Low light inhibits native fish movement through a vertical‐slot fishway: Implications for engineering design

Light intensity within a vertical‐slot fishway was manipulated to determine the effect on fish movement. Three treatments (darkness, low light, artificial light) were tested with natural daylight used as a control. Light intensity varied from 0 to 1,692 lux for the three treatments and from 1 to 4,550 lux for the control. Light intensity outside the fishway ranged from 31 to 80 900 lux. A total of 64 385 fish were collected from six species. The abundance of Australian smelt Retropinna semoni (Webber), unspecked hardyhead Craterocephalus stercusmuscarum fulvus Ivantsoff, Crowley and Allen, bony herring Nematalosa erebi (Günther), carp gudgeon Hypseleotris spp. and Eastern gambusia Gambusia holbrooki (Girard) moving upstream reduced significantly under low‐light conditions. Conversely, movement of macroinvertebrates (freshwater shrimp Macrobrachium australiense Holthuis and freshwater prawn Paratya australiensis Kemp) increased at low‐light intensities. The number of fish moving under artificial light (28 617) was similar to that under natural daylight (33 919). Movements of Australian freshwater fish and macroinvertebrates were found to be influenced by changes in light intensity. Instream structures that alter light conditions, such as road culverts, may thus act as behavioural barriers to fish movement, and this could be mitigated by the provision of natural or artificial light.