Million Tesfaye

Effective fisheries management depends on a clear understanding of the recruitment dynamics of young-of-year (YOY) fish. Analysis of two-decade ichthyological data from Czechia and the Netherlands revealed an unexpectedly small cohort of YOY pikeperch (Sander lucioperca) in late summer. The origin of these individuals remained speculative. Building on the long-term patterns documented in these regions, we provide a new empirical contribution by analyzing the daily otolith increments of 29 pikeperch individuals sampled in the Lipno Reservoir (Czechia) in late August 2023. The otolith-based aging revealed that the extremely small pikeperch were not more than 35 days old, indicating they hatched at the turn of July and August. Even with the uncertainty in the exact timing of the first otolith increment formation, this approach disclosed the origin of the extremely small cohort as late-hatched individuals produced by wild late-spawning pikeperch. These extremely small, delayed pikeperch coexist with their normal, earlier-spawned conspecifics, which are often already piscivorous. Although they mostly do not survive the first winter, they still contribute to the recruitment indirectly by serving as prey for conspecifics. Our results highlight the need to revise the current understanding of pikeperch spawning phenology in the temperate zone.

Fish stocking is a common management practice used to mitigate the effects of overfishing and other threats to wild fish populations. It is mainly employed in freshwater ecosystems with complex and variable strategies (different timings, fish lengths and origins), making its effectiveness challenging to assess. To assess and evaluate the prevalence of artificially stocked individuals in a population with a complex management history (pikeperch— Stizostedion lucioperca— from Lipno Reservoir, Czechia), laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) was used to measure the micro‐elemental composition at the core of otoliths to verify the natal origin of fish (wild and stocked individuals) and confirm the origin of fish sources (facilities). Of the ten elements investigated, nine (Ba/Ca, Cu/Ca, K/Ca, Li/Ca, Mg/Ca, Mn/Ca, Na/Ca, Rb/Ca and Sr/Ca) exhibited significant inter‐origin variation, whereas Zn/Ca failed to provide a meaningful discriminatory signal. A machine learning classification algorithm, trained with data from 100 fish of known origin, was used to classify 70 fish of unknown origin. The five most relevant variables for correctly classifying fish origins were Rb/Ca, Li/Ca, Sr/Ca, Ba/Ca and Mg/Ca. The model achieved an overall accuracy of 99%, with 97% accuracy for wild fish and 100% for stocked fish. For identifying exact fish sources, overall accuracy for stocking facilities was approximately 89%, with individual facility detection accuracy ranging from 28% to 100%. The model determined that most fish of unknown origin were autochthonous (57 individuals, 81.4%), with a smaller proportion identified as stocked fish (13 individuals, 18.6%). Given the relatively small contribution of stocked fish to the pikeperch population, the efficiency of the stocking programme should be further scrutinised to avoid ineffective management strategies. More broadly, these results highlight that otolith microchemistry combined with machine learning provides a robust framework for evaluating stocking efficiency and supporting evidence‐based fisheries management.

Using pikeperch ( Sander lucioperca ) as a model species, we investigated the factors influencing fluctuations in the abundance of piscivorous intra‐annual cohorts and assessed the role of three distinct cohorts: extremely small (ES), ordinary (or mid‐sized) fingerlings, and piscivorous in shaping year‐class strength. Our analysis was based on 13 years of data (2003–2024) collected in the Lipno Reservoir, Czechia. Using LASSO for variable selection among 45 potential covariates and subsequent Generalized Additive Model (GAM) construction, our results revealed that a higher abundance of ES conspecifics and young‐of‐the‐year (YOY) perch, coupled with increased densities of large cladocerans in July, positively impacted the abundance of the piscivorous intra‐annual cohort. The abundance of 1+ pikeperch was positively influenced by the abundance of piscivorous and ordinary cohorts recorded in the preceding year. We concluded that all three intra‐annual cohorts contribute to the year‐class strength of pikeperch, with piscivorous and ordinary cohorts having a more direct effect, while ES intra‐annual cohorts indirectly support the piscivorous cohort by serving as prey.

