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Stritzel Thomson, Jennifer L.

Rubenstein School of Environment and Natural Resources

2006

M.S.

Rainbow smelt (Osmerus mordax) occupy the middle of the pelagic foodweb in Lake Champlain as zooplanktivores and cannibals, while serving as the major forage fish for piscivores. The focus of this study was to (i) relate spatial and temporal distributions of young-of-the-year (YOY) and yearling-and-older (YAO) smelt to zooplanktivory patterns and (ii) to determine the effects of density and availability of larger prey (Mysis relicta and YOY smelt) on smelt individual growth rates. We used hydroacoustics to obtain smelt abundance, midwater trawling to verify acoustic targets and to collect fish for size and stomach content information, and sampled zooplankton with vertical tows throughout the lake from June to September in 2001 and 2002. From these data, diets of smelt and zooplankton consumption were related to zooplankton abundance and species composition and we calculated individual growth rates of smelt. The 2001 smelt yea class was very large, which subsequently resulted in a large number of YAO smelt in 2002. Therefore, in 2001 YOY smelt consumed a major portion of the available zooplankton whereas the majority was consumed by YAO smelt in 2002. Through bioenergetics modeling and zooplankton production estimates, we determined that zooplankton production remained positive in most months and areas during both years despite high consumption of zooplankton by smelt. Seasonally, smelt consumed more zooplankton in September than in June or July. Through AIC analyses, we concluded that YOY growth rates were negatively influenced by YOY density and YAO growth rates were positively influenced by the abundance of Mysis in diets. The findings of this study provide a substantial contribution to our knowledge of the Lake Champlain food web, especially in light of the recent invasion of alewife (Alosa pseudoharengus). Preinvasion levels of smelt consumption and growth rates will be the information needed to document the effect of alewife on the food web as the alewife population becomes more abundant.