Editorial: Adaptation and Response of Fish to Environmental Changes

1. Introduction

Aquatic environments are undergoing unprecedented transformations driven by global warming, habitat degradation, hydrological alteration, salinization, and human activities such as dam construction, invasive species control, and fisheries exploitation [1,2]. Fish, as key components of aquatic food webs and critical resources for global aquaculture, have evolved diverse adaptive mechanisms to cope with environmental heterogeneity—from molecular plasticity and physiological regulation to behavioral adjustments and community restructuring [3,4]. However, the escalating frequency of extreme events (e.g., abnormal droughts, temperature anomalies) and the need for effective conservation interventions (e.g., fishing bans) necessitate a comprehensive, cross-scale understanding of fish adaptive strategies to guide evidence-based management.

The eight studies featured in this Special Issue address critical knowledge gaps in this field, covering different interrelated research themes: (1) retinal structure and opsin-mediated light adaptation (Article 1); (2) physiological, transcriptomic, and immune responses to salinity stress (Articles 2 and 3) [1]; (3) interactive effects of temperature and dissolved oxygen (DO) on fish physiology, behavior, and molecular pathways (Article 4); (4) impacts of abnormal drought on fishery community structure and resource abundance [2]; (5) behavioral responses of fish to pulsed direct-current (DC) barriers [3]; (6) growth, health, and osmoregulatory adaptations of aquaculture species to varying salinities (Article 3); (7) length–weight relationships and spatiotemporal distribution of high-altitude schizothoracinae fishes [4]; and (8) ecosystem structure and function responses to fishing bans [5]. By integrating laboratory experiments, field surveys, transcriptomic analyses, behavioral observations, and ecosystem modeling, these studies provide novel insights into how fish and their associated ecosystems perceive, respond to, and adapt to both natural and anthropogenic stressors, while offering practical guidance for ecological conservation and aquaculture sustainability.

2. Core Contributions of Featured Studies

2.1. Light Environment Adaptation: From Retinal Structure to Non-Visual Photoreception

Light is a fundamental environmental cue regulating fish growth, circadian rhythms, and visual perception (Article 1). Zhang et al. (Article 1) conducted a comprehensive review of fish retinal structure and opsin adaptation, revealing that fish optimize light reception through dual strategies: structural modifications of retinal layers (e.g., thickness changes in the outer nuclear layer and inner nuclear layer) and molecular plasticity of opsin genes. Notably, the study identified melanopsin—a non-visual opsin—as a key mediator of circadian rhythm regulation, body color change, and hormone secretion, expanding the functional scope of fish photosensory systems beyond image formation. This work highlights the synergistic adaptation of fish vision at structural and molecular levels, emphasizing that gene duplication, differential expression, and regional co-expression of opsins enable fish to cope with diverse spectral environments (e.g., blue-dominated deep waters vs. green-dominated coastal zones). For aquaculture, these findings provide a theoretical basis for optimizing light environment management to enhance fish growth and welfare.

2.2. Salinity Stress Responses: Multiscale Regulation from Transcriptome to Organ Health

Salinity fluctuations, driven by sea-level rise and freshwater salinization, profoundly affect fish osmoregulation, metabolism, and immune function (Articles 2 and 3) [5]. Ruan et al. (Article 2) used transcriptome analysis to investigate grass carp Ctenopharyngodon idellus (Valenciennes, 1844) responses to acute salinity stress, identifying differentially expressed genes (DEGs) enriched in immune pathways (antigen processing and presentation, apoptosis) and metabolic pathways (glycolysis/gluconeogenesis) in intestinal and kidney tissues. Concurrent serum biochemical analyses showed increased Na⁺/Cl⁻ levels and decreased glucose/lactate concentrations, indicating enhanced ion regulation and altered energy metabolism under salinity stress. Complementarily, Chacón-Guzmán et al. (Article 3) demonstrated that spotted rose snapper Lutjanus guttatus (Steindachner, 1869) achieves optimal growth and gill health at low salinity, attributed to reduced osmoregulatory energy expenditure and complete avoidance of monogenean parasitic infestations (prevalence reached 87.5% when salinity was 24). Wang et al. [5] further supplemented these findings by showing that small yellow croaker Larimichthys polyactis (Bleeker, 1877) modulates antioxidant enzymes, non-specific immune enzymes, and Na⁺/K⁺-ATPase activities to adapt to salinity stress. Collectively, these studies reveal species-specific salinity tolerance thresholds and underlying mechanisms, providing critical guidance for brackish water aquaculture and the development of salt-tolerant varieties.

