Abstract:
Invasive species and anthropogenic sources of allochthonous trophic subsidies can have substantial eco-logical
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 le-niusculus 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. leniuscu-lus) 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. le-niusculus 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
allochtho-nous subsidy and will also prey upon invasive P. leniusculus where they are present.
