Offshore Spawning Migrations of Anguillid and Conger Eels in Two Oceans: Convergent Strategies for Large-Scale Recruitment Success

Catadromous freshwater eels of the genus Anguilla are important fisheries species and are well-known for making long migrations to reproduce in their offshore spawning areas in the open ocean.  However, the American conger eel also appears to migrate offshore to spawn in the southwestern Sargasso Sea of the Atlantic, near the spawning area of the American eel, based on historical catches of their small larvae.  It has also now been discovered that the common Japanese conger eel migrates offshore to spawn just west of the Japanese eel spawning area in the western North Pacific using DNA identification of recently hatched larvae.  A tropical anguillid, the giant mottled eel, also migrates offshore to spawn in the same area in the Pacific, and the European eel also spawns in the Sargasso Sea.  This indicates that both anguillid and conger eels appear to have independently established migrations to spawn offshore in similar locations in both the north Atlantic and Pacific subtropical gyres using  long-distance migratory behaviors, because these species are not closely related, and their long migrations probably must originate from species with shorter migrations.  Spawning offshore may be a strategy to allow a large number of offspring to develop and grow in the lower-predator open ocean and then to arrive to more predator-dense regions along the continental shelves in great abundance and at large sizes as they migrate to their recruitment habitats in freshwater and estuaries for Anguilla and coastal marine habitats for Conger.  In contrast, almost all other marine eels do not appear to migrate far offshore for spawning.    The realization that migrations to offshore spawning areas have evolved in both marine and freshwater eels indicates that there is an important adaptive value for species that are able to establish these long migration loops in which their adults must be able to migrate long distances and then also find the same region in the vast ocean where they were born. Both conger eels and anguillid eels are extensively harvested in East Asia and are also harvested in the Atlantic region, but populations have decreased in recent decades possibly because of anthropogenic impacts within their continental habitats, or due to changing conditions offshore in the ocean affecting the survival and growth of their larvae.  Increased understanding about the spawning ecology and migratory strategies of anguillid and conger eels will facilitate conservation efforts for these commercially important fishes that are endangered worldwide.