Stock discrimination of European conger eel (Conger conger L.) using otolith stable isotope ratios

European conger eel (Conger conger) is a common and widely distributed fish in the NE Atlantic and Mediterranean. Although there is increasing evidence that stocks of European conger eel are in decline, there is little published material on the population structure or management of the species. Stable isotope ratios, namely δ18O and δ13C, measured by standard mass spectrometric techniques in whole otolith samples of juvenile conger eels sampled in April/May 2006 from NE Atlantic (Azores, Madeira and North Portugal) and Mediterranean (Mallorca) exploited stocks provided location-specific signatures. Isotopic ratios for conger eel were similar to those reported for other marine species (δ18O‰, mean ± SE: 1.64 ± 0.04, 1.87 ± 0.02, 1.26 ± 0.03, 2.41 ± 0.02 and δ13C‰: −1.78 ± 0.08, −2.09 ± 0.07, −3.43 ± 0.15, −2.73 ± 0.12, respectively for North Portugal, Azores, Madeira and Mallorca). Relationships between isotopic ratios (δ18O and δ13C) and otolith mass were not significant. Seawater temperature could partially explain the differences in δ18O of otoliths among the fishery locations. The δ18O signatures of the Portuguese mainland coastal specimens were, however, lower than expected values, possibly as a result of the freshwater input of the Douro River located in the vicinity of this fishery area. The inter-site variation of carbon isotopic signatures was likely related to slight differences in diets or DIC of the water. The distinct isotopic signatures suggest low levels of connectivity between the fishing grounds and that conger eels are relatively sedentary during the juvenile phase. Isotopic signatures in the otoliths of C. conger clearly discriminated between the NE Atlantic and the Mediterranean fishery areas providing support for treatment of these fisheries as different management units.