Fish Biodiversity Research Articles

Seaweed-Associated Diatoms (Bacillariophyta) in Dokdo of South Korea: I. Subphyla Melosirophytina, Coscinodiscophytina, and Class Mediophyceae

Dokdo is an island located in the easternmost part of Korea, which has high levels of biodiversity of birds and fish, especially marine invertebrates. However, the biodiversity of microalgae, especially diatoms (Bacillariophyta), is relatively unknown, despite their ecological importance as primary producers of the marine food web and bioindicators of environmental conditions associated with climate change. To understand the biodiversity of seaweed-associated diatoms from Dokdo, we collected macroalgae present at a depth 5–15 m by SCUBA diving on 17 October 2017. There were a large number of diatoms (over 130 species), even though it was a one-time survey. As it includes too many taxa to cover at once, voucher flora for other taxonomic groups will be provided through the continuous serial papers. This is the first series of seaweed-associated diatoms, with 26 species belonging to the subphyla Melosirophytina and Coscinodisophytina, and the class Mediophyceae. Among these, seven species including one new taxon were reported for the first time in Korea, which, along with the geopolitical characteristics of the survey area, proved that there is no domestic interest in seaweed-related diatoms. In particular, the appearance of species that have been reported in subtropical waters, such as the order Ardissoneales, requires continuous monitoring of marine seaweed-associated diatoms to confirm whether their colonization in Dokdo waters was due to climate change or species-specific water temperature tolerance.

Long-term contamination by non-native fish assemblages in a Neotropical floodplain.

Biological invasions are a major threat to biodiversity in species-rich regions. Therefore, it is important to understand mechanisms behind the long-term establishment of non-native fish species in aquatic environments in the Neotropical region. Here, we associated fish biomass, species richness, and the proportion of non-native species (contamination and Kempton's indices) to quantify the non-native pressure over fish biodiversity in lakes and rivers of the Parana River floodplain, seasonally, from 2000 to 2017. We divided species into native and non-native assemblages sampled in spatio-temporal gradients. Temporal trends were examined using linear regressions and generalised additive models. Fish biomass in gillnets increased for both native and non-native fish species, but their Kempton indices were inversely correlated. Extinction of native species occurred locally with biotic differentiation of non-native species in lakes, rivers, and ecosystem contamination. A constant increase in fish biomass resulted in overwhelming biodiversity of non-natives at the end of the time series evaluated. Native biotic resistance to introductions was not detected in deterministic trends. The observed patterns were consistent with previous studies showing native biotic homogenisation and extinction of species in response to biological invasions, landscape fragmentation, and riverine impoundments. Increases in abundance and species richness of non-native fish were the biodiversity drivers that resulted in non-native species outweighing native species in the Parana floodplain.

The condition status index for doline lakes in urban areas

Karst represents approximately 15% of the planet’s surface, hundreds of millions of people live on and rely on these aquifers for water supply and agricultural irrigation. In karstic landscapes, groundwater is exposed in sinkholes, inundated caves, and artesian wells, which are two-way communication spots. When the phreatic level is exposed, the groundwater can change substantially as a result of anthropogenic impacts, modifying the water quality and the environmental integrity by incoming excess nutrients, contaminants, pathogens, and other hazardous substances such as metals and microplastics. In this paper, we develop and test a multimetric index to evaluate the condition status of dolines located within urban areas, including seven indicators: trophic index, fecal bacteria, fecal viruses, microplastics, heavy metals, zooplankton biodiversity, and fish biodiversity. Lastly, we made a proof of concept for the index in the dolines on the island of Cozumel (Mexico), resulting in evaluations from fair to good. The index is powerful due to its sensitivity to pathogens and exotic invasive species. This additive weighted index allows to assess the condition status of dolines in urban areas anywhere in the world; if required, modifications are possible.

Fish–Seascape Associations Within an Offshore Protected Area in the Arabian Gulf

ABSTRACTCoral reef ecosystems support high fish biodiversity through ecological interactions with structural complexity across multiple spatial scales including coral colony architecture and the surrounding seascape structure. In an era where the complexity of coral reef ecosystems is being diminished, understanding the importance of structural characteristics beyond single focal patches has the potential to better inform actions for protecting, restoring or creating habitat for reef‐associated species. A seascape ecology approach was applied to explore the associations between multiple scales of seascape structure and fish assemblage response variables within a small (49.6 km2) offshore no‐take MPA, Sir Bu Nair Island Protected Area, in Sharjah, United Arab Emirates. Fish–seascape associations were modelled with single regression trees. Both in situ and remote sensing–derived variables produced the best models with highest contributions from coral cover, amount of hard‐bottom habitat type and structural complexity of the seafloor terrain. Fish species richness was significantly higher where coral cover exceeded 35%. The hard‐bottom areas with coral supported diverse assemblages dominated by carnivorous and omnivorous fishes. The Sir Bu Nair Island Protected Area provides a critical refuge for threatened and regionally overexploited species including those with low resilience to fishing. The ecological success of this protected area is key to safeguarding regional marine biodiversity and recovering fish populations to enhance food security.

