First record of smoltified rainbow trout Oncorhynchus mykiss (Walbaum, 1792) in the main riverbed of the Serbian part of the Danube River

Abstract

Several fish species in the family Salmonidae have evolved multiple life history forms that exploit different degrees of anadromy (McMillan, Pess, & Katz, 2007) and one of these is rainbow trout (Oncorhynchus mykiss). In some cases, anadromous O. mykiss undergo smoltification and migrate, however those migrations are completely within freshwater (Holecek, Scarnecchia, & Miller, 2012). Silvering of juvenile salmonids as a part of smoltification is a key determinant of migratory behavior (Haner, Faler, Schrock, Rondorf, & Maule, 1995). The silvering of the anadromous trout, rainbow trout and salmons, which functions as cryptic colorations in open water (Björnsson, Stefansson, & McCormick, 2011), is the change that occurs during the process of smoltification. However, biological changes that take place during smoltification do not preclude continued life in fresh water (Björnsson, Einarsdottir, & Power, 2012). Spawning of rainbow trout in Serbia has never been recorded. In recreational fishing, the rainbow trout has some significance only in those rivers where aquaculture facilities are located (Simonović, 2001). However, they are occasionally caught in the Danube (M. Nikčević, M., personal observation) and are assumed to be escapees from aquaculture. This study reports on the occurrence of smoltification in rainbow trout O. mykiss along the main bed of the Danube River in its Serbian section.