ThreatsRead full entry
The Signal Crayfish grows faster, reaches sexual maturity earlier, is more aggressive, and has the ability to dominate the use of many important factors like food and refugia (FÃ¼reder et al. 2006). The Signal Crayfish have also been shown to have a relatively higher population growth time than the Noble Crayfish. Even in watercourses where the Signal Crayfish is free from plague, it seems likely that the Signal Crayfish will out compete the Noble Crayfish, in the long term (Kataria 2004). This species produces fewer young that the Signal Crayfish and so is quickly out-competed in terms of population numbers. This species is also outcompeted by the invasive species Orconectes limosus.
The annual catch of this species has dropped to a fraction of what it was prior to the introduction of crayfish plague (Westman 2002; Bohman Nordwall and Edsman 2006). This species was extensively produced but the plague has caused a significant decline of more than 95% during a period of nearly 150 years (Skurdal and TaugbÃ¸l 2002). The range of this species remains compromised by the crayfish plague (Holdich and Pockl 2007).
Dredging of waterways in localised areas has further threatened this species as it leaves the water cloudy and disturbs the habitat (FÃ¼reder et al. 2006). Acid rain, also causing a decrease in water quality, is thought to be responsible for a decline in breeding success, as the egg cases of young crayfish are unable to form properly (Collins et al. 1983). Low calcium concentrations may be a factor limiting the distribution and production of this species in soft-water lakes (Rukke 2002).
In Sweden, this species is estimated to have undergone a decline of ~78% over the last 22.5 years based on a decline from 1,724 known subpopulations in 1994, to 1,000 subpopulations in 2002 (Fiskeriverket and NaturvÃ¥rdsverket 1998; Database of Crayfish Occurrences 2005). In Finland, the rate of decline is currently at about 20% every 20 years, though it may be greater (M. Pursiainen pers. comm. 2010). In Norway, this species is estimated to have undergone ~61% decline over the last 22.5 years from a 40 tonnes yield in 1966, to 10 tonnes in 1999 (Skurdal et al. 1999). While there is no available quantifiable data on the trends in Denmark, the situation is likely to be similar. Rates of decline in other countries are similar to that of the Scandinavian countries: ~67% decline over 10 years in Belgium (Arrignon et al. 1999), ~56% decline in Germany over 22.5 years, ~95% decline over 22.5 years in Hungary, and ~86% decline in the Netherlands over 22.5 years. Notable declines are reported for most of the other countries, other than Belarus where numbers are said to be increasing; Croatia where numbers are stable but non-native species have recently been introduced so future declines are expected; Slovakia where numbers are thought to be stable. These figures indicate that this species is likely undergoing a rate of decline of around 50-80% over a 22.5 year period. However, there are a number of large-scale re-stocking programs for this species and in some areas of its range, numbers are stable so the true rate of decline is likely closer to 30-50% globally.