Anglers believe that topping up ponds, lakes, and rivers, with artificially bred fish is a good thing, as it replaces the ones that they kill, and makes their chances of catching a fish so much easier. However, the British Environment Agency says that the practice is damaging.
Increasing numbers can reduce the diversity of water plants, and increase disease amongst fish.
Native salmon can lose out to farmed fish through introduced disease and parasites. They also suffer from increased competition, predation, and loss of their genetic ability to cope with local conditions.
Introduced carp are popular with anglers, partially because they can grow quite large and therefore give the opportunity for anglers to pose for the camera. However, these carp push out perch, rudd and tench, and there is a subsequent decline in water clarity and plant growth. Even the introduced carp themselves can suffer. They:
"Many places produce good catches because they have artificially large stocks of fish, but these can harm local wildlife."
"By relying mainly on repeated artificial stocking, the vital link between sustainable wild fisheries and high environmental quality is lost."
STOCKING CAN DAMAGE WATER AND HABITAT QUALITY, INCREASING RISK OF DISEASE, AND DEATH
"Stocking can be an easy and immediate way to meet anglers’ demands for higher catches. Yet stocking above a certain level can have a damaging effect on a lake’s water and habitat quality. High densities of bream and, especially, carp can dramatically reduce the diversity of water plants, which will also affect other aspects of biodiversity. Without taking the necessary precautions, increasing numbers of fish may also increase the risk of disease, and could potentially kill fish."
"Some fishery owners may choose to have high densities of fish to meet their customers’ demands for higher catches, with little scope to manage the water for other forms of wildlife."
"Stocking with farmed fish can threaten locally-adapted native salmon by posing serious genetic, disease, parasite, competition and predation risks. Exotic species such as rainbow trout can also cause problems."
SIGNIFICANT PREDATION RISK TO SMALL NATIVE FISH
"Where natural spawning is not available and a put-and-take fishery is the only option ... However, they do pose a significant predation risk to small native fish when introduced into natural lakes (or rivers)."
INTRODUCED CARP PUSH OUT OTHER FISH,
AND SUFFER THEMSELVES
"Good perch, rudd, tench and crucian carp waters have become progressively less common as more and more lakes have been developed as carp waters. This has led to a corresponding decline in weed growth and water clarity. In these waters the carp themselves often have difficulty in spawning successfully, seldom reach large sizes and are prone to parasites. Stocking may also introduce diseases such as spring viraemia of carp, which ruin the fishery."