Tourists or residents? Life history strategies of Rainbow Smelt in Lake Champlain

ABSTRACT Objective Rainbow Smelt Osmerus mordax are the most important native prey fish in the coldwater food web of Lake Champlain, but their history in the lake is not well understood. Writers in the 1800s described a nonresident population of smelt that reached sizes greater than 315 mm that was sympatric with a population of individuals that matured at smaller sizes. No studies, however, have investigated mechanisms that cause the dimorphic size structure of Rainbow Smelt in Lake Champlain. Based on our results, larger fish appear to have life history strategy differences from most of the population, which allow them to grow to larger sizes, faster. We proposed three alternative hypotheses to explain the presence of these large Rainbow Smelt in Lake Champlain: (1) anadromous and landlocked Rainbow Smelt may both be present in the lake, (2) all Rainbow Smelt may be landlocked with the two morphotypes resulting from different diet preferences and/or differences in environmental niche occupancy, or (3) the largest Rainbow Smelt are long-lived and therefore had longer time for growth. Methods We tested these hypotheses using otolith microchemistry, diet (stomach content and stable isotope), and age analyses. Our length-at-age analyses showed that age structure was not bimodal, but there are fish present in the lake as large as those in the 1800s. Results Microelemental analysis indicated that Rainbow Smelt in Lake Champlain were landlocked, with no marine signature. Stomach contents of average-sized Rainbow Smelt were exclusively zooplankton and macroinvertebrates, while the larger Rainbow Smelt were primarily piscivorous and had greater δ15N values, which suggests a higher trophic position. A combination of hypotheses 2 and 3 most likely explains current population structure. Elevated manganese : magnesium ratios in larger Rainbow Smelt suggest that they may utilize a hypoxic environmental niche, a potential key life history strategy allowing for faster growth, suggesting that there may be intraspecific variation in how Rainbow Smelt use the basins of Lake Champlain. Conclusions Based on our results, larger fish appear to have life history strategy differences from most of the population that allow them to grow to larger sizes, faster. Future studies are needed to uncover the nuances of the mechanisms for growth.