About Gymnocephalus cernua (Linnaeus, 1758)
The ruffe (Gymnocephalus cernua) has coloring and patterning similar to the walleye, with an olive-brown to golden-brown back, paler sides, and yellowish white undersides. It can grow up to 25 cm (9.8 in) long, but most individuals are around half that maximum size. Relative to its small size, the ruffe is very aggressive. It has a large, spiny dorsal fin that is likely distasteful to its predators. There are two dorsal fins on its back: the front fin has hard, sharp spines, while the back fin has soft spines called rays. The clearest identifying features of the ruffe are its large, continuous dorsal fin and slightly downturned mouth. This species is native to the basins of the Caspian, Black, Aral, Baltic, and North Sea. It is also native to Great Britain, parts of Scandinavia, and regions of the Arctic Ocean basin extending eastward to the Kolyma River drainage. It has been introduced to parts of Western Europe including France and northern Italy, to Greece, and to the North American Great Lakes. The ruffe has a very high reproductive rate. Most ruffe reach maturity in two to three years, but individuals living in warmer waters can become mature and reproduce in their first year of life. A single female can lay between 130,000 and 200,000 eggs each year. To spawn, ruffe leave the deep, dark waters they normally prefer and travel to warmer shallow water. The main spawning season runs from mid-April through approximately June. Ruffe eggs typically measure 0.34 to 1.3 mm (0.013 to 0.051 in) in diameter, with egg size varying based on the size of the female. If a single female produces a second batch of eggs in the same spawning season, these eggs will be smaller than the eggs from her first batch. First batch eggs range from 0.90 to 1.213 mm (0.0354 to 0.0478 in), while second batch eggs measure 0.36 to 0.47 mm (0.014 to 0.019 in). When a female lays two batches in one season, the first is usually produced in late winter or early spring, and the second in late summer. Eggs hatch 5 to 12 days after laying when kept at temperatures between 10–15 °C (50–59 °F). After hatching, newly hatched embryonic juvenile ruffe measure between 3.5 and 4.4 mm (0.14 and 0.17 in) long. Freshly hatched embryos stay sedentary for 3 to 7 days, during which time they grow to around 4.5 to 5.0 mm (0.18 to 0.20 in) in length. One week after hatching, young ruffe begin to actively swim and feed, and they do not form schools at this young age. Ruffe then grow gradually until they reach full maturity at 2 to 3 years old. Full-grown adult ruffe usually reach around 20 cm (7.9 in) in total length, with a maximum recorded length of 29 cm (11 in). Ruffe grow faster in clear, brackish waters. On average, both male and female ruffe generally do not live longer than 7 to 11 years.
