Angling baits and invasive crayfish as important trophic subsidies for a large cyprinid fish

Invasive species and anthropogenic sources of allochthonous trophic subsidies can have substantial ecological consequences for freshwater ecosystems, including modifying the diet of consumers and altering food web structure. Here, the diet of an omnivorous cyprinid fish, European barbel Barbus barbus, was assessed in relation to the presence of invasive signal crayfish Pacifastacus leniusculus and pelletized fish-meal in four rivers in England. Pellets are often used in large quantities by river anglers and thus could provide an important trophic subsidy, not only to the fish but also indirectly via P. leniusculus. Carbon and nitrogen stable isotopes were used to estimate the proportion of diet assimilated from natural sources and from P. leniusculus and pellets by B. barbus of lengths between 420 and 800 mm. Pellets generally made a large contribution to the overall biomass of B. barbus (up to 59 % of population diet) and in the two rivers where they were present, P. leniusculus were also an important resource (up to 30 % of population diet). The proportion derived from macro-invertebrates (excluding P. leniusculus) was substantially lower. Stable isotope mixing models further demonstrated considerable intraspecific variability in B. barbus diet within the rivers, with pellets comprising up to 79 % of the biomass of individual B. barbus in rivers where P. leniusculus was absent. Where present, P. leniusculus effectively replaced and thus reduced the contribution of pellets to individual fish diet. Thus, isotopic evidence from three of the four rivers indicates that B. barbus populations are heavily reliant (> 50 %) upon angler-introduced baits that act as an important allochthonous subsidy and will also prey upon invasive P. leniusculus where they are present.