The behaviour of Pacific salmonid smolts during passage over two experimental weirs under light and dark conditions.

Little is known of how fish respond to the hydraulic environment associated with diversion or bypass structures at hydroelectric power installations. To address this lack of knowledge, this paper presents results from a study to assess how three species of Pacific salmonid smolt (Oncorhynchus spp.) responded to distinct gradients of velocity and depth associated with two submerged weirs as they passed through an experimental flume at McNary Dam (Columbia River, USA) under illuminated and dark conditions. Migrating smolts entered one of two available treatment channels as coherent schools from which individuals would either disassociate from the group and pass over the weirs, or would reject them by swimming upstream. Alternatively, fish maintained position at the upstream end of the flume by swimming into the flow. The response of smolts to velocity and depth gradient and light condition varied between species, and route of passage was influenced by fork length. Initial channel selection and school size was not influenced by weir type, although schools resided longer within the short-weir channel. The majority of smolts (70%) entered the treatment channels facing downstream (negative rheotaxis), but switched orientation at the crests of the weirs. This switch in orientation occurred farther downstream in the short-weir treatment and for the largest smolts. The variation in response of different species of smolts to hydraulic gradients has important implications for the design of screening mechanisms used at hydroelectric power installations to divert migrant juvenile salmonids. Published in 2006 by John Wiley & Sons, Ltd