The underlying mechanisms responsible for ecological plasticity and consequent invasive character of non‐native freshwater fish species, variations in growth and life history traits in gibel carp Carassius gibelio (Bloch) were compared in natural and artificial water bodies of Turkey. Females significantly outnumbered males in all natural and most artificial waters. Discriminate function analysis differentiated gibel populations into three separate groups (natural lakes, artificial water bodies and running waters), with significant differences among separated groups in growth index, standard length and age at maturity, relative fecundity and gonado‐somatic index, but not in egg diameter and both generalised and relative condition. Growth features (e.g. growth index and relative condition) and reproductive features (e.g. relative and absolute fecundity) positively correlated with water body area. No correlations were found for any growth or life history trait with depth, latitude and altitude. With the exception of smaller size at maturity, all traits were higher in populations from artificial water bodies than those inhabiting running waters, suggesting gibel carp is required to exert more reproductive effort to invade natural ecosystems than artificial waters.

Gibel carp, Carassius gibelio (Bloch), impacts on native fish species have been reported but little studied despite a long history of introductions in Europe. This species is able to reproduce gynogenetically, which involves the use of sperm from males of other species to activate egg development, so reproductive competition is a likely but virtually unstudied impact of gibel carp on native fishes. This study evaluates the impact of introduced C. gibelio on the population biology of native fishes over a 6‐year period in a mesotrophic drinking water reservoir in north‐western Turkey. A dramatic decrease in the relative density (i.e. catch per unit effort) of native species correlated significantly with an increase in C. gibelio relative density. Growth characteristics (back‐calculated ages, growth index and relative condition) and length at maturity did not differ significantly among years in C. gibelio and native fishes. Relative density, duration of spawning, reproductive effort and gonado‐somatic index of C. gibelio increased with some water quality variables [total phosphorus (TP); chlorophyll‐a (Chl‐a)] and coincided with decreasing trends for natives. However, TP and Chl‐a were not correlated with growth features in C. gibelio or natives fishes. The results suggest that the decline in the reservoir’s native cyprinid populations is likely due to a combination of degrading environmental conditions and a disparity in reproductive effort, with introduced C. gibelio invasion facilitated by gynogenetic reproduction and an observed interference with native fishes during spawning.

Summary Length–weight relationships were calculated for nine freshwater fish species belonging to two families using 1020 specimens from 15 water sources in the western part of Anatolia, Turkey. This study is the first reference on length–parameters for these species, eight of which are endemic, and with new maximum length records for three of the species.

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Length-weight relationships are given on six freshwater fish species (Phoxinus phoxinus, Cobitis vardarensis, Neogobius gymnotrachelus, Proterorhinus marmoratus, Petroleuciscus borysthenicus and Gambusia holbrooki) from four streams flowing into Lake Sapanca, north-western Turkey.