By Robert Burland, Alberta Environmental Protection, Lethbridge
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Aquatic plant growth causes major operational problems in Alberta's irrigation canals, restricting flows and reducing canal capacity. In 1987, an inter-agency committee consisting of representatives from the federal and provincial governments and from the Alberta irrigation districts was formed to evaluate the feasibility of using grass carp to control aquatic plants in the province's irrigation canals.
Grass carp are not similar to the common carp, which obtains its food by rooting in the bottom sediments, causing increased water turbidity. The grass carp is an open-water feeder, spending most of its time in the upper half of the water column. It does not stir up sediments because it grazes vegetation from the leaves downward.
Grass carp can survive a wide range of water temperatures. This is an important factor in northern climates because water temperature influences the amount of plant material it consumes. Under about 10šC, feeding is irregular or absent. Above 18šC feeding greatly increases until about 22šC, when it can consume its body weight per day.
In the spring of 1989, 1000 grass carp were shipped from Vegreville to dugouts in the Raymond area of southern Alberta. Individual dugouts received differing amounts of fish so that observations could be made on the effects of different stocking densities. Observations were also made on the grass carp's preference for individual plant species.
The grass carp were kept in dugouts over winter and were introduced into an irrigation canal in 1990. Efficacy and fish behaviour studies in the irrigation canal and in dugouts continued through 1992. Other aspects of the study included assessments of risks and environmental effects of the grass carp, surveys of aquatic plant infestations in southern Alberta irrigation districts, studies on fish barriers for canals, and monitoring of predation on grass carp.
Findings of the grass carp study are as follows: _ Grass carp will survive on a diet of aquatic plants under Alberta conditions. _ Grass carp are effective in controlling aquatic plants that are common in Alberta. _ Effective stocking densities for grass carp in irrigation canals appear to be in the range of 50 to 100 kg of fish per hectare. _ Grass carp survive southern Alberta winters quite well when in dugouts under ice cover. Survival ability over winter is as good if not better than that of rainbow trout. _ Stocked grass carp are subject to predation by Northern pike and by piscivorous birds such as cormorants. Precautions need to be taken to prevent predation if small fish are stocked.
Readers should be aware that the use of grass carp is still at a research stage. Grass carp cannot be legally introduced into canals or other surface waters in the prairie region.
For further information concerning grass carp research in Alberta, contact: Duncan Lloyd, Chairman, Committee on Biological Control of Aquatic Vegetation, c/o Alberta Agriculture, Food, and Rural Development, Agriculture Centre, Jail Road, Lethbridge, Alberta, T1K 4C7, Phone:(403) 381-5539. Robert Burland, Alberta Environmental Protection, Room 245, Provincial Bldg., 200-5th Ave. South, Lethbridge, Alberta, T1J 4C7.