2.3. Temperature-Dissolved Oxygen Interaction: Physiological, Behavioral, and Molecular Synergies

The interaction between temperature and DO is a key determinant of fish welfare and productivity in intensive aquaculture (Article 4). Wang et al. (Article 4) found that crucian carp Carassius auratus (Linnaeus, 1758) exhibits significant physiological and behavioral alterations under combined temperature–DO stress: elevated temperature (30 °C) increases cortisol and blood glucose levels, while hyperoxia (7.5 mg L⁻¹) mitigates these stress responses by suppressing cortisol secretion (reductions of 34.7–118.1%) and enhancing glucose metabolism. Molecular analyses confirmed that the upregulated expression of stress-related genes (HSP70, HSP90, HIF-1α, Prdx3) correlated with behavioral abnormalities (e.g., surface swimming, air-breathing) and muscle texture deterioration. Notably, hyperoxia also alleviated heat-induced textural degradation, highlighting the potential of oxygen regulation as a mitigation strategy for climate-related stress in aquaculture. This study establishes a multidimensional biomarker framework for monitoring thermo-oxygen stress, offering insights for intelligent environmental regulation in recirculating aquaculture systems (RASs).

2.4. Extreme Hydrological Events: Impacts on Fishery Community Structure and Resource Abundance

Abnormal droughts, exacerbated by climate change, disrupt hydrological rhythms and threaten river-connected lake ecosystems (Article 5). Peng et al. (Article 5) conducted a two-year field survey on Poyang Lake, China during an extreme drought period (2022–2023), finding that while fish species richness (91 species) and diversity indices (Shannon–Weiner: 3.30; Simpson: 0.95) remained stable, the community structure underwent significant restructuring—characterized by reduced dominance of small, sedentary, and herbivorous fish (e.g., Hemiculter bleekeri Warpachowski, 1888). A sharp decline in catch per unit effort (cumulative reduction of 49.9%) indicated severe impacts on resource abundance, undermining the effectiveness of the 10-year fishing ban. The study reveals that drought-induced habitat filtering and niche reconstruction drive community changes, with migratory species (e.g., Coilia nasus Temminck & Schlegel, 1846) showing greater resilience than sedentary taxa. These findings emphasize the vulnerability of floodplain lake fisheries to hydrological extremes and the need for integrated water resource management to safeguard fishery recovery.

2.5. Anthropogenic Disturbance: Pulsed DC Barriers for Fish Behavior Regulation

Pulsed DC fish barriers are increasingly used to guide fish migration and control invasive species, but their effectiveness depends on species-specific behavioral responses (Article 6). Yi et al. (Article 6) investigated the effects of pulse frequency (4–10 Hz) and water velocity (0.2–0.6 m/s) on crossing probability of bighead carp Hypophthalmichthys nobilis (Richardson, 1845) and Eurasian carp Cyprinus carpio Linnaeus, 1758, demonstrating the following three key findings: (1) Crossing probability decreases with increasing pulse frequency, with the most significant reduction (8–10 Hz) achieving ~95% deterrence at low water velocity. (2) Higher water velocity increases crossing probability, particularly for stronger swimmers (H. nobilis). (3) The barrier causes minimal physiological harm (only four out of 160 fish trembled for >2 s). These results provide critical parameters for optimizing barrier design and balancing deterrence effectiveness with fish welfare, and offer a non-invasive tool for managing invasive carp populations.

