Commercial Fishing as a Complementary Action for Invasive Fish Management in the Cedillo Reservoir ^†

Abstract

Introduction: The most common method for controlling invasive fishes is mechanical removal, although it is time-consuming and operationally demanding. In Portugal, commercial inland fisheries are permitted, but the extent to which they could represent a complementary tool for invasive species control remains unknown. Objective: We compared fish assemblages in two sections of a branch of the Cedillo Reservoir where commercial fishing is allowed (i.e., downstream the Lentiscais bridge) and where it is prohibited under the regulations of the International Tagus Natural Park (i.e., upstream). Methodology: Fish were sampled in 2023 and 2024, using a total of 116 gillnets: 72 downstream and 44 upstream. The proportion of blank nets between sections was compared using Fisher’s exact test. Variations in community composition were assessed using NMDS and PERMANOVA, while homogeneity of multivariate dispersion was evaluated with PERMDISP. Total CPUE was compared between sections using Mann–Whitney tests. At the species level, CPUE were assessed using permutation-based Mann–Whitney U tests adjusted for multiple comparisons using the FDR procedure. The direction and magnitude of between-section differences were quantified using the rank-biserial correlation, and 95% confidence intervals were estimated by section-stratified bootstrap resampling. Results: In total, 20 gillnets yielded no fish, and there was no significant difference in their proportions between sections. Total CPUE per gillnet was significantly higher upstream than in the commercially fished section. Fish assemblage composition differed significantly between sections, and there were no dissimilarities in multivariate dispersion, indicating a genuine, although partial, separation between assemblages. The species contributing most to the dissimilarity between the two sections were Silurus glanis, Cyprinus carpio, Sander lucioperca, and Luciobarbus bocagei, which are also among the main target species for national inland commercial fisheries. Among these species, L. bocagei and S. glanis showed significantly lower CPUE in the commercially fished section, while S. lucioperca showed higher CPUE. Variations in C. carpio CPUE were barely significant. Conclusions: These results suggest that commercial fishing may at least partially influence fish catches and assemblage structure in this reservoir branch. Future studies should partition the influence of commercial fishing from other drivers of assemblage variation to further evaluate whether it may represent a complementary tool for managing fish invasions under specific management strategies.