Effects of extreme hydrological disturbances on the resilience, resistance, and functional redundancy of fish assemblages in a subtropical estuary.

Natural maintenance of biofouling on coral nurseries used across the Great Barrier Reef.

Fish diversity and functional diversity of the catches from gillnet fisheries of the Chao Phraya River (CPR) and the Bang Pakong River (BPR) in the central of Thailand were analyzed, using the monitoring data from 2010 to 2023 and 2007 to 2023, respectively. The study focuses on the inter-annual assessment of fish community dynamics at the whole-river scale, which aims to provide essential insights for the sustainable management of both rivers. Long- and short-term variations as well as the stress of fish assemblages were also assessed. A total of 93,998 individual fish were recorded from 121 species in the CPR, and 108 species in the BPR, in which the species from family Cyprinidae were most diverse in both rivers. Both rivers exhibited substantial species diversity and functional trait diversity, with the CPR generally supporting higher abundance and diversity indices than the BPR. The long-term variation indicated that most species were stable in the catches. Meanwhile short-term variations were influent by fishing pressure year by year. Abundance-biomass comparisons revealed that the fish assemblages in both rivers were minimal disturbance, except for occasional specific years, suggesting localized or temporal stress events. Overall, results indicate that fish assemblages in both the CPR and BPR remain largely intact, with functional diversity contributing to ecosystem resilience despite high fishing pressure and other ongoing anthropogenic influences.

Coastal lagoons are increasingly vulnerable to anthropogenic pressures, with eutrophication emerging as a key driver of ecological degradation. In systems with restricted marine connectivity, understanding how biological communities respond to critical eutrophic events is essential for preserving ecological functions and guiding effective management strategies. This study applies a fish trait-based approach to assess the functional consequences of mass mortality events caused by eutrophication in the Mar Menor coastal lagoon (Western Mediterranean Sea). Based on 588 surveys, we evaluated long-term functional shifts in fish assemblages across three critical eutrophic phases -Pre-Mortality (2018-2019), Mortality (2020-2021) and Post-Mortality (2022-2023)- compared with a reference period (2002-2004). Four functional diversity indices were assessed -functional richness (FRic), specialization (FSpe), originality (FOri), and divergence (FDiv)- and compared their sensitivity with taxonomic metrics and a multi-metric fish index. FSpe and FOri were the most sensitive indices to critical eutrophic events, indicating declines in the uniqueness of traits and ecological functions. Functional homogenization and the loss of specialist taxa were widespread across the lagoon, with minimal influence from site-specific anthropogenic pressure or spatial confinement. These findings suggest limited recovery of ecological functions under and after persistent critical eutrophic conditions. Our study demonstrates the usefulness of incorporating functional diversity metrics to detect and characterize the ecological responses of fish assemblages to eutrophication and mass mortality events in transitional coastal systems.

Cryptobenthic fish as bioindicators: Community shifts along urban pollution.

Mercury biomagnification across food webs with varying non-native fish presence: Implications for native fish conservation in the Upper Colorado River Basin.

Early life stages of marine fishes play a critical role in shaping recruitment variability and population dynamics, yet the environmental processes structuring ichthyoplankton communities remain poorly understood in many marginal seas. Here, we investigated the seasonal dynamics of early life stage (ELS) fish assemblages and their environmental drivers in the offshore waters of Liaoning Province in the northern Yellow Sea using monthly ichthyoplankton surveys conducted from April to December 2021. Community structure and environmental relationships were examined using multivariate statistical analyses, and species distribution models were applied to project potential spawning habitats of the dominant species Engraulis japonicus under future climate scenarios. A total of 36 fish species were recorded, with spawning activity peaking between May and August. The ichthyoplankton assemblage exhibited pronounced seasonal succession characterized by shifts in dominant species composition and community structure. Temperature-related variables, particularly surface and bottom temperature, together with bottom salinity and depth, were identified as primary environmental drivers shaping the spatial and temporal organization of ELS fish communities. Distribution modelling further indicated that suitable spawning habitats for E. japonicus may expand under low-emission climate scenarios (RCP 2.6) but show increasing instability and potential contraction under moderate (RCP 4.5 and 6.0) and high emission scenarios (RCP 8.5). Our results demonstrate that environmental gradients and seasonal hydrographic variability jointly structure ichthyoplankton communities in the northern Yellow Sea, while future climate change may substantially alter spawning habitat availability for key pelagic species. These findings provide new insights into the mechanisms linking environmental variability and early life stage fish dynamics and highlight potential risks to regional fisheries resources under ongoing climate change.

