Migratory history of African longfinned eel Anguilla mossambica from Maningory River, Madagascar: discovery of a unique pattern in otolith Sr:Ca ratios

The migratory life histories were investigated for 30 African longfinned eels (Anguilla mossambica) of the yellow eel stage from the Maningory River, East Madagascar using otolith Sr:Ca ratios and parasite composition. The Sr:Ca ratio patterns of nine eels resembled those of A. mossambica in South Africa and of other anguillids and were classified as “typical” Sr:Ca ratio pattern eels. Typical pattern eels were further classified into freshwater residents (five eels) and habitat shifters (four eels) depending on whether they spent their first several years in saltwater. An unique Sr:Ca ratio pattern was found in 21 eels whereby the Sr:Ca ratios in the yellow eel stage increased drastically to extremely high values of 10 to 30 × 10⁻³which have not been observed for any Elopomorpha species. These individuals were categorized as “elevated” Sr:Ca ratio pattern eels. The elevated pattern eels were characterized by significantly older ages, lower somatic growth rates, a much higher intensity and abundance of the gastrointestinal parasite Heliconema africanum and a lower abundance of the swim bladder parasite Anguillicoloides papernai. The extremely elevated Sr:Ca ratios of the yellow eel stage result most likely from inhabiting freshwater areas characterized by a high strontium concentration because the ratios far exceeded the known values for Elopomorpha species. Cyclical lows in the otolith Sr:Ca ratios of the elevated type eels corresponded to annuli, revealing either an overwintering movement pattern between habitats with different Sr:Ca ratios, or a seasonal change in the ambient environment.