Control and eradication efforts of aquatic alien fish species in Lake Caicedo Yuso-Arreo

Lake Caicedo Yuso-Arreo (Spain) supports dense populations of alien fish species, namely common carp (Cyprinus carpio), largemouth bass (Micropterus salmoides), pumpkinseed (Lepomis gibbosus), and the red swamp crayfish (Procambarus clarkii). These introductions have caused a reduced transparency, decreasing submerged vegetation and population decline of native tench (Tinca tinca). Alien species were caught and removed for 76 days in 2014 and 2015 with the objectives of eradicating carp and reducing largemouth bass and pumpkinseed populations under 20% of 2014 observed density and biomass. Only two individuals of tench were captured, underlining its scarcity. A total of 27,077 individuals (1089 kg) of alien species were removed, but eradication was not achieved for any species. The largest sizes of carp and largemouth bass almost disappeared, while in 2015 an increase in the abundance of large recruits was observed. A hydroacoustic fish stock assessment was carried out at the end of the action in 2015 to obtain data on fish density and biomass in open water as a mean to compare results to measurements from 2010. Although the density of fish increased from 2010 to 2015, biomass fell drastically. In 2017, the alien species community was controlled again, revealing a substantial recovery from previous control efforts, a change in the size distribution towards smaller specimens and interactions among North American species. Additionally, no tench could be identified and the species is considered locally extinct. Thus, the results from this study suggest the need for ongoing control efforts with increased electrofishing and implementation of alternative control methods such as the introduction of biological control agents to achieve the ecological status improvement and environmental restoration goals.