Stoichiometry of fishes and their prey: Implications for nutrient recycling

Fishes are important pools of nitrogen (N) and phosphorus (P), and they have important direct and indirect effects on cycling of these potentially limiting nutrients in lakes. We used a model of fish bioenergetics coupled to a mass balance model of fish N and P budgets to investigate the stoichiometric interactions among fishes, their food, and the nutrient recycling that results from this predator-prey interaction. We tested the hypothesis that the N:P ratios in fish excretion (supply ratios) are high but potentially variable because of fishes' high P requirement and the likelihood of P limitation of fish growth. A survey of 186 cases representing 18 fish species demonstrated that limitation of fish growth due to availability of P in food was exceedingly rare in natural ecosystems. Fish growth rates were energy limited in almost all systems. Fish grew at an average of 26% of the maximum potential rate determined from bioenergetics. As a result, the predicted N:P supply ratio from fish excretion was low and relatively constant (the median mass ratio for piscivores was 6.1, and 13.1 for all other fishes). Excretion rates for N and P by fishes were predictable from body size, food nutrient concentration, and environmental temperature. Nutrient recycling by fishes will tend to alleviate P limitation of phytoplankton growth, but the importance of this effect will be directly proportional to the magnitude of nutrient regeneration rates from fishes relative to other sources available to phytoplankton.