Chatting behind the reef: Fish bioacoustic diversity of tropical back-reefs in Fiji, South Pacific

Back-reef habitats are important and fragile transition zones acting as nurseries for many coral reef fishes. In this framework, Passive Acoustic Monitoring (PAM) can be an important tool to evaluate the diversity and dynamics of sonic fish community. Here, we investigated the diversity, spatial and diel dynamics of fish sounds in back-reef habitats at Makogai Island in Fiji, South Pacific. Synchronized underwater recorders were deployed in 4 bays collecting data for about 4 days. The abundance of 12 different sub-categories of fish sounds were quantified. Signals were acoustically characterized and the level of discrimination between the sub-categories was evaluated by Discrimination Function Analysis. Generalized Additive Models showed that the abundance of signals was related to the bay and the hour. Moreover, the Shannon Diversity and Equitability Indices were calculated using acoustic and visual census data to describe fish biodiversity of each bay. The two bays with greater biodiversity based on visual census also showed a greater acoustic diversity at dawn and night. Our results highlight the importance of PAM to reveal the diversity of fish community in back-reef habitats, providing a baseline to understand future changes in these crucial environments.

Exploring Fish Biodiversity and the Socioeconomic Status of Fishers in the Karatoya River, Sadar Upazila, Panchagarh District, Bangladesh

These studies were carried out to study species diversity of fishes and socio-economic conditions of the fishers of Karatoya River at Sadar upazila in Panchagarh district Bangladesh. Pertinent information was collected from 65 fishermen, who were directly engaged in catching fish around the study area for six months from December 2022 to June 2023. The results demonstrated that 41 fish species belonging to 8 orders and 15 families were found in the Karatoya River. Cypriniformes was found as the most dominant order, constituting 50.6% of the total fish population, considering species abundance, and Siluriformes constituting 12.7% and Perciformes constituting 12.0%, which were occupied the second and third position in abundance. The order Channiformes, Clupeiformes, Mastacembeliformes and Tetraodontifomres were constituted 6.6%, 6.0%, 4.8%, and 4.2%, respectively. Beloniformes constituted the lowest number of species (30%). It was observed that many fish species in the Karatoya River were severely impacted by the environmental and human-induced hazards. However, the age of the fishermen was ranged from 22 to 65 years. The Hindus (70%) were featured to be the highest percentage than the Muslims (30%), while no Christians and Buddhists were found. Various levels of education were documented in the study area, where majority of the fishermen were illiterate (69%), up to primary 18%, up to secondary 8% and the small portion (5%) were above secondary. The fishermen’s housing status was predominated by kaccha (66%), followed by semi pacca (23%) and pacca (11%). Majority of the fishers were landless (39%), whereas 15% fishers had 1-10 decimal (dec.) lands, 18% had 11-20 dec., 11% had 21-30 dec., 14% had 31-40 dec. and badly 3% of them had possessed >40 dec. lands. Among the fishermen, 45% of them were found to be involved in fishing activities, 40% in agriculture and 15% in sand business as day-to-day labour. There were 4 types of nets, 3 types of traps, 4 types of spears and 2 types of line fishing operated in this area. The study revealed that the fisheries production was not sufficient according to the demand. The reasons behind this might be due to the fact that the fishers around the Karatoya River were mainly poor, illiterate, lack of proper training and also nonexistence of consciousness of their sanitary and health care facilities. Therefore, it is very important to provide the required Organizational and Institutional help, support and extension facilities for the enhancement of fish biodiversity as well as development of sustainable livelihood of fishers in the Karatoya River as well.

Evaluation of Fish Biodiversity in Estuaries Through Environmental DNA Metabarcoding: A Comprehensive Review

Traditional methods face difficulties in capturing elusive species and navigating the complexities of estuarine environments. The adoption of eDNA metabarcoding has revolutionized biodiversity assessment, offering non-invasive and comprehensive sampling. Challenges specific to estuaries, such as sediment loads and salinity fluctuations, were initially addressed through improved DNA extraction, optimized primers, and specialized bioinformatics tools. The integration of multiple genetic markers and quantitative eDNA approaches has enhanced resolution, enabling the detection of rare and cryptic species. Standardized sampling protocols and validation through traditional methods contribute to the reliability of eDNA metabarcoding. This technology holds promise for informing conservation and management strategies in estuarine ecosystems by providing timely and cost-effective biodiversity data.

