| QUANTIFYING ZOOPLANKTON CONSUMPTION OF LARVAL AND JUVENILE RAINBOW SMELT USING A MERCURY MASS BALANCE MODEL |
| PLOURDE, J., Sirois, P. Aquatic Sciences Laboratory, Departement des sciences fondamentales, Universite du Quebec à Chicoutimi, 555 boulevard de l'Universite, Chicoutimi (QC) G7H 2B1, Canada, jerome.plourde@uqac.ca, pascal_sirois@uqac.ca; Trudel, M., Pacific Biological Station, Fisheries and Oceans Canada, 3190 Hammond Bay Road, Nanaimo (BC) V9T 6N7, Canada, marc.trudel@dfo-mpo.gc.ca |
| Consumption rate have seldom been directly estimated in the field for larval fish. Such estimation is valuable to assess the feeding demand during early life and the top-down effect on zooplankton populations. The general objective of this study was to quantify zooplankton consumption by young-of-the-year and yearling rainbow smelt, a key forage species in several aquatic ecosystems. A mercury mass balance model has been used to determine rainbow smelt consumption rate in lake St-Jean, a large boreal reservoir. In 2009, fish were sampled seven times from June to October to measure methylmercury concentration in smelt and determine their diet. Zooplankton was sampled two times during the same period and was sorted to the lowest taxonomic level to determine methylmercury concentration in each prey species. Results showed that rainbow smelt feed almost exclusively on zooplankton between June and October. In early summer, young-of-the-year smelt ingested more than 30% of their body weight whereas in early autumn, consumption declines to less than 5%. On the other hand, yearling smelt ate 15% of their body weight early in the summer and consumption declines to less than 5% in October. These consumption rates corresponded roughly to 1 mg of zooplankton per day in early summer to 150 mg/day in early autumn for young-of-the-year smelt. This can be compared to an average of 300 mg/day for yearling smelt during summer and 100 mg/day in October. These results provided new data on zooplankton predation by rainbow smelt in lake St-Jean and will contribute to evaluate the carrying capacity of the lake for future enhancement programs. |
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