Editorial for the Special Issue “Toxicology of Anthropogenic Pollutants on Fish”

Anthropogenic pollution is among the most pervasive and persistent pressures affecting aquatic ecosystems worldwide, and fish are central to this issue not only because they are directly exposed to contaminants throughout their life cycle, but also because they connect environmental quality, ecosystem functioning, fisheries’ sustainability, and food safety. In recent years, fish toxicology has expanded beyond conventional lethality-based assessments to include sublethal, mechanistic, behavioral, ecological, and even transgenerational responses, reflecting a broader and more realistic understanding of contaminant impacts in aquatic environments. Against this background, the Special Issue “Toxicology of Anthropogenic Pollutants on Fish” was launched to gather recent advances on the effects of human-derived pollutants on fish across different habitats, contaminant classes, and levels of biological organization. The papers published in this Special Issue reflect the breadth of contemporary fish ecotoxicology, covering classical and emerging contaminants, including ammonia, antibiotics, pharmaceuticals, herbicides, trace metals, and mercury, and addressing their effects from molecules to communities in freshwater, estuarine, marine, tropical, and temperate systems. Together, these contributions highlight the broad relevance of anthropogenic pollution to fish health and aquatic ecosystem integrity.

A clear strength of this Special Issue lies in its emphasis on ecological assessment under field conditions. Stoica et al. examined fish communities in the heavily impacted Bistrița River and showed that fish-based assessment alone may underestimate ecological impairment, whereas the inclusion of zoobenthos improves the detection of water quality degradation [Contribution 1]. Their study provides an important methodological message: although fish are indispensable indicators of environmental quality, they should not always be used in isolation when evaluating complex riverine systems. Similarly, Sunjog et al. assessed four native fish species from the Danube River under untreated wastewater exposure using an integrated suite of biomarkers, including metal and metalloid accumulation, DNA damage, micronuclei, hepatic enzyme activity, and erythrocyte maturation [Contribution 2]. Their results highlighted marked interspecific and tissue-specific differences, with white bream emerging as the most sensitive species. Taken together, these studies demonstrate that environmentally realistic assessment benefits from multi-species and multi-endpoint approaches and that fish-based biomonitoring is strengthened when interpreted within a broader ecological framework [Contributions 1 and 2].

Another major theme emerging from this collection is the increasing importance of mechanistic and sublethal toxicology. Shi et al. investigated acute ammonia stress in Pseudobagrus ussuriensis and showed that even short-term exposure induced clear liver injury, suppressed antioxidant defenses, and triggered extensive transcriptional responses [Contribution 3]. Their work further identified the PI3K–Akt signaling pathway as a prominent molecular target associated with stress-induced hepatic damage. In marine medaka, Huang et al. demonstrated that environmentally relevant sulfamethoxazole exposure altered embryonic and larval development through changes in heart rate, hatching dynamics, morphology, larval growth, lipid accumulation, behavior, transcriptional profiles, and microbiome composition [Contribution 4]. These findings are especially valuable because they integrate developmental, physiological, molecular, and microbial endpoints into a single toxicological framework. In another experimental contribution, Lee et al. reported that environmental concentrations of fluoxetine and atrazine, alone and in combination, increased latency to aggression and reduced agonistic display behaviors in male Betta splendens [Contribution 5]. This study highlights the ecological significance of behavioral endpoints and reinforces the need to account for contaminant mixtures, which more closely reflect real environmental exposure scenarios than single-chemical designs. Together, these papers illustrate how fish toxicology is increasingly moving toward the multi-level interpretation of contaminant effects, linking visible phenotypic responses with underlying molecular and physiological mechanisms [Contributions 3–5].

