Crayfish Management in the Upper Thames

Invasive non-native species are important direct drivers of global biodiversity loss (Millennium Ecosystem Assessment 2005) and in many parts of the world are the first or second most important threat to freshwater biodiversity and ecosystem function (Lodge et al 2000). The impact from invasive non-native species can be severe, with altered ecosystem functioning, the loss of native biodiversity, and the disruption of ecosystem services, all of which can have major socio-economic consequences (see Vitousek et al 1996). Aquatic ecosystems are particularly vulnerable to invasive non-native species because of the potential for rapid spread and the difficulties of detection, monitoring and control.

An estimated one-third to one-half of the world's crayfish species are at risk of serious population decline or extinction, partially due to the accidental or deliberate introduction of non-native species (Taylor 2002). Crayfish might seem uncharismatic, and they are not at the forefront of public enthusiasms, but they are very important components of the freshwater environment itself relatively neglected by conservationists. The United Kingdom has only one species of freshwater crayfish, the white-clawed crayfish (Austropotamobius pallipes). The introduction of the crayfish plague (Aphanomyces astaci) in the early 1980s has resulted in the elimination of many populations and most are now concentrated in northern and central England (Holdich 2003). It is a priority species under the UK Biodiversity Action Plan. The non-native signal crayfish (Pacifastacus leniusculus) is believed to be a significant contributing factor in the decline of the white-clawed crayfish, through the introduction of fungal disease, the crayfish plague to which indigenous species are highly susceptible. The story, although less dramatic in the public eye, parallels the impact of grey on red squirrels through parapox virus. Habitat modification is also implicated which again offers a parallel, although less catchy in the public eye, of the impact of agricultural intensification which exacerbates the effect of the invasive mink on the native water vole. The signal crayfish, a North American species first introduced to Europe for aquaculture in the 1960s, is highly invasive and is now established in the wild in most northern European countries. It digs extensive burrows into soft river banks, causing bank erosion with subsequent silting of streams. The white-clawed crayfish requires well oxygenated, clear water and this habitat alteration, combined with disease, pollution and anthropogenic habitat modification, have had considerable impacts on its survival.

Freshwater crayfish are keystone components of freshwater ecosystems playing important trophic roles as omnivorous consumers and also as prey for larger predators. Signal crayfish have been shown to have negative effects on native freshwater fauna and flora. A recent study of signal crayfish in Scottish rivers recorded as much as a 40% decline in fluvial macro-invertebrate density and significant impacts on community structure (Crawford et al 2006). They also have impacts on larger vertebrate species by direct predation (e.g. consumption of amphibian eggs, Axelsson et al. 1997) and through competition for food and refugia particularly with juvenile stages of over-wintering Atlantic salmon (Griffiths et al 2004) and stone loach (Barbatula barbatula) and bullheads (Cottis gobio) in River Ouse riffles (Guan & Wiles 1997; also see Ibbotson et al 1997).

Crayfish are amongst the largest freshwater invertebrates and commonly dominate the benthic biomass in the United Kingdom (Momot 1995). They are predated by fish (perch, chub, trout, pike and eel), mammals (mink, rat and otter) and birds (herons and crows) particularly during periods of low river flow (Holdich 2003). Juvenile crayfish are even predated by insect larvae (Hogger 1988). They are important components of the diet of indigenous otters (Lutra lutra) and potentially important components of the diet of invasive non-indigenous American mink (Mustela vison).

The study

Our approaches to this project are two-fold. Firstly we are undertaking a catchment-wide survey for white-clawed and signal crayfish, as well as the distribution of otters and American mink. Secondly, we are undertaking a replicated signal crayfish removal experiment in the Upper Thames. For this experiment we have selected four 1 km stretches of river, comprising two 'removal' and two 'control' stretches. Crayfish are removed from the removal stretches via trapping. At the control stretches, crayfish are captured by trapping, marked and released (under license from Natural England). Furthermore, each 1km section is divided into a central 500 m section, flanked by two 250 m sections (an upstream and a downstream section). In the control stretches individuals are marked and released in all sections, but the marks (made by piercing the tail in a specific location with a sterile needle) are specific to the section and month in which the crayfish is captured. In removal stretches, only individuals captured from the 500 m central section are removed, with individuals from the upstream and downstream sections being marked and re-released as for the control stretches.

Using this experimental design we can investigate the movements and growth rates of crayfish in all four of the stretches of river. If the removal of crayfish from the central 500 m of the removal stretches, for instance, benefitted any un-captured crayfish from this section, or encouraged immigration of individuals from the surrounding sections, then we would expect individuals captured in the middle section to be increasingly large in size, or to bear marks from either the upstream or downstream locations. Comparing growth rates and movements of crayfish between the removal and control stretches will provide valuable data demonstrating whether trapping is a viable control strategy for this species.

At each of the sites in this study we are conducting broad-brush habitat surveys and surveying the abundance and diversity of fish species and species of aquatic macrophytes (for more details see Dr Alison Poole's pages) on a monthly basis. In this way we hope to quantify any benefits resulting from crayfish removal for the co-existing biota, and to gain a detailed insight into the biodiversity impacts of signal crayfish.