Reconstructing lifetime nitrogen baselines and trophic position of Cynoscion acoupa from δ15N values of amino acids in otoliths

Habitat connectivity and trophic shifts during the lifetime of an individual fish are important determinants of fish population growth and persistence, yet remain little understood for many species. We investigated whether insights into individual lifetime migration, trophic position (TP) and environ mental nitrogen dynamics could be achieved using compound-specific nitrogen isotope analysis of oto lith proteinogenic amino acids (AAs). By comparing acoupa weakfish Cynoscion acoupa otoliths and muscle tissue from the monsoonal Amazon area in Para with otoliths from semi-arid Rio Grande do Norte (RGN), Brazil, this study illustrates estuarine to coastal shelf habitat use and trophic ecology during juvenile and adult stage growth. Muscle tissue and otoliths gave comparable TPs for both life stages, while weighted mean delta N-15 values of all source AAs differed between tissues. These differences reflected large seasonal and spatial changes in nitrogen biogeochemical cycles and anthropogenic nitrogen influences from the Amazon River onto the coastal shelf of Para. AA delta N-15 values of fish otoliths from the Para region indicated changes in TP and sources of nitrogen between life stages, whereas analysis of fish otoliths from the RGN region indicated similarities in individual TP and sources of nitrogen through ontogeny. However, in both areas, individual adult TP ranged between 3 and 4, whereas juvenile TP remained around 2.8 to 3.0 in Para and RGN, respectively. Since otoliths preserve a record of baseline delta N-15 values over the lifetime of individual fish it may be possible to infer migration and TP across pre historic ecosystems from AA isotopic analysis of an cient otoliths.