Assessing feeding history as recorded in tropical fish scales

1. Osteoglossum bicirrhosum Assessing feeding history as recorded in tropical fish scales Tomas F. Domingues 1, M. C. Martinelli 2, L. A. Martinelli 3 and J. R. Ehleringer 1 1 - University of Utah, 2 - IPAM - Brazil, 3 - Universidade de São Paulo - CENA - Brazil Introduction Stable isotope analysis can be a useful technique providing information about the life history of organisms, if the organisms have a recording device to register events throughout their life time. We evaluated the linear distance along fish scales as a recorder of a fish’s feeding habits. Scales are mainly composed of collagen with a long turnover time, allowing fish scales to potentially be accurate indicators of dietary inputs. Mineralization creates an ossified layer on scales. The presence of calcium carbonates in this layer may pose a difficulty for evaluating feeding history. To test the hypothesis that scales register fish feeding history, we evaluated scales from two large fish species occurring in the Amazon Basin, a region that shows strong seasonality in food resource availability. No temporal trend was common to most A. gigas evaluated, and a high variation in d13C and d15N was observed. It was also found that shifts in isotopic composition were frequent. Individual preferencesand exploration of different habitats may explain the high variability among individuals while shifts in isotopic composition may be caused by migration between lateral lagoons and the main channel. The d13C values of the calcified layer were not statistically different from that of the collagenous layer indicating that carbonates, if present, do not alter the carbon isotopic signature of the scale. However, d15N, C% and N% values were statistically different between ossified colagenous layers. The link between forest and fish The interaction between fish and forest is modulated by the annual flooding of the Amazon River. For most fish species food availability changes with water level, since seasonally flooded forests are a large portion of the aquatic system and represent a very different ecosystem than the main river. During high waters, fish disperse and colonize lateral lagoons, which may became isolated from the main channel during low waters. Arapaima gigas and Osteoglossum bicirrhosum are both large predators, with the former feeding on fish and the latter on allochthonous insects. When analyzed together, scales of A. gigas with different sizes, showed a trend that was opposite that found for O. bicirrhosum. Because scales from different locations showed distinct isotopic composition, it may be possible to identify the origin of captured fish. Methods Samples were taken from the posterior portion of the scales, following a straight line from its focus to its edge, representing the chronological nature of scale growth. A dual inlet Finnigan Mat IRMS coupled with a Carlo-Erba EA, operating at continuous flow was used to determine carbon and nitrogen isotope ratios and concentrations. Analytical precision was ± 0.15 ‰. O. bicirrhosum showed a progressive depletion for both 13C and 15N as fish aged. This trend may be explained by a progressive importance of allochthonous food to this species diet. The annual water level fluctuation does not seem to influence the feeding behavior of this species. • Conclusions • Fish scales do record feeding history, with a suggestion of dietary change with the size of the fish. • A. gigas showed signs of being influenced by the seasonality of the Amazon Basin, perhaps reflecting fish that were once restricted to lakes now having access to open river water. Arapaima gigas