NITROGEN AND PHOSPHORUS EXCRETION FROM THE NOBLE CRAYFISH, ASTACUS-ASTACUS L, IN RELATION TO FOOD TYPE AND TEMPERATURE

The excretion of nutrients from adult freshwater crayfish, Astacus astacus L., was studied in relation to two different food types (commercial fish pellets and potatoes) and three temperatures (10, 15 and 20-degrees-C). Conductivity measurements revealed a steady diurnal excretion rate, with no clearcut day/night effect. The amounts of N and P excreted were significantly influenced by food type as well as by temperature between 15 and 20-degrees-C. The daily amounts of nitrogen and phosphorus discharged in dissolved excreta (urine and diffusion products) from crayfish reared at 20-degrees-C (as mu-g/g DW per day) were 254 (N) and 3.4 (P) for crayfish fed pellets and 52 (N) and 1.3 (P) for those fed potatoes. The corresponding values for nutrients in faeces were 34 (N) and 29 (P) when fed pellets and 20 (N) and 1.7 (P) when fed potatoes. The total yearly load from one ton of adult crayfish fed fish pellets (20-degrees-C) would be 24.9 kg N and 2.8 kg P (for crayfish fed potatoes 6.2 kg N and 0.26 kg P). The algal growth potential on the excreta was determined by an algal assay using the green alp Selenastrum capricornutum Printz. The daily nutrient output from 1 kg of crayfish (fed pellets) would theoretically support a primary production of about 28 g of the alp. The results are compared to corresponding data on Atlantic salmon (Salmo salar L.), based on experiments and literature studies.