MAKING AQUATIC WEEDS USEFUL

Grass carp feeding on aquatic weed. The fish can feed only on the ends of plants, as the teeth used to macerate the plant tissues are located in the throat. (Weed Research Organization)

4.5 kg per year in tropical regions can be attained. Weight increases up to 6 g per day have been measured for grass carp feeding on hydrilla in Florida.•

With this rapid growth, grass carp quickly consume stands of aquatic weeds. Small fish, less than 1.2 kg, may eat several times their body weight of plant material daily. Large fish, under favorable conditions, consume their body weight daily. The digestive tract is short, only 2-3 times the body length. Only about 65 percent of the plant material eaten is digested. The remainder passes through undigested and is excreted as dense pellets. In experimental ponds, SO kg of fresh hydrilla was consumed for each kilogram of fish produced. In the absence of aquatic weeds, grass carp can be fed on terrestrial plants; in Malaysia it is common practice in the intensive culture of grass carp to feed them napier grass.

Weed control is rapidly achieved if more than 75 fish per hectare are present. Lower stocking rates reduce the rate of weed control, but result in larger, more valuable food fish. Quite apart from weed control, the grass carp can be cultured for food. Yields of 164 kg of fish per hectare in temper~te

•in reviewing this chapter Lee Chan Lui (see Research Contacts) reported that "In Malacca it has been conclusively established that grass carp can grow from fingerlings (20 g) to 2-2.5 kg in 6 months. Once this size is reached the fish can grow approximately 1 kg per month for the next 6 months, i.e., the fish can reach a size of 8-8.5 kg in a year. It's without doubt one of the fastest (if not the fastest) growing fish in the world."

GRASS CARP 17 zones and 1,SOO kg per hectare in tropical weed-infested waters have been obtained. This is usually done in polyculture in which several different fish species are grown together, each living off a different food in the pond. This is amply described in a recent book.*

Grass carp may be harvested from lakes or streams using gill nets, trammel nets, hoop nets, and seines. They are very agile and frequently leap over capture nets. They can also be caught with fish traps, hooks baited with vegetation, or the fish poison rotenone.

Limitations

When introduced into a new country, grass ^carp should be initially maintained in quarantine, and only released when known to be free of serious pest parasites and diseases. They should also be tested to determine any adverse effects they might have on native aquatic animal life. During the first few weeks after hatching, grass carp are not herbivorous. They eat zooplankton and other microscopic animals. It is not until the fry become fingerlings that they begin feeding on filamentous algae and small stems of tender plants. In some parts of the United States and other countries, such as New Zealand, it has been argued that the fry might eat the eggs or fry of native or sport fish, such as trout and bass, and reduce their populations. As of late 1975, however, there was little evidence that this occurs. Nonetheless, some governments have objected to the introduction of grass carp because they fear that it may spawn excessively and adversely affect recreational fishing for trout, bass, and other sport fish. Projects are now under way in Florida and in New Zealand to test for such adverse effects. Among researchers it is generally believed that the fears are unjustified.t

Research into grass carp genetics has recently led to methods for producing fish of the same sex. Although at the present time these are more expensive to produce than fish of mixed sexes, when introducing grass carp to new countries and waterways it is far safer to use them. Since no reproduction can occur, the fish will die out after one generation. This permits a trial introduction: if judged undesirable, the fish can be allowed to die out;

if judged desirable, new stocks can be added.;

In most waterways outside its native China the grass carp has failed to spawn naturally; however, it did spawn at least once after introduction into

*Bardach, et al. 1972. (See Selected Readings.)

tSome have reported that grass carp benefit other fish by removing weeds and recycling nutrients, but one paper from the Societ Union [Vinogradov and Zolotova. 1974. (See Selected Readings)) records that the grass carp seriously reduced fish production in fish farming, and enhanced the growth of undesirable plants.

:j:lnformation supplied by J. Stanley. (See Research Contacts.)

18 MAKING AQUATIC WEEDS USEFUL

The "Bois d'Arc," Arkansas, USA, was filled with submerged weeds (Ceratophyllum demenum) when grass carp were added in 1971. ...

the Volga, Di, Terek, Arnu-Darya, and Kubon Rivers, as well as in the Kara Kum Canal in the Soviet Union, the Tone River in Japan and the Balsas River and Bodegas Lake system in Mexico. To date, these are the documented cases of the grass carp's reproducing naturally outside China. It therefore seems unlikely that the fish will spawn extensively and thereby become a pest.

Until recently, in fact, researchers reported difficulty in spawning the grass carp artificially. Now, however, the techniques have been developed and, at least in Arkansas where several million fry are produced each year, artificial spawning is routine.

Grass carp fmgerlings must be carefully protected during rearing since they are vulnerable to predators and heavy losses can occur.

As a food f1Sh, the grass carp has a disadvantage: Although it has fewer bones than many edible fish, it does have intramuscular bones and must be filleted skillfully.

Grass carp grow poorly on plants that are low in nutrients. They also eat little and their growth rate slows in waters where the oxygen content is low;

nevertheless, they can survive highly eutrophic waters. Furthermore, they grow poorly when the water temperature drops below 10°C.

GRASS CARP 19

... Three years later the same lake is weed-free. (W. M. Bailey)

Once released into large waterways grass carp, because they eat aquatic vegetation, are not easy to catch by usual hook-and-line techniques.

Even when released into warm, placid waters where they are unlikely to reproduce, the upstream and downstream regions must also be considered, for if contiguous waters approximate the grass carp's natural habitat, the fish may be able to reproduce there.

Although water hyacinth, salvinia, and other problem weeds that float are sometimes eaten, they are not preferred, and the grass carp is not a satisfactory agent for controlling them. Neither is it good for controlling tall, hard, emergent aquatic weeds, such as cattails-though if the plants are deliberately cut off below the water line, the grass carp will kill them by eating all new shoots as they form.