Mesopelagic fish often dominate the micronekton in the mesopelagic zone. Despite their ecological importance, information about their diversity, distribution and abundance in some areas of the Indian Ocean remains patchy and incomplete. Using data collected from three acoustic/trawl surveys spanning the western (WIO), central (CIO) and eastern (EIO) Indian Ocean regions in 2018, we investigated the composition and structure of the fish assemblages that were sampled using different pelagic trawl types, namely Macrozooplankton and Multpelt, which targeted the deep scattering layer (DSL) and shallow scattering layer (SSL). We assessed the structure of mesopelagic assemblages across the region at the level of genus and family, and analysed relationships between the samples with concurrently collected hydrographic data using multivariate statistics. Mesopelagic fish taxonomic richness varied across the region, as follows: WIO – 68 genera (13 unique), 34 families (4); CIO – 48 genera (6), 35 families (5); and EIO – 86 genera (26), 44 families (9). Although there was a pronounced difference in the structure of the assemblages by survey region, gear type and time of day (and many interactions between these), the assemblages were significantly impacted by water temperature, dissolved oxygen and aspects of the chlorophyll a environment— which varied across the sampled areas. Intriguingly, the spatial patterns (and diversity) observed when the mesopelagic fish assemblages were examined at the genus level were likewise reflected when the data were analysed at the family level. The implications of these results are discussed.

Baited remote underwater videos (BRUVs) are increasingly used to study fish assemblages. Therefore, understanding the biases and limitations of this methodology is critical, including questions about the derived metrics of abundance and independence among replicates. However, assessing these biases requires identifying individual fish, which is typically challenging. Here, we take advantage of the distinct spot patterns on Great Barracuda (Sphyraena barracuda) to identify individuals on BRUVs to compare the most common BRUVs abundance metric (MaxN) to an alternative method based on identified individuals (IND). We also assessed whether individual S. barracuda were seen on multiple cameras to determine if a commonly used spacing (500 m) was sufficient to assume independence between replicate BRUVs. S. barracuda were identified based on spot patterns on >2000 videos at 43 sites across 12 countries. MaxN and IND were highly positively correlated (0.93), but there were significant differences between the measures (p < 0.001) that became particularly apparent when MaxN was >4. IND abundances were, on average, 1.4 times greater than MaxN. Five sites had at least one S. barracuda appear on multiple BRUVs (<4% of videos), but only one occurrence during the same deployment on two cameras separated by >500 m. Thus, 500 m is a reasonable standard for independent sampling units for species with similar mobility to S. barracuda. While identifying individuals is time-consuming and not feasible for many species, it does improve the accuracy of abundance estimates, and we recommend its use when possible and where estimates of true abundance on videos are important.

Reef fish functional trait responses to wastewater-driven environmental gradients.

Nursery pressures in megacities: Interplay of anthropogenic disturbance and natural drivers in shaping ichthyoplankton community dynamics in urbanized coasts.

Application of environmental DNA technology in monitoring species diversity at multiple biological groups in marine protected area.

Zebra mussels ( Dreissena polymorpha ) have caused significant ecosystem changes across North America since their introduction, yet their effects on fish populations remain poorly understood, particularly in Great Plains reservoirs. We quantified effects of zebra mussel invasions on water quality and fish assemblages across 25 reservoirs &amp;gt;5 km 2 in Kansas, U.S., where invasions have occurred over the past 2 decades. Using data from invaded reservoirs before and after they were invaded, and data from uninvaded reservoirs, we evaluated changes in water quality parameters with random forest models, and we evaluated trends in long-term fish abundance data and creel data with generalized linear mixed models. Overall, we found minimal impacts of zebra mussel invasion on water quality, fish species abundance, or angler use. Random forest models estimated that zebra mussel invasions had little importance in predicting water quality, while reservoir identity was the strongest predictor. The 95% confidence intervals for abundance trends in invaded reservoirs included zero for all species except largemouth bass, which demonstrated a small, and likely not biologically meaningful, decrease following invasion. Furthermore, all fish species showed similar estimates for abundance trends in invaded and uninvaded reservoirs. Angler use also showed no response to zebra mussel presence. Although zebra mussel invasions might have other negative consequences (e.g., on water treatment facilities), our results suggest that invasions in Kansas reservoirs have not considerably affected water quality or fish populations in the past 2 decades.

Temporal and spatial changes in fish assemblages associated with the placement of a rock armoured pipeline complex in northwestern Australia.

