Biological Weed Control in Alberta Using Triploid Grass Carp

Heavy, dense stands of submerged aquatic vegetation are a major problem in many Alberta pond and dugout environments. Controlling this vegetation using the weed-eating grass carp may be one answer to this problem.

The problems are particularly acute in the shallow, warm water environments of farm ponds during the summer months. Extensive weed growth frequently clogs intake pumps and filter systems, interferes with recreational activities, and causes water quality to deteriorate.

Water quality problems occur in the late summer and continue into the winter as dense weed growth decomposes. This decomposition leads to oxygen depletion and nutrient release, which stimulates microscopic algae that contribute to obnoxious tastes and odours.

Traditional methods of controlling excessive aquatic weed growth include mechanical harvesting (chaining, raking, weed cutters, hand harvesting) or the application of chemicals. Mechanical methods are often labour-intensive and costly. Chemical treatments are also expensive and can directly or indirectly harm other aquatic life and water quality.

Regional health authorities and agencies directly involved with the livestock industry have recently identified the quality of on-farm domestic and livestock dugout water supplies as a concern. When you consider that the majority of farm dugouts have multiple functions, the concern becomes significant. The same dugout may be used for domestic water, livestock watering and commercial or recreational pond fishery.

Biological control programs may provide promising alternatives to traditional methods of weed control. Species of insects, snails, fish and even aquatic mammals like sea manatees have been investigated worldwide for their potential as agents in controlling aquatic weeds. One of the most extensively studied biological control organisms is the weed-eating (herbivorous) fish known as the grass carp or Silver Amur.

Species Description

The grass carp, one of the largest members of the minnow family, is indigenous to large coastal rivers in Siberia (Amur River) and China that flow into the Pacific Ocean between latitudes 51° North and 23° North. The carp are also extensively cultured in China, Malaysia, Singapore, Borneo, Indonesia, Thailand, Taiwan, Hong Kong and the Philippines.

In some countries, the grass carp is an integral part of fish culture, and fish flesh forms an important source of protein for human consumption. In North America, the grass carp is currently licenced to control aquatic vegetation in irrigation canals, reservoirs, recreation lakes and farm ponds in Mexico and 37 American states.

Grass carp can be readily distinguished from the closely related common carp and some native species that may look like grass carp, such as suckers (see Figure 1). The back of the grass carp is dark grey, and the sides of the body are light with slightly gold sheen. The belly is scaled and dusky, yellowish green to silver white in colour. Fins are either greenish-grey or buff in colour. The fish has moderate to large scales with a dark brown base. Body shape is oblong with a rounded belly and broad head. Teeth have been replaced by specialized structures called pharyngeal teeth located in the back of the throat. These are used for tearing and grinding plant matter.


Figure 1. Triploid grass carp

By contrast, European or common carp have a laterally compressed, robust body (deep bodied from the back of the fish to the belly). Colouration is usually olive-green on the back of adult fish and yellowish on the belly area. Tail fins often have a reddish hue. Common carp are easily distinguished by the presence of barbels (whiskers) located at the corners of the mouth. It should be noted that common carp are benthic (bottom) foraging fish that muddy the water during feeding, whereas the grass carp "clip" vegetation near the base or at mid-water level.

Sucker species are either blackish or grayish on the back, with silver sides and bellies (white sucker) or dark-olive slate on the back and sides with white bellies (longnose sucker). Scales of these fish are smaller that the grass or European carp. Unlike the carp species, suckers have large lipped mouths located well below the head.

Grass Carp in Alberta

Grass carp were first introduced into southern Alberta (49 - 50° north latitude) in 1987 to research their use in the biological control of aquatic vegetation in irrigation canals. Later research was conducted on the control of aquatic vegetation in farm ponds (dugouts) and golf course environments.

Aquatic Weed Control

Grass carp have proven to be effective in controlling a large number of different aquatic weeds, including Chara (stonewort), water plantain, sago pondweed, Canada waterweed, and filamentous algae.

A common pond species called white water buttercup is least preferred by the fish and may only be uprooted when other, more palatable aquatic species are depleted. It appears that the young new buttercup growth is consumed by grass carp. The young growth of pond cattails, sedges and rushes may be eaten and ultimately controlled, over time, as older plants die back.

Fish Growth and Survival

The amount of vegetation consumed by grass carp is related to fish size, fish numbers, water temperatures, weed density and species composition, as well as the length of time the fish have been in the ponds. Smaller fish (25 - 40 cm) will consume more feed (35 - 50 per cent body weight per day) than larger fish (45 cm), 20 to 30 per cent body weight per day.

Dugout studies conducted more than two years in Southern Alberta found average weed control in areas with grass carp to be 73 - 80 per cent. In some ponds, because of fewer palatable aquatic plants, weed control was as low as 20 per cent. Fish survival over the summer months was high in both years of the study, with averages of all dugouts at 91 per cent in 1994 and 97 per cent in 1995. When fish were transferred to aerated overwintering dugouts, survival ranged from 82 to 100 per cent.

According to scientific literature and various tests conducted by many researchers, the amount of food consumed by grass carp is directly related to temperature. For example, at a water temperature of 13°C, grass carp can consume 5 per cent of their body weight per day while at temperatures of 18 to 25°C, grass carp consume 24 per cent of their body weight.

Alberta studies suggest that grass carp have optimal feeding temperatures of 18°C or higher, with moderate feeding activity between 13°C and 18°C and limited feeding below 13°C. In southern Alberta, grass carp in farm dugouts did feed on artificial rations (fish feed) during temperatures as low as 7°C.

Water temperatures in southern Alberta dugouts were within active feeding ranges for four months of the year (77 days of optimal >18°C and 43 days of sub-optimal between 13 - 18°C). In comparison, northern dugouts (Peace River) had active feeding temperatures for three and a half months of the year (46 days of optimal >18°C and 60 days of sub-optimal, between 13 - 18°C). This difference suggests that a cooler environment in the northern part of the Province will result in lower aquatic weed consumption rates by grass carp. However, it must be noted that pond water temperatures are also controlled by pond depth, wind exposure and water source.

Info obtained from Alberta Agriculture

  http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex3446