m s-¹ and slot widths of 2-3 mm) for protecting drifting larval Murray cod (Maccullochella peelii). Larvae were tested in a large flume under combinations of approach velocity (0.1 or 0.2 m s-¹), slot width (2 or 3 mm), and proximity. Entrainment rose sharply with velocity; slot size had a smaller interactive effect. The most protective combination (0.1 m s-¹ and 2 mm) reduced entrainment by up to 94% relative to unscreened conditions. Three-dimensional flow measurements helped explain how velocity vectors interact to influence larval fate. The results demonstrate that Australia's current standards, although developed for juveniles, can provide strong larval protection when strictly followed, but that even modest departures can sharply increase risk. More broadly, since the criteria tested here are less conservative than those adopted in many other countries, where empirical evidence on larval behaviour does not exist, targeted research could determine whether existing guidelines warrant revision.