An assessment of the invasion of Rainy Lake, Voyageurs National Park, by rainbow smelt (Osmerus mordax): 1990–2002

Kallemeyn, Larry

Exotic Rainbow Smelt (Osmerus mordax), which had been present since 1972 in the upper Rainy Lake watershed, first appeared in Rainy Lake in 1990. Based on studies conducted elsewhere it appeared their invasion might have a significant effect on the aquatic ecosystem of Rainy Lake and might exacerbate an existing mercury contamination problem since there was some evidence that the rate of mercury accumulation in top level predators accelerates if they switch to a smelt diet. There were four parts to this assessment of the effect of the smelt invasion: (1) sampling methods were identified and tested, (2) food habits analyses were conducted to determine how the smelt were fitting into the food web, (3) long-term monitoring data and bioenergetics modeling were used to assess the possible effect of the smelt on the growth and condition of Walleye (Sander vitreus) and Northern Pike (Esox Lucius), and (4) mercury concentrations in smelt and native prey species were compared, as were pre- and post-invasion concentrations in Walleye and Northern Pike. Smelt abundance rose rapidly, peaking in the mid-1990s at which time hydroacoustic analysis indicated the biomass was similar to levels in the Great Lakes. By the early 2000s smelt abundance had fallen significantly. Smelt occupied their characteristic position in the food web – feeding primarily on zooplankton, icthyoplankton, and Mysis diluviana and other invertebrates and in turn being preyed on by larger piscivores. Diet overlap was significant between smelt and smaller Walleye and Sauger (<200 mm), suggesting competition might occur if shared resources became limited. Contrary to expectations, the smelt invasion did not appear to have contributed significantly to changes in growth and condition and bioaccumulation of mercury in Walleye and Northern Pike.

An assessment of the invasion of Rainy Lake, Voyageurs National Park, by rainbow smelt (Osmerus mordax): 1990–2002

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