As a significant piscivorous predator, the pikeperch ( Sander lucioperca ) plays a critical role in regulating aquatic ecosystems. Understanding its size‐related feeding behaviour is thus vital for assessing its impact. To evaluate its role in the ecosystem, stomach content analysis provides useful insights into predator–prey dynamics. This study investigated the stomach content and dietary patterns of pikeperch in the Lipno Reservoir, Czech Republic, across two distinct time periods: the 1966–1970 (19's) and the 2010–2022 (20's). We analysed the stomach contents of 875 pikeperch specimens and revealed a notable shift in diet composition over time. The results demonstrated a significant decrease in overall prey consumption and a marked increase in the proportion of cyprinids in the diet, whereas the percentages of perch ( Perca fluviatilis ) and ruffe ( Gymnocephalus cernua ) decreased. Cannibalism was observed in the 20's dataset, suggesting a reduction in prey abundance in the reservoir or a shift in community structure. The predator–prey length ratio (PPR) decreased as pikeperch grew larger. The PPR values in Lipno Reservoir were lower than those reported in other studies, indicating potentially slower growth rates in this fish. Although percids were the main prey of pikeperch, the electivity index showed positive values only for pikeperch as the prey. Overall, this study highlights long‐term changes in pikeperch diets, both in the numbers and species of prey and in PPR.

Freshwater ecosystems are highly susceptible to degradation, underscoring the importance of assessing their health using ecological indicators. In this study, we developed an Ecopath mass‐balance model for Lake Tinshu Abaya, Ethiopia, and analyzed its trophic structure and energy flows. The model integrated seven functional groups using data on fish catch, diet composition, and plankton biomass collected from March to August 2022. The seven functional groups were detritus, phytoplankton, herbivorous zooplankton, carnivorous zooplankton, zoobenthos, Nile Tilapia ( Oreochromis niloticus ), and birds. Four trophic levels were determined, ranging from 1.00 for primary producers and detritus to 3.56 for birds of prey. The calculated values for the EE of the primary producers (phytoplankton: 0.87; detritus: 0.75) indicated that they were highly exploited compared to the secondary producers, the herbivorous zooplankton (0.40), the carnivorous zooplankton (0.065), and the zoobenthos (0.60). The computed primary production to total respiration ( P / R ) ratio of 1.19 suggested that the ecosystem is in a developing, net autotrophic state. The ecosystem exhibited a high degree of internal recycling (Finn's Cycling Index = 15.2%), reflecting efficient energy and nutrient utilization. The trophic connectivity index of 0.56 suggested a moderately interconnected food web. Collectively, these metrics indicate that Lake Tinshu Abaya is a developing ecosystem in a transitional stage, not yet having reached full ecological maturity.

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.

Pikeperch (Sander Lucioperca) belongs to main predatory fish species in freshwater bodies throughout Europe playing the key role by reducing planktivorous fish abundance. Two size classes of the young‐of‐the‐year (YOY) pikeperch are known in Europe and North America. Our long‐term fish survey elucidates late‐summer size distribution of YOY pikeperch in the Lipno Reservoir (Czechia) and recognizes two distinct subcohorts: smaller pelagic planktivores heavily outnumber larger demersal piscivores. To explore molecular mechanisms accompanying the switch from planktivory to piscivory, we compared brain transcriptomes of both subcohorts and identified 148 differentially transcribed genes. The pathway enrichment analyses identified the piscivorous phase to be associated with genes involved in collagen and extracellular matrix generation with numerous Gene Ontology (GO), while the planktivorous phase was associated with genes for non‐muscle‐myosins (NMM) with less GO terms. Transcripts further upregulated in planktivores from the periphery of the NMM network were Pmchl, Pomcl, and Pyyb, all involved also in appetite control and producing (an)orexigenic neuropeptides. Noncoding RNAs were upregulated in transcriptomes of planktivores including three transcripts of snoRNA U85. Thirty genes mostly functionally unrelated to those differentially transcribed were alternatively spliced between the subcohorts. Our results indicate planktivores as potentially driven by voracity to initiate the switch to piscivory, while piscivores undergo a dynamic brain development. We propose a spatiotemporal spreading of juvenile development over a longer period and larger spatial scales through developmental plasticity as an adaptation to exploiting all types of resources and decreasing the intraspecific competition.