Novel trophic subsidies from recreational angling transform the trophic ecology of freshwater fishes.

Abstract

1. Angling is a globally popular leisure activity. There are over 31 million anglers in Europe, many of which target species of the Cyprinidae family in lowland freshwater ecosystems using methods generally involving bait (e.g. groundbaits, seeds and pellets), with large bait inputs possible in periods of high angling activity. While these bait inputs act as novel trophic subsidies (‘angling subsidy’), substantial knowledge gaps remain on their influence on freshwater food-webs, including on fish trophic niche size and position. 2. The effects of angling subsidies on the trophic ecology of cyprinid fish populations and their macroinvertebrate prey resources were investigated in field studies comparing waters of high angling activity (‘subsidised fisheries’) versus low angling activity (‘non-subsidised fisheries’), and complemented by a pond experiment using two cyprinid species in subsidy absence/presence. Methods were based on stable isotope analysis, with angling subsidies being δ13C enriched and, generally, δ15N depleted compared to macroinvertebrate prey resources. 3. In the subsidised fisheries, while there were minimal influences of the baits on macroinvertebrate stable isotope values, the effects of the subsidies on all fish species were to substantially δ13C enrich and δ15N deplete their isotopic niches. However, patterns of inter-specific niche divergence remained similar between the species in subsidy presence. 4. In the pond experiment, there was strong isotopic association between the two fish species and macroinvertebrate putative prey in subsidy absence. In treatments that then exposed both species to angling subsidies, their stable isotope values shifted to enriched δ13C. 5. Synthesis and application: Where angling activity is high, angling baits can provide strong trophic subsidies to freshwater fish, but with minimal effects on other trophic levels. Their regular input into freshwaters can provide some substantial benefits for fish (e.g. increased growth rates) and fisheries (e.g. elevated carrying capacity, higher catch rates), but can also increase nutrient enrichment and potentially raise concerns on angling ethics. Thus, in allowing the use of these baits, especially in relatively high quantities, managers must balance the benefits they can deliver to fish and fisheries versus the adverse effects their use can have on freshwater organisms and ecosystem functioning.