APPLICATION OF STABLE ISOTOPE GEOCHEMISTRY FOR RECONSTRUCTING THERMAL HISTORY AND NATAL ORIGIN OF LAKE MICHIGAN ALEWIFE

Determining the role of climate on the ecology of aquatic organisms such as fishes is a main concern, especially in the context of Climate Change. Much work has focused on temperature because of its direct and indirect effects on reproduction, growth and recruitment mechanisms. Measurements of oxygen and carbon isotope ratios from biogenic carbonates have been successfully employed for paleoclimatic reconstructions, however, applications to fish ecology are much more limited. Because aragonitic otoliths (ear stones) grow incrementally throughout the life of a fish and oxygen isotopic ratios are related to physicochemical parameters of the environment, they permit temperature reconstructions of natal origins. We applied measurements of oxygen and carbon isotope ratios for determining the impact on recruitment of temperature experienced during the larval stage of young-of-the-year (YOY) alewives (Alosa pseudoharengus) captured at the beginning of fall of 2001 and 2002 in three sites of Lake Michigan and Muskegon Lake. Isotope profiles were produced by microsampling along otolith growth axes using a computer-automated apparatus. We find that carbon and oxygen isotope values can successfully discriminate specimens from the two lakes and permit detection of natal temperature. Most of the surviving YOY captured around Lake Michigan are those individuals who spawned in environments warmer than 19°C, which suggests that they were spawned and resided in drowned-river mouths, embayments, or tributaries. However, our conclusions remain limited due to the small sample size. Traditionally, the complex shape and size of biogenic carbonates have been the limiting factors for producing high-resolution profiles. The problem associated with shape has been solved by the use of a state-of-the-art micromill technique, but the otolith size of species such as alewife still represents a challenging issue. Carbonate quantities that correspond to the larval stage do not exceed 25 mg, which are difficult to be analyzed by a routine coupling system between a mass spectrometer and automated carbonate device.