Reef fish assemblages are dynamic and heterogeneous across both temporal and spatial scales. Understanding these patterns is essential for effective management strategies that aim to protect marine communities. We investigated whether previously described cross and longshore patterns in reef fish assemblages persisted after a decade of high ocean temperature variability driven by the El Niño-Southern oscillation and a significant marine heatwave in 2011. Two nearshore marine parks near Perth, Western Australia, located 70 km apart, were sampled in 2012 and 2023 using Baited Remote Underwater stereo-Video systems. At each location, distinct inshore, midshore, and offshore reef lines were targeted across a depth gradient of 3-23 m. Inshore-to-offshore assemblage patterns described in 2012 remained temporally consistent in 2023, with each reef line representing distinct compositions at both locations. The mean relative number of species, the mean relative abundance of fish per deployment, and the mean relative abundances of most individual feeding guilds, were also similar between years. Although assemblage and feeding guild compositions differed at a few locations between years, these changes were likely driven by the localised variation in the distribution of some focal species across reef lines. Planktivores were the only feeding guild to significantly increase in abundance over time. As cumulative anthropogenic and climate pressures increase, it is likely assemblage modification will occur more often. To protect the full range of fish assemblages, it would be appropriate for future spatial management to include fully protected sanctuary zones across inshore, midshore, and offshore reef lines at both locations. • Cross-shore assemblage patterns of increasing species richness and fish abundance was consistent in 2012 and 2023 • Reef fish assemblage compositions had minor differences between 2012 and 2023 • Planktivores were the only feeding guild to increase in abundance • For future management, including no-take sanctuary zones across all three reef lines would be most appropriate

The Cheshmeh-Langan River is a semi-arid mountain river in Iran that has been strongly affected by Inter-Basin Water Transfer (IBWT) operations since 2005. This study investigates the combined effects of IBWT and climatic drought on river hydrology, habitat characteristics, and fish habitat integrity. Field surveys conducted in 2022 compared environmental conditions, mesohabitat composition, and fish–habitat associations between two segments (upstream control and downstream impacted). Long-term meteorological and hydrological droughts were assessed using standardized precipitation (SPI) and streamflow drought (SDI) indices. Habitat Suitability Curves (HSCs) were developed for key native fish species across different life stages. Results indicate a significant shift in the hydrological regime following IBWT operation. The coupling between precipitation and discharge weakened, with the correlation between SPI and SDI declining from r = 0.67 before water transfer to r = 0.34 afterward. Mean SDI values shifted from + 0.69 in the pre-transfer period to –0.93 post-transfer, demonstrating the emergence of a persistent, transfer-induced hydrological drought. This altered flow regime substantially changed habitat exploitation, as shown by HSCs. Capoeta pyragyi fingerlings and Oxynoemacheilus frenatus showed different habitat associations between the segments, while C. pyragyi adults used the same habitats but showed lower abundance downstream. Additionally, downstream fish assemblages exhibited clear species turnover and expansion of the non-native species Petroleuciscus esfahani , likely facilitated by IBWT. • IBWT alters flow regimes, causing chronic anthropogenic hydro-drought. • Langan IBWT reduces habitat suitability for two native fish species. • IBWT spreads Petroleuciscus esfahani downstream, raising biotic homogenization. • Habitat-based benchmarks guide adaptive environmental-flow management. • Hydrology–habitat coupling quantifies ecological risk under regulation.

Identifying suitable habitat shifts and climate refugia under climate change is essential for the conservation of freshwater fish, particularly in mountainous river systems. However, there is a lack of comprehensive evaluation frameworks to quantitatively assess the degree to which regional fish assemblages are affected. In addition, asymmetric habitat shift patterns in fish populations of mountainous rivers remain insufficiently explored. To address these gaps, we developed a climate-responsive evaluation framework—CR-TOPSIS—based on the Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS), and applied it to assess current and future habitat changes for the Top 15 climate-sensitive species and other key protected species in the Qinba mountainous region, integrating environmental DNA surveys and targeted traditional capture, cross-validated against regional checklists and expert review with MaxEnt modeling. Projections were made under two climate scenarios (SSP126 and SSP585) for the period 2070–2100. Results indicated pronounced asymmetric habitat shift patterns, with leading-edge expansion consistently exceeding trailing-edge contraction across climate scenarios. Species distribution centroids exhibited a clear northward shift, averaging 1.76 km under SSP126 and 2.64 km under SSP585, reflecting enhanced redistribution under stronger warming. Coldwater and bottom-dwelling species experienced disproportionate habitat loss under high-emission scenarios, whereas eurythermal and pelagic-spawning species showed comparatively higher adaptive potential. Core climate refugia, defined by 100% spatial overlap across scenarios, covered approximately 0.51 × 10³ km² and were primarily concentrated along the midstream Hanjiang River and its tributaries, remaining stable under both climate pathways. This study demonstrates the utility of integrating molecular monitoring and species distribution models to detect climate-sensitive shifts, evaluate species vulnerability and conservation prioritization in montane freshwater ecosystems. • Developed a CR-TOPSIS framework to evaluate climate sensitivity of freshwater fish species. • Integrated eDNA data and MaxEnt modeling to predict future habitat suitability under SSP126 and SSP585. • Revealed consistent asymmetric habitat shifts, with leading-edge expansion exceeding trailing-edge retreat. • Identified coldwater and benthic species as highly vulnerable to warming and hydrological change. • Mapped climate refugia concentrated in the midstream Hanjiang River, providing conservation targets.

