Carbon, nitrogen, and phosphorus stoichiometry of cyprinid fishes

We investigated the carbon, nitrogen, and phosphorus levels in whole fish and gut samples of several species of cyprinids, relating our findings to nutrient flux models. Some differences in whole-fish nutrient content across species, lakes, and seasons, as well as differences across fish length and mass, were found. N and P contents were highest in fathead minnows and lowest in pearl dace, with northern redbelly dace and finescale dace intermediate. Larger fish had higher percent C and lower percent N and P. However, all differences in whole fish C, N, and P chemistry were small. Cyprinids had the following mean composition: carbon. 46%; nitrogen, 9.7%; and phosphorus. 1.5%. The cyprinid molar C:N:P ratio was 242:16:1. These values make cyprinids relatively low in phosphorus compared to other fish that have been previously studied, especially members of the Percidae and Centrarchidae. Gut contents were lower in N and P than the whole fish, and C:N and C:P ratios were correspondingly higher in gut contents than in the whole fish. Thus, minnows must concentrate both of these nutrients within their biomass compared to what they eat. The N:P ratio of minnows and minnow gut contents had nearly identical means. All chemical variables showed lower variation in the fish than in the gut contents, supporting a homeostatic model of nutrient flux. Stable nitrogen isotope analysis found that minnows were ~3‰ (parts per thousand) heavier than their gut contents, providing evidence that gut contents analyzed were derived mainly from ingested material. A homeostatic nutrient model appears to be an appropriate one for fish.