Egg drift and hatching success in European river lamprey Lampetra fluviatilis: is egg deposition in gravel vital to spawning success?

Abstract

1.The European river lamprey Lampetra fluviatilis (Linnaeus, 1758) is a threatened species, formerly widespread throughout western Europe, for which loss and degradation of habitat is one of the main causes of decline. As with other lamprey species, areas of gravel substrate with moderate flows are considered necessary for spawning and egg development of river lamprey. 2.This study investigated the dispersal of river lamprey eggs downstream of a spawning area and the hatching success of eggs in the laboratory under a range of potential abiotic conditions (substrate type, water flow and dissolved oxygen level) which eggs could experience in nature. 3.Lamprey eggs were found to drift for a minimum of 50 m downstream of spawning excavations, facilitating dispersal in riffle habitat and to bankside depositional zones. Under conditions mimicking natural microhabitats, but without predation, median egg hatching success was 85.0% in 'spawning habitat' conditions, but surprisingly, was 50.2% in 'larval habitat' conditions with natural silt. 4.The study suggests that egg dispersal out of spawning excavations may be common in this species and demonstrates that habitat located downstream of spawning areas, even larval habitat characterized by fine sediment and moderate to low flow rates, could play an important role in larval recruitment. This suggests that even small areas of gravel or degraded spawning habitat may enable a higher degree of spawning success than has previously been assumed to be necessary for conservation or recolonization of this species.