An enigma: how can freshwater eels (Anguilla spp.) be such a successful genus yet be universally threatened?

Freshwater eels are of tropical marine origin and have spread worldwide. Of the 19 recognised species or subspecies, all 6 temperate species are listed within IUCN Threatened Species categories, together with 7 tropical species. Many reasons have been advanced to explain the significant declines of eels including habitat fragmentation and loss, spread of parasites and diseases, pollution, impacts of hydro dams, overfishing and oceanic changes. The present paper reviews some of the unique life history characteristics of Anguillids—fecundity and semelparous spawning, adaptability, resilience, and being energetically conservative—but despite such remarkable adaptations, according to the IUCN the genus is facing possible extinction. It is suggested that Anguillid life history strategies rely on two additional components—environmental determination of sex, and the need for surplus production (the Surplus Production Hypothesis, SPH). Surplus production is required to counteract the many hazards that eels face throughout an often long and complex life history. Recruitment of temperate species has fallen by over 90%, and resulting lower densities of juvenile eels in tidal and lower river reaches provides less incentive for upstream migration and dispersal. Female eels are generally found further inland than males, with development of sex associated with lower densities. With fewer juveniles migrating to inland areas, the numbers of females will become reduced, and further compound the reduced spawner biomass and consequent reduced recruitment of glass eels. The SPH hypothesis emphasises the importance of conservative harvest of glass eels, habitat connectivity, the need to ensure maximum female escapement, and the importance of stocking inland waterways with juvenile eels. [ABSTRACT FROM AUTHOR]