Between-lake variation in the elemental composition of roach (Rutilus rutilus L.)

Stoichiometry theory states that the elemental composition of individual organisms is a taxon-dependent trait, and that organisms maintain relatively constant body nutrient composition, independent of food nutrient content or ingestion rate. Some recent studies have pointed out that assuming a strict homeostasis in fish is an over-simplification, so that nutrient ratios in fish bodies may not be as constant as previously thought. In this study, our aims were (a) to assess the overall nutrient composition of roach, (b) to reveal any significant differences in elemental composition of roach between populations from different lakes and (c) to relate these potential differences to environmental parameters and fish condition. The mean elemental composition of roach (whole fish) was 45.0 ± 5.3 % carbon (C), 10.2 ± 0.8 % nitrogen (N), and 2.4 ± 0.5 % phosphorus (P), while their molar C/N/P ratio was approximately 50:10:1. The elemental composition of roach was not as constant as homeostatic stoichiometry models would predict. Significant differences were found in the carbon, nitrogen, and phosphorus contents of fish sampled from different lakes. Productivity of lakes (expressed as chlorophyll-a) was positively correlated with  %C, C/N, C/P, and N/P ratios, and negatively correlated with  %N and  %P. In turn, mean lake temperature was negatively correlated with  %C and C/N ratio, but not with other nutrients or nutrient ratios. Condition factor was negatively related to the  %C of fish, but it did not correlate significantly with  %N,  %P, or nutrient ratios. Our results on intraspecific nutrient variation illustrate the potential differences between populations and emphasize the importance of lake-specific approaches.