Intraspecific competition is a fundamental selective force in animals, leading to various specializations that influence ecological interactions. Diet composition and trophic position at the early life stages substantially influence fish growth, survival, and recruitment success. Yet, most studies focus disproportionately on adult stages, leaving critical knowledge gaps in our understanding of early life history. To address this, we used young of the year (YOY) pikeperch (Sander lucioperca) as a model species and investigated the intraspecific interaction and degree of trophic partitioning between three intra-annual cohorts (extremely small (ES), ordinary and piscivorous YOYs) using stable isotope (SI) and gut content analysis (GCA). Analysis of SI metrics unveiled that an ontogenetic diet shift was linked to increasing body size, leading to significant trophic niche variation among intra-annual cohorts. The piscivorous cohort occupied the highest trophic position, followed by the ordinary and ES cohorts. There was no overlap in the isotopic niche between the intra-annual cohorts, considering the 40% standard ellipse area. The GCA showed two distinct feeding patterns: the ES cohort exclusively consumed zooplankton, while the ordinary cohort had a more diverse diet, feeding on zooplankton and benthic macroinvertebrates. The piscivorous cohort (≥ 80 mm) predominantly fed on their conspecifics and YOY perch (Perca fluviatilis). Our study demonstrates that YOY pikeperch intra-annual cohorts exhibit a broad size range and unique ontogenetic feeding patterns, with vital implications for population dynamics and ecological interactions. These differences are likely due to different hatching dates, environmental factors, and individual ability to become predatory. Furthermore, this work emphasizes the need for comparative studies to better understand trophic dynamics and uncover the ecological factors shaping the early life stages of fish.

• Klíčová slova
• gut content analysis, intraspecific competition, ontogeny, piscivory, stable isotopes,
• Publikační typ
• časopisecké články MeSH

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.

• Klíčová slova
• Acoustic telemetry, Aquatic ecosystems, Dietary plasticity, Environmental variation, Interspecific competition, Niche partitioning, Predator interactions, Resource utilization, Spatial niches, Stable isotopes, Trophic niches,
• Publikační typ
• časopisecké články 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.

• Klíčová slova
• Bait, Ichthyological method, Large-bodied predator, Non-native area, Predation pressure,
• Publikační typ
• časopisecké články 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
• ekosystém * MeSH
• Esocidae fyziologie   MeSH
• izotopy dusíku analýza   MeSH
• izotopy uhlíku analýza   MeSH
• sladká voda MeSH
• sumci * MeSH
• svaly chemie   MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• Názvy látek
• izotopy dusíku MeSH
• izotopy uhlíku MeSH