The ecology of oceanic dispersal and survival of anguillid leptocephali

Recruitment declines of anguillid eels are difficult to understand because both anthropogenic impacts on juveniles and adults and oceanic changes affecting larval survival or dispersal may be contributing. Anguillid larvae may passively disperse widely from offshore spawning areas but late-stage larvae or glass eels apparently must swim directionally to reach recruitment habitats. Their long larval durations vary among tropical (similar to 3-4 months) and temperate species (5 months to >1 year). The bodies of anguillid leptocephali are filled with transparent gelatinous material, possibly reducing predation rates and providing an energy reserve for swimming and metamorphosis. Leptocephali feed on marine snow making their first-feeding success linked to primary producers contributing to marine snow production. Alternations between ubiquitous cyanobacteria dominating in low-nutrient conditions and eukaryotic phytoplankton such as diatoms that are important for marine snow production dominating in high-nutrient conditions may influence early-larval survival at first-feeding due to many eggs simultaneously hatching within sympatric spawning areas. Fewer spawning eels resulting from population reductions and variations in early-larval survival may offer some explanations for lower and fluctuating recruitment in recent decades.