Nutrient digestibility of angling baits for carp, Cyprinus carpio, with implications for groundbait formulation and eutrophication control R. ARLINGHAUS Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany M. NIESAR Department of Inland Fisheries, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany Abstract To evaluate the suitability of angling groundbaits as a fish feed and the potential for nutrient loss, the macronutrient digestibility of four groundbaits was investigated in carp, Cyprinus carpio L., using a sieving method of faeces collection and chromic oxide. Apparent digestibility coefficients (ADCs) of protein and lipids were >80% for all angling baits. ADCs of nitrogen-free extracts and phosphorus (P) differed significantly between the test diets, probably because of divergent feed compositions and ingredient treatments. The generally high digestibility estimates indicated that fish feeding on angling groundbait will benefit from this food source. The differences in nutrient digestibility suggested that type of groundbait and ingredient used will have a major effect on potential eutrophication and fish production caused by groundbaiting. The principle of groundbaits to protect the environment should be to minimise the P-content of the groundaits and maximising P-digestibility and P-retention efficiency. KEYWORDS: boilie, coarse fishing, cultural eutrophication, groundbaiting, phosphorus retention, recreational fisheries. Introduction Groundbaiting is commonly practised in freshwater recreational (i.e. non-salmonid) angling to attract fish [e.g. bream, Abramis brama (L.), carp, Cyprinus carpio L., or tench, Tinca tinca (L.)] to the angling site (Wolos, Theodorowicz & Grabowska 1992; Arling- haus & Mehner 2003). In Germany, the mean amount of bait used per angler was estimated at 7.3 kg yr )1 (Arlinghaus 2004a). Given about 3.3 million active anglers in Germany (Arlinghaus 2004a), the total amount of bait used in 2002 was about 24 000 t. When large quantities of anglersÕ baits are introduced to a fishery, these items may become important in the diet of the fish (Speczia´r, To¨lg & Bı´ro´ 1997). The energy and nutrients provided by groundbait can lead to considerable additional production of fish (Schreck- enbach & Bra¨mick 2003; Niesar, Arlinghaus, Rennert & Mehner 2004). This additional fish production in turn has a direct effect on yield and angling carrying capacity of the fishery (i.e. yield and carrying capacity increases if groundbait is used in large amounts). At the same time, nutrient inputs by egested, excreted or uneaten groundbait may substantially contribute to anthropogenic euthrophication (Arlinghaus & Mehner 2003; Niesar et al. 2004), and/or result in local deoxygenation and reduction of benthic invertebrates (Cryer & Edwards 1987). Accordingly, angling clubs in Germany and the UK are beginning to ban the practice of groundbaiting in some fisheries (Niesar 2003). However, these fisheries management measures are not established objectively because of a lack of thorough scientific studies (Niesar et al. 2004). Against this background, nutrient digestibility is an important aspect of evaluating the suitability of feedstuffs. Highly digestible angling groundbait is desirable for three reasons: (1) high digestibility allows the feeding fish to benefit (i.e. grow) from the artificial Correspondence: Robert Arlinghaus, Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Mu¨ggelseedamm 310, 12587 Berlin, Germany (e-mail: arlinghaus@igb-berlin.de) Fisheries Management and Ecology, 2005, 12, 91–97 Ó 2005 Blackwell Publishing Ltd 91