Opportunistic migration and habitat use of the giant mottled eel Anguilla marmorata (Teleostei: Elopomorpha)

Freshwater eels have fascinated biologists for centuries due to the spectacular long-distance migrations between their freshwater habitats and their spawning areas far out in the ocean. Although freshwater eels originated in the Indonesian region, remarkably little is known about the life history of tropical freshwater eels. The diverse migratory patterns and habitat choice between marine and freshwater environments by the giant mottled eel Anguilla marmorata Quoy & Gaimard, 1824 were examined by analysing the otolith strontium (Sr) and calcium (Ca) concentrations collected in Asian waters. The wide range of otolith Sr:Ca ratios indicated that the habitat use of A. marmorata was opportunistic among fresh, brackish, and marine waters. The present study first confirmed the occurrence of marine-resident eels that have never migrated into a freshwater habitat in A. marmorata. A. marmorata may have the same behavioural plasticity as temperate and other tropical anguillid species regarding whether to enter freshwater or to remain in estuarine and marine environments. Freshwater eels migrate flexibly among freshwater, brackish water, and seawater environments and it is now evident that their movement into freshwater is not an obligate migratory pathway but should be defined as an opportunistic catadromy, with marine and estuarine residents as ecophenotypes.