Evolutionary Applications ISSN 1752-4571 ORIGINAL ARTICLE Changes in selection and evolutionary responses in migratory brown trout following the construction of a fish ladder Thrond Oddvar Haugen,1,2 Per Aass,3 Nils Christian Stenseth1 and Leif Asbjørn Vøllestad1 1 Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, Oslo, Norway 2 Norwegian Institute for Water Research (NIVA), Oslo, Norway 3 Zoological Museum, The Natural History Museums and Botanical Garden, University of Oslo, Oslo, Norway Keywords Abstract contemporary evolution, evolutionary rates, human-induced evolution, life-history Brown trout (Salmo trutta) are extensively harvested and its habitat highly evolution, migration, Salmo trutta. influenced by human encroachments. Using a 40-year time series of mark– recapture data we estimate vital rates for a piscivorous trout population. This Correspondence population spawns upstream of a waterfall, which historically acted as a migra- T. O. Haugen, Norwegian Institute for Water tion barrier for smaller trout. In 1966, the waterfall was dammed and a fish Research (NIVA), Gaustadalle´ en 21, NO–0349 ladder constructed. All fish ascending the fish ladder were individually tagged Oslo, Norway. Tel.: 4722185100; fax: 4722185200; e-mail:
[email protected] and measured for a variety of traits. The fish ladder overall favoured access to upstream spawning areas for middle-sized trout, resulting in stabilizing selec- Received: 12 November 2007 tion acting on size at spawning. Over time, natural and fishing mortality have Accepted: 3 February 2008 varied, with fishing mortality generally decreasing and natural mortality increasing. The average and, particularly, variance in size-at-first-spawning, and doi:10.1111/j.1752-4571.2008.00031.x growth rates during the first years of lake residence have all decreased over the 1966–2003 period.