ECOLOGICAL INSIGHTS FROM δ18O AND δ13C OF AMERICAN EEL ANGUILLA ROSTRATA OTOLITHS — A COMBINATION OF LABORATORY AND FIELD APPROACHES

Abstract

Fish species with complex life cycles, extensive migrations, or broad distributional ranges, such as the American eel, Anguilla rostrata, commonly have poorly understood ecologies because of a lack of information about their ambient environment. Stable isotopes of otoliths offer lifelong records of ambient conditions and life history, making them a promising tissue for ecological studies. However, unexplained variation in δ18O geothermometers among fish species has necessitated use of species-specific geothermometers, thus limiting the tool’s broad utility. Furthermore, interpretations of δ13C of otoliths are complicated by multiple driving factors (e.g., diet, DIC) and disputes over the possible influence of metabolic rate on both isotopes. In this thesis, I used three controlled laboratory experiments to isolate and examine: (1) the relationship between temperature and δ18O of otoliths, (2) the relative contribution of diet versus DIC to δ13C of otoliths, and (3) metabolic rate influences δ13C and δ18O. In experiment 1, I developed an eel-specific δ18O fractionation equation (geothermometer); the slope was shallower than equations for other species. In experiment 2, otoliths reflected the full δ13C range of four isotopically distinct diets, regardless of diet quality. Mixing models showed diet contributed ~50% of total δ13C to otoliths. In experiment 3, I used the novel approach of manipulating swimming activity to induce a gradient of metabolic rates. The results showed no relationship between metabolic rate and δ13C. I propose that temperature may explain the apparent connection between metabolic rate and δ13C. Finally, I applied the laboratory findings to an ecological study of stocked American eels in the St. Lawrence River. The δ18O of eel otoliths could be used to estimate water temperature within 0.4oC of their habitat. I used δ13C to determine habitat use after stocking and found that eels change habitat once they reach a specific age and size. Collectively, my thesis integrates three long-term laboratory experiments and 6 years of field data to elucidate the role of temperature, diet, and metabolic rate on the oxygen and carbon isotope compositions of otoliths with implications for future ecology studies of species with complex life histories.