Size-mediated, density-dependent cannibalism in the signal crayfish Pacifastacus leniusculus (Dana, 1852) (Decapoda, Astacidea), an invasive crayfish in Britain

Abstract

The role of cannibalism in crayfish populations is not well understood, despite being a potentially key density-dependent process underpinning population dynamics. We studied the incidence of cannibalism in an introduced signal crayfish Pacifastacus leniusculus population in a Scottish lowland river in September 2014. Animals were sampled using six different sampling techniques simultaneously, revealing variable densities and size distributions across the site. Cannibalism prevalence was estimated by analysing the gut contents of crayfish >20 mm CL for the presence of crayfish fragments, which was found to be 20% of dissected individuals. When seeking evidence of relationships between the sizes of cannibals and ‘prey’, the density of conspecifics <56% the size of a dissected individual yielded the best fit. The relationship between cannibalism probability and crayfish size and density was equally well described by three different metrics of crayfish density. Cannibalism increased with crayfish size and density but did not vary according to sex. These results suggest that large P. leniusculus frequently cannibalize smaller (prey) conspecifics, and that the probability of cannibalism is dependent upon the relative size of cannibal-to-prey and the density of the smaller crayfish. We suggest that removing large individuals, as targeted by many traditional removal techniques, may lead to reduced cannibalism and therefore a compensatory increase in juvenile survival.

Between‐lake variation in the trophic ecology of an invasive crayfish

“…As abundance increases, changes in crayfish behaviour to reduce the risk of antagonistic interactions with conspecifics might cause a shift in habitat use or time spent foraging (Svanbäck & Bolnick, ), causing individuals to consume resources at lower trophic levels. However, a recent study suggested that cannibalism in crayfish increases with population density (Houghton et al., ). The negative effect of population abundance on trophic position was only evident in smaller lakes which might be linked to an increase in encounter rates between conspecific individuals, since these are likely to increase in smaller areas if abundance remains the same.…”

“…Our results appear to support the first theory, especially since it was found that individual trophic position also nick, 2007), causing individuals to consume resources at lower trophic levels. However, a recent study suggested that cannibalism in crayfish increases with population density (Houghton et al, 2017).…”

“…In addition, we predicted that size‐related shift in trophic position would be affected by environmental parameters. For instance, we hypothesised that population abundance (and therefore potential intraspecific competition) would enhance this size‐related shift in trophic position due to increasing cannibalism by largest individuals at high densities (Houghton, Wood, & Lambin, ). Population niche width and the level of omnivory will increase with lake productivity, reflecting the wider diversity of available resources. This relationship will be less evident in larger lakes where littoral and allochthonous resources are restricted (Stenroth et al., ). Population niche width and the level of omnivory will decrease as predation pressure and/or crayfish abundance increase due to reduced access to resources (Araújo et al., ; Jackson et al., ), and that this relationship will be less evident in larger lakes due to reduced encounter rates (and therefore, reduced competitive and predation pressures; Stenroth et al., ).…”

“…In addition, we predicted that size-related shift in trophic position would be affected by environmental parameters. For instance, we hypothesised that population abundance (and therefore potential intraspecific competition) would enhance this size-related shift in trophic position due to increasing cannibalism by largest individuals at high densities (Houghton, Wood, & Lambin, 2017). 3.…”

Trapping for invasive crayfish: comparisons of efficacy and selectivity of baited traps versus novel artificial refuge traps

“…For population control programmes to be successful, Bomford and O'Brien (1995) suggested a number of criteria have to be met, including that all reproductive animals must be at risk of capture, with their capture still probable at low population density. For invasive crayfish, an issue is the low rates of capture and removal of juveniles (<30 mm carapace length), despite them often comprising a high proportion of population abundance (Houghton et al, 2017). Thus, trapping methods that are biased towards the capture of only mature crayfish tend to result in poor control efficiency due to much of the population remaining unaffected (Peay, 2004).…”