Management of Introduced Crayfish
Crayfish Trapping Found Useful in Restoring Aquatic Biodiversity
Introduced crayfish are causing loss of biodiversity around the world. Trapping programs are showing some potential to help control the ecosystem damage done.
Crayfish are a staple in diets around the world. They are also regularly used as fish bait and are commonly found in the aquarium trade. This has led to various species being introduced into areas where they are not native. Over time crayfish damage ecosystems where they have been introduced. For many years trapping was considered an ineffective method of crayfish control, at least in the case of the signal crayfish. But some recent work is changing that thinking.
Introduced Crayfish in Europe
The problems caused by introduced signal crayfish (Pasificastacus leniusculus) and the red swamp or Louisiana crayfish (Prcambarus clarkii) crayfish in Europe are well documented. The deliberate introduction of these species into European waters as part of various aquaculture projects has led to habitat degradation and loss of native species to disease.
Introduced Crayfish in North America
In the United States and Canada, where most crayfish species are resistant to crayfish plague, the biggest problems have been with moving species around the continent, largely as discarded live bait from fishing. But the same problems have occurred with local native species being out-competed for food and native vegetation being destroyed. Fish eggs also form part of the diet for crayfish, meaning losses of biodiversity of both plants and animals. And additional problem has been that rusty crayfish (Orconectes rusticus) are closely related to Virile or Northern Soft-shelled crayfish (Orconectes virilis) and Northern Clearwater crayfish (Orconectes propinquus) and hybrid crayfish have been found.
Trapping Studies
Early trapping studies in Europe used traps that were designed for sustainable fisheries. While populations would initially decline, they rebounded once trapping ceased. Work done in the Norfolk Brecks in the UK with various trap designs has resulted in significant reduction in signal crayfish numbers there. Long-term trapping will be required to ensure the signal crayfish population does not rebound but the ecosystem of the River Lark is already healthier and damage to riverbanks is greatly reduced.
The rusty crayfish has been the focus of a US trapping project similar to that carried out in the UK. Results have been good with catches declining by 95% over the five years of the trapping study. The difference in this study was that fisheries management was also employed to reduce rusty crayfish numbers. In a balanced ecosystem, large predatory fish, such as small mouth bass (Micropterus dolomieu) and rock bass (Ambloplites rupestris) eat small crayfish helping to keep the population in check. When there are significant numbers of large male crayfish predation on fish eggs is greater and fish populations decline.
The international movement of crayfish is a prime example of how much can go wrong when non-native species are introduced into an area. In this case, there is hope of at least gaining some control over the problem.
