New clues for freshwater eels (Anguilla spp.) migration routes to eastern Madagascar and surrounding islands

A total of 4,172 freshwater eels have been collected by electrofishing in upper estuaries from Madagascar (East coast), Mascarene (Reunion and Mauritius Is.), Comoros (Mayotte Is.) and Seychelles (Mahe and Praslin Is.) Archipelagos, between October 2003 and February 2006. Eel species composition in the sampling stations was contrasted between eastern Madagascar (Anguilla mossambica 96.0%, A. marmorata 3.9% and A. bicolor bicolor 0.2%), the Comoros (A. marmorata 56.1% and A. bicolor bicolor 43.9%), the Mascarene (A. marmorata 91.4%, A. bicolor bicolor 5.4% and A. mossambica 3.2%) and the Seychelles Archipelagos (A. bicolor bicolor 100.0%). This gradient in species composition, even concerning the short time-range of our sampling, argued for separate migration routes between species. A total of 168 eels were aged by reading their otolith microstructure, and otolith growth rates were calculated from pre-leptocephalus stage (post-hatching) to metamorphosis, until freshwater check. For all species, mean otolith growth rate (OGR) was related to specific migration routes: A. bicolor bicolor is distributed in the lowest latitudes and showed the highest OGR during leptocephalus stage, whereas A. mossambica, endemic of the Malagasy area, has the most southern distribution and showed the lowest OGR. OGR during leptocephalus stage was negatively correlated to the leptocephalus stage duration, showing a decrease of global metabolism with time, classical in leptocephali. This relationship was found significant for A. marmorata and A. mossambica, probably because all these larvae crossed successively the same environments, but not for A. bicolor bicolor, probably because their larvae crossed different pelagic environments, opening the hypothesis of larvae from different origins.