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Pêche récréative et fonctionnement des lacs : conséquences des pratiques d'amorçage et aide à la gestion opérationnelle
Summary
A study of recreational fishing in French gravel-pit lakes found that two of three angling bait types tested contained microplastics at an average of 17.4 particles per kilogram. Some carp anglers were using over 4.6 kg of bait per day, meaning microplastics are being deliberately introduced into freshwater systems as a by-product of angling, on top of the significant nutrient pollution these baits also cause.
Terrestrial subsidies to aquatic ecosystem have a strong impact on their functioning. In addition to natural subsidies, human activities also generates allochthonous inputs. Recreational fishing, through groundbaiting practices, which consists in introducing organic matter to optimise fish harvest, generates human subsidies which properties and impact on aquatic ecosystem are poorly known, despite their potential implication on lake eutrophication. This study aimed to determine the composition and properties of angling bait subsidies in order to quantify their consumption by fish, to assess impacts on water quality and to propose groundbaiting management measures. In the first chapter, the analyses of stoichiometric composition of four angling bait types revealed a high level of nitrogen and phosphorus, variable according type, which phosphorus contents up to 17 times higher than those observed in natural subsidies (tree litter). In the second chapter, the microplastic concentration was quantified. Two of the three angling bait types analysed contained microplastics, with an average concentration of 17.4 particles.kg-1. Through the analysis of stable isotope values, the third chapter highlighted a contrasted angling bait consumption by cyprinid fishes in gravel pit lakes supporting different fishery types. Some individuals used angling baits as their major energy source, accounting for up to 80.5 % of the resources consumed. In the fourth chapter, a field survey coupled with a questionnaire to anglers allowed to assess a strong seasonality in the angling bait quantities used according to fishing technique practiced. Specialised carp anglers used in average 4.6 kg of angling baits per day. Associated to a strong summer input, these introduced quantities generated an important eutrophication, modelled with a numerical lake model functioning. This model also identified the limitation of individual amount of baits as the most efficient groundbaiting management measure, which allowed to drastically reduce the total phosphorus and chlorophyll-a concentrations. This work allowed to identify angling bait as a subsidy with qualitative, quantitative and temporal properties highly variables which modulate the impact on ecosystems. In some cases, angling bait subsidies lead to a substantial water quality degradation, which justifies the need to set up some groundbaiting