2.6. High-Altitude Adaptation: Spatiotemporal Distribution and Growth Patterns

High-altitude aquatic ecosystems are characterized by extreme environmental conditions (e.g., low temperature, low oxygen), selecting for unique adaptive traits in endemic fishes (Article 7). Liu et al. (Article 7) investigated the length–weight relationships and spatiotemporal distribution of three schizothoracinae fishes (Schizothorax nukiangensis Tsao, 1964, Ptychobarbus kaznakovi Nikolskii, 1903, Schizopygopsis thermalis Herzenstein, 1891) along the Nujiang River on the Qinghai–Tibet Plateau. The study revealed species-specific growth patterns: S. nukiangensis exhibited negative allometric growth, while P. kaznakovi and S. thermalis showed near-isometric or positive allometric growth. Elevation and water temperature were identified as key drivers of distribution, with S. nukiangensis dominating low elevations (1800–2700 m), P. kaznakovi mid-elevations (2700–3800 m), and S. thermalis high elevations (>4000 m). These findings underscore the role of ecological niche differentiation in enabling the coexistence of closely related species in extreme environments, providing a baseline for conserving high-altitude fish biodiversity.

2.7. Fisheries Management: Ecosystem Responses to Fishing Bans

Fishing bans are critical conservation tools to mitigate anthropogenic pressure on aquatic ecosystems, but their long-term impacts on ecosystem structure and function remain poorly understood (Article 8). Xiang et al. (Article 8) used the Ecopath model to compare the Geheyan Reservoir ecosystem (Yangtze River Basin) before (2017) and after (2022) a fishing ban, revealing significant improvements in ecosystem complexity and stability. Key findings include the following: (1) Trophic levels increased from 3.36 to 3.89, reflecting enhanced food web complexity with recovery of top predators (e.g., Siniperca chuatsi (Basilewsky, 1855), trophic level 3.65). (2) Ecotrophic efficiency (EE) of major commercial species (e.g., silver carp, common carp, shrimp) improved, indicating optimized energy utilization. (3) Ecosystem maturity metrics—total primary production-to-total respiration ratio (6.93) and connectivity index (0.25)—improved, confirming enhanced stability. However, energy transfer bottlenecks persisted between trophic levels II and IV (average transfer efficiency: 6.60% in grazing chains, 5.95% in detrital chains), highlighting the need for targeted interventions to optimize nutrient cycling. This study demonstrates the effectiveness of fishing bans in promoting ecosystem recovery and provides a framework for evaluating conservation policies in reservoir and river-connected ecosystems.

3. Conclusions

The eight studies in this Special Issue collectively reveal the complexity and diversity of fish and ecosystem adaptive responses to environmental heterogeneity, spanning molecular, physiological, behavioral, community, and ecosystem scales. From high-altitude schizothoracinae fishes adapting to extreme cold and low oxygen, to aquaculture species optimizing growth under varying salinities and reservoir ecosystems recovering from fishing pressure, these investigations highlight the role of phenotypic plasticity, genetic adaptation, and ecological niche differentiation in enabling resilience. The integration of traditional ecological methods with advanced molecular techniques (e.g., transcriptomics) and ecosystem modeling (e.g., Ecopath) has greatly enhanced our ability to quantify and predict these adaptive responses.

As global environmental change accelerates, future research should prioritize interdisciplinary approaches to address remaining knowledge gaps, particularly focusing on multistressor interactions and long-term resilience. Translating scientific insights into practical strategies—such as targeted conservation measures for vulnerable species, optimized aquaculture environmental management, and adaptive water resource policies—will be critical for safeguarding fish resources and ensuring the sustainability of aquatic ecosystems. By continuing to unravel the mechanisms underlying fish and ecosystem environmental adaptation, we can better prepare for the challenges of a changing world and promote the coexistence of humans and aquatic biodiversity.