Defining mesophotic depth: Analysis of fish assemblages and ecological boundaries in the Mexican Pacific

We used an ecological niche theory framework integrating physiological and behavioural ecology to understand interspecific differentiation of juveniles of four Antarctic fishes: Trematomus bernacchii, Trematomus pennellii, Trematomus nicolai and Pagothenia borchgrevinki. We focused on basal metabolic traits and the exploration-avoidance axis of behaviour, two key dimensions of species fitness and drivers of niche differentiation. Metabolic metrics included routine metabolic rate (RMR) to understand resting energy requirements and Fulton's condition factor (CF) as a proxy for energy stores; together, these metrics give a baseline understanding of the energetic status of a species. Behavioural assays included the novel object test (NOT) and the novel tank test (NTT) to assess exploratory activity and anxiety. We found relatively low interspecific differentiation in basal metabolic demands-RMR did not differ across species and only T. nicolai exhibited higher CF than the other species. This suggests that T. nicolai may have greater on-board energy stores available to cope with environmental stressors, although more research is needed to confirm this. While we generally saw behavioural differentiation across species, we did see evidence of a possible conserved freezing strategy in response to novelty, where T. bernacchii, T. pennellii and P. borchgrevinki reduced their activity after being exposed to a novel object. In both the NOT and NTT, species showed distinct exploratory tendencies, with T. bernacchii and T. pennellii spending more time in risk-associated zones, T. nicolai being more avoidant and P. borchgrevinki exhibiting cautious exploration. We also observed substantial intraspecific variability in physiological and behavioural traits, particularly in T. bernacchii, T. nicolai and P. borchgrevinki, suggesting the presence of divergent behavioural phenotypes within species. Examining behavioural syndromes in these fishes is an important next step as behavioural syndromes are thought to be linked to metabolic status, individual fitness and species and community resilience. Collectively, our findings suggest that while basal metabolic demands appear relatively conserved across species at the juvenile life stage, divergent behavioural strategies could be a critical driver of niche differentiation in Antarctic fish assemblages and in species-specific performance under environmental threats such as climate change.

Ontogenetic niche shifts in fishes are nearly universal but remain poorly understood in many species despite being fundamentally important for the persistence, management, and conservation of fish populations, including those of vulnerable salmonids. Eye lens stable isotope analysis has proven useful in studying ontogeny in some marine species but has rarely been applied in freshwater fishes. We conducted among the first applications of eye lens stable isotope analysis in two salmonids, Arctic Charr (Salvelinus alpinus) and Brook Trout (Salvelinus fontinalis), in four North American lakes at the southern extent of the range of Arctic Charr (Maine, USA). Our goal was to determine if ontogenetic patterns varied between individuals and populations in ways that relate to differential vulnerability. Like studies in marine systems, we found patterns in lens isotopic values that agree with expected ontogenetic patterns to reach known adult trophic niches. Within lakes and individuals examined in this study, Arctic Charr appeared more dependent on pelagic resources than co-occurring Brook Trout through life. Using Bayesian hierarchical linear regressions, we found evidence that ontogenetic shifts in trophic position (measured by δ15N) of Arctic Charr may vary among lakes. Arctic Charr in some populations increased in trophic position through life (population lifetime δ15N posterior mean slope estimate = 1.01) while others showed no substantial changes (population lifetime δ15N posterior mean slope = 0.05), which may relate to differences in habitat and fish assemblage among our study lakes. Our study suggests that individual life stages and populations of salmonids are likely to respond to climate variability (e.g., basal resource shifts) differentially, which could warrant population and life-stage-specific management.