Latin name

Ctenopharyngodon idella

Other names

White amur, amur, carp; French: carpe amour, carpe herbivore, amour blanc; German: graskarpfen; Japanese: sogyo.

Identification

The grass carp has an elongated and rather compressed body, a broad and blunt head, a very short snout without the barbs characteristic of common carp, a short dorsal fin and a moderately forked tail. The forehead is very wide. The mouth is halfway down. The pharyngeal teeth are serrated, with a longitudinal groove on the chewing surface, double rowed. The terminal and unbranched mouth has thin lips and sharp pharyngeal (throat) teeth, particularly suited to its feeding habits. Grass carp are covered with coarse scales. The scales on the upper side of the body have a dark border and a black spot at the base. The back is grey or green, turning white or yellow on the belly, with transparent or dark fins.

Distribution

Originally found in China and eastern Siberia in the Amur River basin, the grass carp has been introduced into more than 20 countries. Only in a few areas has it succeeded or been allowed to reproduce naturally. These include the Danube River in central Europe, the Mississippi River in North America, Russia and southern Africa. In the United States, it was first stocked in reservoirs in Arkansas in 1963 and deliberately released in 35 states, subsequently spreading to other reservoirs where it was unwanted. Many states have banned grass carp from breeding within their borders without the permission of the relevant fisheries management agency.

Habitat

The freshwater grass carp inhabits lakes, ponds, pools and backwaters of large rivers, preferring slow-flowing or standing water with vegetation. In rivers, it remains in deeper water during the day and emerges at night to feed in floodplains and spillways. Grass carp can tolerate temperature fluctuations, extreme salinity and low oxygen levels.

Size

Grass carp grow fast and reach large sizes. In local waters some reach up to 100 pounds. In favourable conditions it can gain 3 to 5 pounds a year. The largest fish caught on a rod and reel was an Arkansas specimen weighing 68lbs 12oz. Growth is fastest in the first 4-5 years of life (8-10cm per year), then (by the 8th year) growth slows to 6-7cm and in the 9th year drops sharply to 2-5cm and remains relatively low in subsequent years. It grows much faster in Chinese rivers than in the Amur basin, weighing 18-80 g in the first year, 500-1500 g in the second year and 1500-3000 g in the third year. Acclimatised in a number of areas, it grows differently depending on climatic conditions, the fastest in the south and in cooling water bodies.

Life history and Behavior

Spawning takes place once a year in the gravel beds of rivers between April and September, depending on the temperature. Adults migrate upstream in search of suitable spawning habitat. The round, semi-oval, amber-coloured eggs hatch without parental protection after 24-30 hours. After absorbing nutrients in the yolk sacs during the first 2-4 days of life, the larvae feed on micro-plankton in still waters. For the winter, juveniles hide in deep burrows in riverbeds.

Food and feeding habits

Predominantly vegetarian. Grass carp earn their name by feeding on aquatic plants and underwater grasses, sometimes adding insects or invertebrates. They use the teeth on their throats to tear off vegetation with jerky movements. Unlike common carp, it does not pollute the water with its food, but its aggressive feeding habits cause other problems. Grass carp tend to break off the tops of grasses, leaving the roots to grow, so they don't destroy as much vegetation. They are also unable to digest all the plant material they ingest, so rather than solving the vegetation problem, they exacerbate it by discarding plant material and spreading it to new areas. They also contribute to increased water turbidity and eutrophication. Finally, heavy paddling can stimulate faster than normal growth of certain plant species. As adults, herbivorous fish feed on shredded higher vegetation. When the water level in the river is high, it feeds mainly on surface vegetation inundated by floodwaters. At low water it switches to aquatic mosses. Eats small amounts of small animal organisms.

Reproduction

In the rivers of central and southern China, spawning occurs in spring and summer from April to mid-August, with the largest mass spawning occurring from late May to mid-June at temperatures of 17-27°C, usually 20-24°C. In the lower reaches of the Middle Amur, the lower reaches of the Ussuri, and the upper reaches of the Lower Amur, spawning occurs in June-July. The length of the spawning season is due to the simultaneous maturation of females and batch spawning. Spawning takes place mainly in the evenings and mornings in places with a fast stream when the water level is high. Spawning stops when the water level drops. In the Amur, where the water level varies from year to year and on which the reproduction of grass carp is largely dependent, in unfavourable years only part of the eggs hatch and some females miss the spawning season. The average fecundity is 800 thousand eggs. Eggs are 2-2.5 mm in diameter before hatching and swell to 5-6 mm in the water. Eggs are semi-pelagic, slightly heavier than water. They sink to the bottom of stagnant or slow-moving waters, and are carried by currents in fast moving waters. The embryos hatch in 34-40 hours at a water temperature of 22-26°C. The dissolution of the yolk stops on the sixth day. On the eighth day, the larvae switch completely to external feeding.