Expert topic: Grass carp
by Rebecca Sherratt, Features Editor, International Aquafeed
Grass carp (Ctenopharyngodon idella), also known as white amur, is a freshwater fish that is reportedly the second-most farmed fish (after silver carp) in the aquaculture industry, with five million tonnes-per-year produced on average globally. Native to Eastern Asia, grass carp can also be found in northern Vietnam and on the Siberian-Chinese border. Due to their popularity grass carp can now be found almost everywhere, having been introduced to the US, UK, Japan, the Philippines, the Netherlands, Switzerland, Poland and Germany, just to name a few. Grass carp can live an average of five-to-nine years, but some have been said to live up to fifteen years old in Silver Lake, Washington, US.
This species can be distinguished by its slightly chubby body and firm lips, as well as dark olive colour that can shift to brownish-yellow on the sides. Their bellies are a paler white colour. Grass carp dorsal fins have between 8-10 soft rays and its anal fin is closer to the tail, when compared to most cyprinids. The grass carp has ridged, pharyngeal teeth that enable it to break down tough plants.
Alongside aquaculture, the grass carp has also been brought over to the west for control of aquatic weeds, thanks to its herbivorous diet. The grass carp eat three times their body weight daily, ensuring that they effectively maintain weeds in bodies of water, whilst also growing big and meaty as a result. Grass carp thrive in backwaters, ponds and small lakes that are brimming with dense aquatic vegetation but may also sometimes ingest insects and detritus (dead particulate organic materials).
10,000 tonnes of grass carp were farmed in 1950, which increased significantly into 100,000 tonnes by 1972. In 2002, 3,419,593 tonnes were farmed, 95.7 percent of this total number gathered from China alone. In 2016, this number again increased to 6,068,014 tonnes.
Breeding grass carp
One key advantage of the grass carp is that is grows especially quickly and has relatively low protein requirements in their diet. Young fish, stocked in the spring, usually at 20cm in size, are capable of reaching over 45cm by autumn. The average length of a fully-grown grass carp is between 60-100cm and the largest grass carp on record is two metres long. They can also grow to become up to 45kg heavy.
In the wild, grass carp are semi-migratory fish and will spawn in fast-moving rivers. They can reach sexual maturity under culture conditions but will be unable to spawn naturally. In their natural climate their eggs float downstream, developing as they follow the flow of the water. Their eggs are slightly heavier than the water in order to carry along the current with ease, but it is said that the eggs will die if they sink to the very bottom of the river.
In culture conditions, eggs must be spawned via introduction of certain environmental stimuli, such as fast-flowing water in tanks, or hormone injections.
Despite their introduction to a wide variety of different countries around the world, it has been noted in various studies that grass carp populations struggle to establish themselves properly outside of Asia. This has led scientists to believe that the grass carp has rather specific reproductive requirements, that are specific to the conditions prevalent in most Asian countries. Under culture conditions, grass carp can accept artificial feeds such as grain by-products and pellets.
Farming grass carp began back in the Tang dynasty in China (618-904AD), after the current emperor of the time discovered that the pronunciation of his name was the same as that of common carp. Once this fact was discovered, farming of the common carp in China was instantly forbidden and so farmers turned to grass carp as a substitute.
Grass carp are notoriously difficult to catch, as they are especially wary fish and grow very large in size. Many fishers try to catch grass carp with rod and reel, which can prove effective when corn or tomatoes are used for chumming.
For larger farming projects, grass carp can be farmed through semi-intensive and intensive culture ponds, as well as through pens and cages in open waters. Broodstocks are typically maintained through collected seeds from the wild or breeding stations. Mature grass carp are placed into spawning tanks after being injected with hormones, most typically LRH-A.
Once eggs are laid, they are transferred to jars or hatching raceways of depths between 0.8-1 metre and widths of 0.8m. The inlets are mounted on the bottom of the raceways with openings at an angle of 15° to promote water circulation. Current flow is consistently maintained during the hatching period to keep both eggs and larvae suspended.
In the nursery stage the stocking density varies between 1.2-1.5 million-per-hectare. In China this stage takes between two-to-three weeks. Soybean milk can be used as a fertiliser to replace organic fertiliser. Fingerlings reach 30mm in length after about two-to-three weeks of rearing.
At this stage, a process called conditioning takes place wherein holding the fish through netting at high density for several hours is required before the fish are then transferred to the fingerling pond. This process enables the fish to better manage higher levels of stress, thereby making them more tolerant and less likely to induce mortalities.
It is especially important that fingerling grass carp are fed well. They are usually fed with Wolffia arrhiza when they are between 30-70mm in length, followed by Lemna minor when they reach 70-100mm. After this, they can be fed with tender aquatic vegetation. Survival rate through the fingerling process usually exceeds 95 percent.
Polyculture often takes place in China, wherein grass carp may be stocked with other species, up until they reach a size of 125-150g. In Chinese intensive culture systems cages are typically 60m2 with a depth of between 2-2.5m. Silver and bighead carp may also be present in the cages, to function as cleaner fish. Feeding efficiency tends to yield better results in pond culture rather than through cage culture, where vegetation is more abundant in quantity. For grass carp to be fed mostly aquatic weeds in cage culture more labour is required on part of the fish farmers.
One significant challenge when farming grass carp is that they are especially susceptible to a variety of diseases. Some of the few diseases that can infect grass carp include gill-rot, erythroderma, haemorrhagic disease, septicaemia, ichthyophthiriasis and bothriocephalosis.
Their susceptibility to diseases has been raised as a potential issue, leading to excessive drug and antibiotic use in the species, which can result in various negative effects for both fish and end-consumers. Various regulations have been put into place by governments to minimise the amounts of antibiotics permittable for use in grass carp.
Another potential issue when farming grass carps is that they tend to leave a significant amount of waste when eating pelleted feeds, leading to potential strain on the local environment. The unutilised leftover feed, as well as the discharged waste produced by the fish, can cause rapid eutrophication.
To combat this issue, the water and plants in water inhabited by grass carp must be regularly monitored. It also helps to maintain the natural diet of grass carp wherever possible, keeping them strictly to plant-based diets. Reasonable stocking densities and strictly monitored feeding management are also steps that can help to minimise risk of eutrophication.
A delicious treat
Grass carp has white, flaky meat that tastes mild and not especially fishy, making it a good choice of meal for those who aren't especially big fish lovers. Grass carp are often steamed or pan fried, but connoisseurs will know that you should not cook the meat for any longer that 15 minutes, as a rapid boil or extended cooking can cause the meat to flake apart. This fish can be served in a variety of ways, which helps cement its status as one of the most widely farmed fish in the aquaculture sector.