|Ontogenetic characterization of the feeding habits of Franciscanas, Pontoporia blainvillei, based on stable carbon and nitrogen isotopes|
Cibele Troina, G. (2013). Ontogenetic characterization of the feeding habits of Franciscanas, Pontoporia blainvillei, based on stable carbon and nitrogen isotopes. MSc Thesis. Universiteit Antwerpen/Universiteit Gent/VUB: Antwerpen, Gent, Brussel. 16, 45 pp.
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We characterized sex- and age-related resource exploitation and ontogenetic dietary shifts in franciscana dolphins (Pontoporia blainvillei), by-caught between 1992 and 2010 in Rio Grande (RS), Brazil. Carbon and nitrogen stable isotope ratios, expressed as d13C and d15N, were measured in the tooth dentin of dolphins from different age classes (calves, juveniles, first adults and adults). d15N values were negatively correlated with Total Body Length (TBL) and age which was attributed to decreasing influence of the suckling period. Correlations of d13C with TBL and age were small but significant, with slight enrichment in 13C in older animals. No significant differences were found in isotope signatures between females and males, indicating that both sexes forage upon the same resources. The isotopic compositions of main prey species in the diet of these dolphins were also analyzed. Bayesian isotopic mixing models were applied to estimate the fractional contribution of prey items to each ontogenetic class. Prey groups had high similarities in their isotope signals, which hindered the ability of the models to isolate the proportional contribution of prey groups separately. The effectiveness of Stable Isotope Analysis to assess foraging ecology was compared to that of Stomach Content Analysis (SCA). Stomach contents have been extensively used to infer the diet of franciscanas in this region for several years, and it is therefore considered representative of their diet. SIAR results indicated that ES_fish and squid had a high contribution to calves and juveniles. However, SCA revealed that squid was more frequent in adults, although their IRI could not be compared to that of fishes through that method due to the different digestion rates of otoliths vs beaks. Results obtained for the younger age classes may be biased due to the high relative contribution of the wide deposition of the first year dentine. Therefore, caution is necessary when interpreting these results. On the other side, adult SIAR results seemed to be more coincident with that of SCA, although some differences in the relative contribution of the main prey were found. The present study is a first attempt to characterize the ontogenetic dietary shifts in franciscanas through the analysis of their isotopic signatures. The results presented here emphasize the importance of integrating previous information about the species’ feeding habits, instead of relying exclusively on the model’s outcome.