Slow growth rates of common carp Cyprinus carpio in English catch-and-release fisheries are related to high stock densities.

Hatchery-reared, domesticated strains of common carp Cyprinus carpio ('carp') are regularly used to enhance angler catch rates in many catch-and-release (C&R) lentic fisheries. These carp are usually selected for specific traits (e.g. stress resilience, high probability of angling capture) and are often stocked at relatively small sizes into these fisheries in very high abundances to support high catch rates. To assess the population-level consequences of this practice, carp scale samples were collected from nine C&R fisheries in southern England, where the carp were always present in very high abundances, and compared with three low-density, naturally recruited populations in the same geographical area, plus global carp populations using data from literature. Scale analyses revealed that carp from the C&R fisheries exhibited significantly slower growth compared with both the three wild populations and the global carp populations, with the fishery carp always producing significantly smaller annual length increments than all the other populations. These results suggest that high stocking densities induce strong density-dependent growth suppression in carp. This suppression occurs despite the potential nutrient subsidy from angler bait.