Citizen science network reveals unique insights into coastal eel abundances over the past 20 years

The European eel holds a central position in discussions of historical population changes among European fish species. Since the late 1970s, the species has suffered a dramatic decline and is described as critically endangered by the IUCN Red List of Threatened Species. Assessing stock trends however remains highly challenging, as eels occupy a wide range of freshwater, estuarine, and marine habitats, which complicates the collection data and the evaluation of population fluctuations.
In Denmark, coastal fish abundances have been monitored since 2002 through the Citizen Science network “Nøglefiskere” (Key Fishermen), where volunteer recreational fishers collaborate with DTU Aqua and fisheries organizations to collect standardized catch data from Danish coastal waters. The most recent Nøglefisker report, among other findings, summarizes 20 years of catch per unit effort data on eel, revealing significant temporal and spatial changes.
In this presentation we elaborate on catch results of eels and present regional differences. Overall, the catch data from 2002 until approximately 2015–2017 show stable relatively low catches. Thereafter, however a marked increase has been documented nationally. This increase is mainly driven by catches in eastern Danish fjords and coastal regions such as Roskilde Fjord, Øresund, Køge Bugt, Storstrømmen, Fakse Bugt, Hjelm Bugt, and the Fehmarn Belt, which outmatch a decline recorded in areas including the South Funen Archipelago, Als, and Lillebælt. No clear explanation is available for this increase, but positive management actions are outlined. Furthermore, eels’ general linkage to lunar phases was also visible in catch rates as these were found to vary with lunar phases, being higher around new moon than full moon.
These spatial and temporal patterns emphasize the complexity of eel stock dynamics and underscore the need for continued long-term monitoring through standardized coastal fishery surveys such as the Nøglefisker network.