Fish as bags of macro and micronutrients: factors that influence fish excretion and body stoichiometry

Abstract

Consumer nutrient excretion rates and their body stoichiometry can be influenced by a series of factors. Here, we tested how 12 species of fish from a freshwater stream had their excretion rates affected by body size, temperature, body stoichiometry and diet (feeding group as proxy). We further investigated the magnitude of the temperature effect on freshwater fish excretion through a meta-analysis conducted with 15 studies and 40 individual effect sizes. Finally, we measured 19 fish species whole-body content of calcium (Ca), phosphorus (P), sodium (Na), potassium (K), sulfur (S), magnesium (Mg), manganese (Mn) and zinc (Zn). We tested how body stoichiometry was influenced by the species habitat, trophic guild and taxonomic order, and each pair of nutrients was tested for correlation. Our results show that body size is the primary driver of the excretion rates of nitrogen (N) and phosphorus (P) and, contrary to our expectations, temperature, N:P body stoichiometry and diet were not statistically significant in our study. However, further experiments should try to include measurements of ingestion and egestion rates to increase understanding on how diet and body stoichiometry affects nutrient excretion. Through our meta-analysis, we saw that temperature increase leads to an increase in fish nutrient excretion, but the magnitude of the effect is extremely variable between studies. The acclimatization time and feeding status together answer for 22% of this variation, however most of it is still unexplained and it is most likely related to the species identity and their physiological characteristics. More experiments using standardized methods are needed to better explain what can influence the temperature effect on nutrient excretion. Finally, we found that elements with similar functions were correlated with each other, especially calcium and phosphorus, due to their role in bone formation. Tropical fish presented higher variation in their body composition, which could be due to a more specialized stoichiometric niche or to a bigger sample size (12 tropical species versus 7 temperate species), and was mostly driven by differences in Ca and P content. Fish body composition differed between taxonomic orders, highlighting the influence of phylogeny. Fish body stoichiometry also differed between trophic guilds, but mostly driven by armored fish species.