Signal Crayfish (Pacifastacus leniusculus)

Dans le document The Economic Cost of Invasive Non-Native Species on Great Britain (Page 41-45)

The American signal crayfish was introduced to Britain in the late 1970s primarily to farm for food. However, they quickly escaped or were deliberately released and spread rapidly across England and Wales. The distribution is currently limited in Scotland, though increasing. The signal crayfish is larger than the native white-clawed crayfish, and out-competes the native crayfish, as well as carrying a crayfish plague that kills the native species. Signal crayfish burrow into riverbanks, increasing erosion as well as affecting wild fish stocks (bullhead, stone loach, salmonids and other angling species) whose eggs are predated.

4.2.1 Ma na ge me nt Cos ts

The main costs of the signal crayfish to the British economy are associated with biodiversity.

The decline of the white-clawed crayfish is partly attributed to the presence of the signal crayfish through competition for resources and the spread of the fungal disease Aphanomyces astaci, carried by signal crayfish (Peay 2000, UKBAP, Craig Stenton, pers.

comm.). Therefore, most of the white-clawed crayfish conservation costs can be attributed to the signal crayfish, in particular costs associated with ‘ark’ sites set up to protect the white-clawed crayfish. There are also limited control measures taking place with trapping activities in some areas; around 15,000-20,000 individuals were removed per year in one severely infested area in the 1990s to reduce the impact (Richard Sankey, pers. comm.). A range of

these projects around England were selected and the average cost per annum was calculated at £32,574 (Table 4.6). The total number of ‘ark’ sites in the country is unknown (Andrew Whitehouse, Buglife, pers. comm.), as is the total number of signal crayfish or white-clawed crayfish management projects being undertaken each year, but we assumed that ten projects are carried out each year in England and the management cost can be estimated at £325,740.

Table 4.6. Costs of projects involving signal or white-clawed crayfish in England.

Lead organisation Start Yorkshire Dales Millennium Trust 2007 £223,799 3c £74,600 £77,575 South Cumbria Rivers Trust 2009 £10,500 1 £10,500 £10,500

a: Start of project or date funding was allocated

b: Estimated 50% allocated to conservation of native crayfish -£10,000 c: Duration of project is estimated

d: Corrected for inflation to today’s cost

The population of white-clawed crayfish in Wales has also been significantly reduced and trapping of signal crayfish and conservation activities for native crayfish are also carried out in Welsh rivers. One example is the approximately 30,000 signal crayfish trapped in 2007, and the 6 km of habitat restored under one project (Wye & Usk Foundation 2006). A cost of

£120,000 for this project covered four main UK Biodiversity Action Plan (BAP) priority species, including white-clawed crayfish. Again, the number of signal crayfish management projects in Wales is unknown, however if it is assumed that five projects are conducted each year, at an average cost of £32,574 (as in England) then, a management cost of £162,870 is estimated.

White-clawed crayfish are not native to Scotland, although there are two introduced populations in Scotland (Peay 2006). However, control measures are still undertaken for signal crayfish due to their effects on fisheries and economically important species, such as salmon. There are known populations of signal crayfish in the Upper Clyde, the Kirkcudbrightshire Dee catchment (including Loch Ken), the River Earn (Ribbens and Graham 2004) and the North Esk catchment (Peay et al. 2006). No specific costs could be found for control measures in these areas, although one project in Loch Ken was said to cost

£90,000. (Attempts to obtain more details of the cost of signal crayfish to Loch Ken were not

forthcoming.) Therefore, if it is assumed that five management projects are carried out each year at the same average cost as England (£32,574) (as this is based on a larger sample size), then management costs in Scotland can be estimated at £162,870.

In England and Wales, the Environment Agency spends approximately £500,000 per annum on controlling signal crayfish and another £500,000 on conserving the white-clawed crayfish (Trevor Renals, pers. comm.). However, some of the projects carried out by the wildlife trusts, discussed above, are partially funded by the Environment Agency (Wildlife Trusts, pers. comm.), and some of the white-clawed conservation work cannot be attributed to the presence of signal crayfish and therefore a reduction of 35% is made to the total amount spent on management work giving a total amount spent entirely by the Environment Agency of £650,000 split between England (£450,000) and Wales (£200,000).

