Nutrient recycling by two phosphorus-rich grazing catfish: the potential for phosphorus-limitation of fish growth


In ecosystems where excretion by fish is a major flux of nutrients, the nitrogen (N) to phosphorus (P) ratio released by fish can be important in shaping patterns of algal biomass, community composition, primary production, and nutrient limitation. Demand for N and P as well as energy influences N/P excretion ratios and has broad implications in ecosystems where nutrient recycling by fishes is substantial. Bioenergetics and stoichiometric models predict that natural fish populations are generally energy-limited and therefore N/P recycling by fishes is relatively invariant. Yet, the potential for P limitation of growth has not been examined in herbivorous fishes, which are common in many aquatic habitats. We examined N/P excretion ratios and P demand in two P-rich herbivorous catfishes of the family Loricariidae, Ancistrus triradiatus (hereafter Ancistrus) and Chaetostoma milesi (hereafter Chaetostoma). Both fishes are common grazers in the Andean piedmont region of Venezuela where we conducted this study. Mass balance (MB) models indicate that these fishes have a high P demand. In fact, our Ancistrus’ P MB model predicted negative P excretion rates, indicating that Ancistrus did not consume enough P to meet its P demand for growth. Direct measurement of excretion rates showed positive, but very low P excretion rates and high N/P excretion ratios for both taxa. To obtain measured P excretion rates of Ancistrus from the MB model, gross growth efficiency must be reduced by 90%. Our results suggest that growth rates of both of these herbivorous and P-rich fish are likely P-limited. If P limitation of growth is common among herbivorous fish populations, herbivorous fishes recycle likely at high N/P ratios and act to diminish the quality of their food.

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We thank B. Daley, S. Harper, J. Headworth, L. Knoll, M. Munson, B. O’Shea, and B. Taylor for their help in the field or lab. M. Hughes and A. Bailer helped with error propagation. We thank E. Hall, A. Kay, P. McIntyre, D. Post, R. Sterner, K. Zimmer, and one anonymous reviewer for comments on the manuscript. Our work in Venezuela would have been far more difficult and less rewarding if not for the generosity of the Figueredo family, D. Taphorn, and B. Perez. This work was supported by NSF grant DEB 9615620 to MJV and DEB 9615349 and DEB 0321471 to ASF. Funding was also provided by Miami University’s Philip and Elaina Hampton Fund for Faculty International Initiatives. All experiments and sampling discussed here comply with both USA and Venezuelan regulations.

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Correspondence to James M. Hood.

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Communicated by David Post

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Hood, J.M., Vanni, M.J. & Flecker, A.S. Nutrient recycling by two phosphorus-rich grazing catfish: the potential for phosphorus-limitation of fish growth. Oecologia 146, 247–257 (2005).

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  • Consumer driven nutrient cycling
  • Ecological stoichiometry
  • Fish
  • Streams
  • Loricariidae