Effects of inter- and intra-specific competition on the growth rates of juvenile European barbel Barbus barbus used in the stock enhancement of UK fisheries

Fish stocking can result in increased resource competition for coexisting species in receiving fisheries, with potential implications for the subsequent growth of individuals. The effects of increased exploitative competition for limited food resources were tested on the somatic growth of European barbel Barbus barbus (Linnaeus 1758). A substitutive–additive design was used, utilising common carp (Cyprinus carpio Linnaeus, 1758), tench (Tinca tinca Linnaeus, 1758) and B. barbus conspecifics. Additive treatments revealed that the growth of B. barbus was significantly suppressed following the introduction of C. carpio, with the magnitude of growth suppression directly proportional to increased density and biomass. The strength of this competition was, however, similar to that elicited by T. tinca at the same density, but at lower biomass, suggesting density was the important determinant of inter-specific competition strength and was independent of species. A substitutive treatment testing the effect of intraspecific competition revealed that when B. barbus was introduced at a similar density and biomass to C. carpio, there was no significant difference in the extent of the suppressed growth. There was no effect of the treatments on fish condition and coefficients of variation of the treatments suggested there was no establishment of hierarchies in any treatment, with growth rates consistent between individuals.

► Fish stockings can increase resource competition for extant fish, with implications for the fitness of individuals. ► Experiments revealed increasing interspecific competition resulted in decreased fitness of extant fish, independent of stocked species. ► Intraspecific competition elicited similar competition strength to inter-specific; additional density was more important than biomass. ► Outputs have implications for the current management of stocked fisheries in relation to the fitness and welfare of extant fishes.