Long-term reduction of late-stage European eel larval abundance at the continental slope reflects glass eel recruitment decline

Abstract Between 1971 and 1991, a series of ichthyoplankton surveys were conducted along the European continental slope to assess the abundance of late-stage European eel larvae (Anguilla anguilla). The information from these surveys provides a valuable baseline for assessing the situation prior and during the recruitment collapse as well as for comparisons with the present-day status. The resulting information can supplement existing eel recruitment data, particularly in regions that are currently not sufficiently covered by glass eel recruitment time series (e.g. eastern and southern Mediterranean Sea). In addition, comparing trends in late-stage larval eel abundance with trends at other life stages may facilitate the identification of life phases and areas with increased mortality, helping to understand the importance of oceanic mortalities for the recruitment decline. In November 2022, a survey was conducted from the northern Bay of Biscay to the Strait of Gibraltar to investigate the current abundance of eel larvae along the continental slope and to test whether current data support the hypothesis of correlated changes between late-stage larval eel abundance and glass eel recruitment. Although this study represents a snapshot of the current situation, the results demonstrate that larval eel densities around the Iberian Peninsula have continued to decrease since the 1980s and that this is in line with the trend observed in glass eel recruitment. The consistency of glass eel and late-stage larval data further indicates that no disproportionately increased mortalities occur during the final larval phase.