BACKGROUND: Niche partitioning allows species to diversify resource utilisation and space allocation and reduce interspecific competition. Variations in abiotic and biotic conditions in different ecosystems may further influence resource availability and habitat utilisation, potentially reducing competition. The aim of this study is to investigate the effects of environmental variation on spatial and trophic niche overlap between two freshwater apex predators, the northern pike (Esox lucius) and the European catfish (Silurus glanis), in three different water bodies. METHODS: We used fine-scale acoustic telemetry to assess the spatial niche overlap of pike and catfish, analyzing their spatial and habitat use in relation to the thermocline and their presence in benthic versus open-water habitats. Stable isotope analysis (SIA) was used to quantify trophic niche overlap and dietary differences between the species. We compared the habitat use, spatial niche width and overlap, and trophic differentiation among waterbodies to determine how environmental conditions influence predator interactions. RESULTS: During summer, pike and catfish primarily occupied benthic habitats above the thermocline across all waterbodies and diel periods. However, catfish more frequently used open water above the thermocline, while pike were more often present in both open water and benthic habitats below it. While this general pattern of habitat use was consistent, its extent varied among lakes, suggesting that local environmental conditions shape species-specific habitat selection. Despite these variations, the species exhibited substantial spatial overlap, though its magnitude fluctuated across waterbodies and diel periods. Catfish occupied a broader spatial niche in two waterbodies, while pike had a broader niche in one. Across all lakes, catfish consistently maintained a broader trophic niche than pike. However, pike exhibited higher trophic overlap with catfish than vice versa, with nearly complete overlap in one lake and substantial but incomplete overlap in others. This suggests that pike relies more heavily on shared prey resources, while catfish exploits a broader range of food sources beyond those used by pike.These patterns were primarily driven by the position of the thermocline, prey availability, structural complexity and the greater foraging plasticity of catfish, highlighting the environmental dependence of niche partitioning in these predators. CONCLUSIONS: Our findings demonstrate that spatial and trophic niche overlaps between pike and catfish are highly context-dependent, shaped by abiotic conditions, prey availability, and species-specific foraging strategies. This study highlights the importance of integrating spatial and trophic analyses to understand predator interactions in aquatic ecosystems.

• Keywords
• Acoustic telemetry, Aquatic ecosystems, Dietary plasticity, Environmental variation, Interspecific competition, Niche partitioning, Predator interactions, Resource utilization, Spatial niches, Stable isotopes, Trophic niches,
• Publication type
• Journal Article MeSH

European catfish is a large-bodied apex predator, a key species in native areas, but invasive in others where it negatively impacts local aquatic fauna necessitates catfish regulation. However, traditional ichthyological methods face challenges in capturing it. The study presents a detailed description of the efficient long-line method, refined through 48 sampling campaigns across twelve European water bodies. This method proves cost-effective and technically undemanding, requiring an average of 5.6 bait fish to catch one European catfish per day. The long-lines outperform other techniques, with the highest Biomass per unit effort (BPUE) of 6.205 kg of catfish per man-hour and minimal by-catch (0.276 kg per man-hour). In contrast, fyke nets, the second most efficient method, achieve a BPUE of 0.621 kg of catfish per man-hour with 3.953 kg of by-catch per man-hour. To optimize long-line catches, a 15 m distance between branch lines and regular relocation is recommended. Live fish is the most effective bait with no significant differences observed among species. However, earthworms, a less controversial alternative, are also efficient, especially for smaller catfish. Our recapture approach using various ichthyological methods revealed no hook avoidance behavior by catfish after a previous catch or avoidance by a certain part of the population. The long-line method is suitable for population regulation, scientific research, and conservation efforts and is the most effective means of capturing live European catfish.

• Keywords
• Bait, Ichthyological method, Large-bodied predator, Non-native area, Predation pressure,
• Publication type
• Journal Article MeSH

Stable isotope analysis (SIA) is widely used to study trophic ecology and food webs in aquatic ecosystems. In the case of fish, muscle tissue is generally preferred for SIA, and the method is lethal in most cases. We tested whether blood and fin clips can be used as non-lethal alternatives to muscle tissue for examining the isotopic composition of two freshwater predatory fish, European catfish (Silurus glanis) and Northern pike (Esox lucius), species of high value for many freshwater systems as well as invasive species in many others. Blood samples from the caudal vein, anal fin clips, and dorsal muscle obtained by biopsy punch were collected from four catfish and pike populations (14-18 individuals per population). Subsequently, these samples were analyzed for δ13C and δ15N. The effects of alternative tissues, study site, and fish body mass on the isotopic offset were investigated. Both species showed a correlation between the isotopic offset and the tissue type, as well as the study site, but no significant relationship with the body mass. The isotopic offsets between tissues were used to calculate the conversion equations. The results demonstrated that both blood and fin clips are suitable and less invasive alternative to muscle in SIA studies focused on European catfish and Northern pike. Blood provided better correspondence to muscle isotope values. However, our results clearly demonstrated that isotopic offsets between tissues vary significantly among populations of the same species. Therefore, obtaining a muscle biopsy from several individuals in any population is advisable to gain initial insights and establish a possible population-specific inter-tissue conversion.

• MeSH
• Ecosystem * MeSH
• Esocidae physiology   MeSH
• Nitrogen Isotopes analysis   MeSH
• Carbon Isotopes analysis   MeSH
• Fresh Water MeSH
• Catfishes * MeSH
• Muscles chemistry   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Nitrogen Isotopes MeSH
• Carbon Isotopes MeSH