The introduction of non-native fish species to our waters is not a new phenomenon.
As far back as Roman times, fish such as common carp were moved from their native range for food. The formation of ‘The Society for the Acclimatisation of Animals, Birds, Fisheries, Insects and Vegetables’, in 1860, led to further introductions, such as the wels catfish Siluris glanis and the zander Sander lucioperca amongst others.
Some species, such as the wels catfish have been introduced to fisheries to provide an additional attraction and draw for anglers. Providing these fish are legally stocked into the right waters, they cause no harm to the wider environment. It is when non-native fish are introduced into inappropriate waters, such as those with a connection to a river or in a floodplain, that they can cause environmental damage.
In addition to non-native fish species that have been introduced to boost angling, there are others, now present in the wild, that were originally from the aquarium trade. Such introductions are illegal and these are the most likely to pose a threat to our native fish stocks. They are often released into unsuitable waters, which allows the introduced fish to spread. An example of the potential damage that can be caused to our native fisheries can be seen from the threat posed by the topmouth gudgeon Pseudorasbora parva. This small cyprinid, originating from SE Asia, has already been found in over 30 locations throughout England and Wales.
Primarily found here in stilwaters, the topmouth gudgeon can reproduce rapidly with numbers in excess of 65 individuals per square metre. At these levels, competition with the native fish species for food and spawning habitat is a major concern. A recent study showed the impacts such a population had on a small stillwater fishery, where no young native fish were found in fishery surveys. Also, the growth of roach and carp was severely limited by the competition from the topmouth gudgeon. Once introduced, eradication of this species is both costly and extremely difficult.
Non-native fish introductions can also upset the natural balance within our fisheries. If, for example, a non-native predator is introduced to a fishery, it can cause native prey fish numbers to decline, reducing the amount of food available for native predators, such as pike and perch.
Introducing non-native fish also carried the risk of introducing disease. The relatively recent detection of the rosette agent, Sphaerothecum destruens, in the highly invasive fish, the topmouth gudgeon, Pseudorasbora parva, highlights this risk. In the United States, this pathogen has prevented other species form spawning and there are concerns as to the effects that it may have on our native fish species.
At our National Fisheries Laboratory in Brampton, Cambridgeshire we are working to monitor the introduction of new parasites on non-native fish and during the last year we have found three new parasites that have arrived this way. Some of these may turn out to not be a threat to our native fish stocks, but we need to be aware of their presence. If they damage our fisheries we need to be ready to eradicate or control their spread.
More information on some of the non-native fish that have been found in our waters can be found below:
The message is clear: non-native fish can pose a real threat to our fish stocks. If you catch any of these species please report them to Gareth Davies.
If you suspect fish are being stocked illegally then please report the incident immediately on 0800 807060.
