Morphologically, this species is very similar to the roach (Rutilus rutilus), with which it can be easily confused. It can be identified by the yellow eye colour. The eye of the roach has a big red spot above the pupil, that can be more or less conspicuous. The rudd has an upturned mouth allowing it to feed easily at the top of the water. The placement of the dorsal fin is more to the rear which is even visible in very young fish. There are normally only one or two scales between the tip of the pelvic fins and the anal fins, while on the roach there are five. Also the skin of the rudd is yellowish green, while the roach is bluish on the flanks. Also the upturned mouth is visible even in young fish. Furthermore the rudd's number of soft rays in the dorsal fin (8–9 compared to 10–12). There can be confusion with the ide also, which has smaller scales however.
There is a variety of the common rudd, popular among pond-keepers, called the golden rudd, which has a gold-coloured body.
The rudd can grow to a size of about 45-50 cm with an average of about 25 cm.
Rudd prefer clear waters rich in plants. They also feed on aquatic vegetation when the temperature exceeds 18°C. They hunt for living prey in the upper levels. They prefer mesotrophic waters, while the roach is sometimes found together with the perch in waters that are nutrient poor.
- Chadderton, W. Lindsay. "Management of invasive freshwater fish: striking the right balance!". Hamilton, NZ: Department of Conservation. Retrieved 10 May 2010.
- "Scardinius erythrophthalmus". Integrated Taxonomic Information System. Retrieved 19 March 2006.
- Froese, Rainer and Pauly, Daniel, eds. (2006). "Scardinius erythrophthalmus" in FishBase. April 2006 version.
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