Several papers in this Special Issue also illustrate the growing convergence between fish toxicology, environmental monitoring, and public health. Díaz et al. analyzed trace metal concentrations in commercially important fish sold in a local market in the Ecuadorian Amazon and showed that all sampled species exceeded the recommended thresholds for arsenic, mercury, and lead, while one species also surpassed the guideline for aluminum [Contribution 6]. By combining contaminant measurements with health risk assessment, this study extends the scope of fish ecotoxicology from ecosystem diagnosis to human exposure concerns in a region where fisheries are closely linked to food security. In contrast, Nobili et al. assessed trace metals in twaite shad from two Northern European populations and found generally low concentrations that complied with international food safety standards [Contribution 7]. Beyond their relevance to consumer safety, their results provide useful baseline information for a near-threatened species and support its potential role as a regional bioindicator. Considered together, these studies show that pollutant burdens and associated risks vary substantially among species, habitats, and geographic regions, and they underline the need for region-specific interpretation rather than broad generalization [Contributions 6 and 7].

The Special Issue also expands the perspective of fish toxicology from organism-level responses to food-web and habitat-scale processes. Monteiro et al. compared fish assemblage structure and mercury bioaccumulation between a natural floodplain lake and a constructed irrigation canal in central Brazil [Contribution 8]. They found that the artificial wetland supported a simplified fish assemblage and lower mercury concentrations overall, but trophic magnification slopes were comparable between the two systems. This is a particularly valuable contribution because it links contaminant dynamics with habitat transformation and community restructuring. It also suggests that strong anthropogenic modification may alter contaminant burdens without necessarily disrupting the efficiency of trophic transfer, an observation with important implications for wetland management and ecological risk assessment [Contribution 8].

At a broader conceptual level, the review by Sun et al. provides an important forward-looking perspective for the Special Issue [Contribution 9]. By synthesizing current knowledge on the transgenerational and multigenerational effects of emerging contaminants on fish, the authors emphasize that pollutant exposure in parental generations may impair offspring development, reproduction, endocrine regulation, neural function, behavior, and physiology, and that such effects may persist even after exposure ceases [Contribution 9]. Their review also identifies key research gaps, particularly regarding F3 generation evidence, mixed-contaminant exposure, and epigenetic inheritance. This contribution is especially relevant because many of the original studies in the present collection document sublethal alterations that may have lasting biological significance even when immediate mortality is limited. In this sense, the review helps position the studies in this Special Issue within a broader and increasingly important research trajectory: the shift from short-term toxicity testing toward long-term, mechanistic, and ecologically meaningful risk evaluation [Contribution 9].

Overall, the papers assembled in this Special Issue reveal several consistent trends in current fish toxicology research. First, there is a clear movement toward integrative assessment, combining histopathology, oxidative stress, genotoxicity, transcriptomics, microbiome analysis, behavior, and ecological monitoring [Contributions 1–9]. Second, environmentally relevant concentrations and realistic exposure scenarios are becoming increasingly central, improving the ecological relevance of toxicological inference [Contributions 3–5 and 9]. Third, the value of fish as bioindicators remains indisputable, but the most informative assessments are those that incorporate species traits, trophic context, complementary biological communities, and habitat condition [Contributions 1, 2, 7 and 8]. Fourth, contaminant effects on fish are not only ecological but may also have implications for conservation, fisheries management, and human health [Contributions 6–8]. Taken together, these developments indicate that fish ecotoxicology is evolving from a largely descriptive discipline into a more predictive, mechanistic, and ecosystem-oriented field.

As Guest Editor, I would like to express my sincere gratitude to all authors who contributed their work to this Special Issue and to the reviewers for their thoughtful and constructive evaluations. The studies brought together here differ in model species, pollutants, endpoints, and ecosystems, yet they share a common objective: to improve our understanding of how anthropogenic pollution affects fish and, through fish, aquatic ecosystem health more broadly. It is hoped that this Special Issue will serve as a useful reference for researchers in fish toxicology, aquatic ecology, environmental monitoring, and risk assessment, and that it will encourage further work on realistic exposure conditions, mechanistic pathways, and the long-term ecological consequences of pollution in aquatic environments.