4.2.2 Re s tora tion Cos ts

The damage caused by signal crayfish burrowing into river and canal banks and causing erosion is an increasingly common problem as the range of signal crayfish expands (Simon Cain, pers. comm.). The species can be found in anything from silty river waters to canals, and will burrow into banks up to a depth of two metres. The effect of this behaviour accelerates the natural erosion process and in severe cases one metre of river bank could be lost per year (Richard Sankey, pers. comm.). In certain areas, bank restoration work has taken place. One such place is the River Lambourn, from Lambourn village to Newbury, where 800 m of restoration work was carried out following damage over a four month period.

It cost £105,000, including £25,000 in kind contributions providing a cost of £131.25 m-1 (The River Restoration Centre 2007). For a longer length of the river (3 km) along the same waterway it was estimated that it cost around £1 million to restore the bank and a mill structure (Richard Sankey pers. comm.). The river in this case had a particularly serious problem with signal crayfish and there had been significant degradation of the banks over the last 10-15 years. Current evidence of bank restoration work due specifically to signal crayfish is very limited, though some work is undertaken that also reduces the susceptibility of the bank to signal crayfish burrowing. In addition, no data were found to suggest that bank restoration work occurred on a regular basis, and therefore it is assumed that only 20%

of the cost of the major scheme described above is directly attributable to the presence of signal crayfish and an annual spend is estimated at £200,000.

4.2.3 Angling

Signal crayfish can cause a nuisance to some anglers through the loss of bait or the crayfish predating on stock (Abby Stancliffe Vaughan, pers. comm.). This can cause a significant loss of income for those businesses that rely on angling, as fishermen will go elsewhere if the fish stock is being predated. One example, from Loch Ken where signal crayfish have had a large impact of fish stocks for angling may cost about £250,000 per year in control measures and lost angling revenue (S. Peay, pers. comm.). There is also a cost caused by the need to clean equipment between fishing in different sites to prevent the spread of crayfish plague.

However, these costs are expected to be minimal and there will be other reasons to ensure that equipment is clean and other causes of loss of bait, etc. Commercial fisheries may implement crayfish control programmes to reduce the impact of predation on the young fish.

Even so predation may still reduce the number of fish reaching maturity, and the presence of crayfish can reduce the ability of fish farmers to supply fishing areas if they are near white-clawed crayfish populations. Again this will reduce the income of fish farmers. Specific data on the costs relating to these effects is very limited, but based on the Loch Ken estimated costs, a cost of £1,000,000 per year is estimated (£550,000 England, £325,000 Scotland,

£125,000 Wales).

4.2.4 Re s e a rch

In addition to the projects discussed above that concentrate on management strategies to protect white-clawed crayfish, or eradicate signal crayfish, further research projects are carried out into control methods for signal crayfish or conservation strategies for the white-clawed crayfish. At least two research projects into crayfish plague, with funding from Defra and the Environment Agency, were running in 2009 at an average cost of £37,463 per annum. Assuming that there are five research projects taking place each year, into the effects of signal crayfish and crayfish plague, an annual research cost is estimated at


4.2.5 Tota l Cos ts

Other economic costs are attributable to the presence of signal crayfish in Great Britain, such as the loss of aesthetic value related to native white-clawed crayfish and damage to river banks through burrowing. However, no data could be identified that valued white clawed crayfish or the amount of damage done to river banks, even though some figures were available on the costs of bank restoration work. The data that was available is summarised to give the following totals of annual costs to the economy due to the presence of signal crayfish.

Table 4.7. Total annual costs of signal crayfish.

England Scotland Wales GB

Management £776,000 £163,000 £363,000 £1,302,000

River Bank Restoration £100,000 £50,000 £50,000 £200,000

Angling £550,000 £325,000 £125,000 £1,000,000

Research £112,000 £38,000 £37,000 £187,000

Total £1,538,000 £576,000 £575,000 £2,689,000

Dans le document The Economic Cost of Invasive Non-Native Species on Great Britain (Page 41-45)