1. We measured N and P excretion rates of 470 individuals belonging to 18 freshwater fish species widespread in Western Europe. We assessed the effect of body mass on excretion rates at both the intra- and interspecific levels.
2. The high variability in per capita N and P excretion rates was mainly determined by differences in body mass. The scaling coefficients of allometric relationships for both N and P excretion rates were significantly lower than 1 (mean ± SE, 0.95 ± 0.04 and 0.81 ± 0.05, respectively).
3. The slope of the allometric relationship between fish mass and nutrient excretion rate was significantly different among species. We did not detect any influence of phylogenetic conservatism on fish mass and on excretion rates. Further investigations are needed to understand the biological determinants of these differences.
4. This high intra- and interspecific variability in per capita excretion rates, coupled with differences in fish body mass, produce marked differences in biomass-standardised excretion rates. These results thus indicate the necessity for further experimental and in situ investigations on the consequences of nutrient recycling by fish in freshwater ecosystems.