Non‐Native Invasive Fish Species (NIFS) in Inland Waters of India: Management Issues, Strategies and Suggestions

ABSTRACTIn India, there are 626 non‐native invasive fish species (NIFS), with 16 becoming invasive in inland waters, adversely impacting native biodiversity, and fisheries by altering the abundance of NIFS in commercial catches. Scientific findings highlight that the movement of NIFS interacts with agricultural activities and the evolving environment, facilitating their spread and establishment. The scenario of NIFS invasion disrupts biological communities, ecosystem services and fisherfolk livelihoods. Despite the implementation of existed guidelines, issues related to NIFS have persisted and escalated over time scale putting increased pressure on indigenous aquatic biological resources in inland waters. This paper suggests the need of framing a new management plans, guidelines and regulatory policies to control NIFS invasions, protecting riverine fish biodiversity and ecosystem services. Proposed management strategies include functional response (FR) and adaptive response (AR)‐based approaches, which are discussed to address NIFS management in inland waters.

Characterizing spatial patterns among freshwater fishes and shrimps of the Poso River (Sulawesi, Indonesia) using DNA barcoding

Fish biodiversity assessments are vital for understanding threats and aquatic ecosystem health. In the Poso River, Central Sulawesi, changes in biodiversity are influenced by habitat alteration, non-native species, and overfishing. This study evaluates fish and crustacean biodiversity in the Poso River system to address these challenges for improving fisheries management, conservation, and fish passage integration into hydropower development. The present biodiversity study utilized a comprehensive methodology that encompassed both traditional taxonomic approaches and DNA barcoding, specifically targeting the mitochondrial cytochrome c oxidase subunit-1 (COI) gene for accurate species identification and spatial boundary validation. The survey was conducted in the upstream, hydropower area and downstream areas of the river. We found 27 species of fish in the Poso River, including both native (e.g., Anguilla celebesensis, Oryzias nebulosus, Rhyacichthys aspro) and non-native species (e.g., Melanochromis auratus, Amphilophus trimaculatus, Oreochromis niloticus). Two International Union for Conservation of Nature (IUCN) red listed endangered species were also observed (e.g., Adrianichthys poptae and Mugilogobius sarasinorum). DNA barcoding was performed to examine species spatial boundaries. The fish population in the Poso River was dominated by non-native species, accounting for 85.70% of the total population. The upstream reaches had the highest fish abundance and diversity, while the downstream reaches had the lowest. There was, however, no significant difference in species richness and diversity across different locations and seasons. The dominance of non-native species in the Poso River necessitates the improvement of existing fishways within the hydropower dams potentially through the development of selective technology that can block the distribution of these non-native species. This research highlights the critical issue of non-native species proliferation and the potential threat they pose to native fish populations, providing valuable insights for conservation and management efforts in Indonesia and similar ecosystems worldwide.

Effect of environmental DNA sampling resolution in detecting nearshore fish biodiversity compared to capture surveys.

Sampling and sequencing marine environmental DNA (eDNA) provides a tool that can increase our ability to monitor biodiversity, but movement and mixing of eDNA after release from organisms before collection could affect our inference of species distributions. To assess how conditions at differing spatial scales influence the inferred species richness and compositional turnover, we conducted a paired eDNA metabarcoding and capture (beach seining) survey of fishes on the coast of British Columbia. We found more taxa were typically detected using eDNA compared to beach seining. eDNA identified more taxa with alternative habitat preferences, and this richness difference was greater in areas of high seawater movement, suggesting eDNA has a larger spatial grain influenced by water motion. By contrast, we found that eDNA consistently missed low biomass species present in seining surveys. Spatial turnover of communities surveyed using beach seining differed from that of the eDNA and was better explained by factors that vary at small (10-1000s meters) spatial scales. Specifically, vegetation cover and shoreline exposure explained most species turnover from seining, while eDNA turnover was not explained by those factors and showed a distance decay pattern (a change from 10% to 25% similarity from 2 km to 10 km of distance), suggesting unmeasured environmental variation at larger scales drives its turnover. Our findings indicate that the eDNA sample grain is larger than that of capture surveys. Whereas seining can detect differences in fish distributions at scales of 10s-100s of meters, eDNA can best summarize fish biodiversity at larger scales possibly more relevant to regional biodiversity assessments.

Climate change drives fish communities: Changing multiple facets of fish biodiversity in the Northwest Pacific Ocean

Global marine biodiversity is experiencing significant alterations due to climate change. Incorporating phylogenetic and functional diversity may provide novel insights into these impacts. This study used an ensemble model approach (random forest and boosted regression tree), to predict the habitat distribution of 47 fish species in the Northwestern Pacific under contemporary (2000–2014) and future scenarios (2040–2050, 2090–2100). We first examined the relationship between eleven functional traits and habitat changes, predicting the spatial distribution of functional traits within fish communities. A significant correlation was observed between temperature preference and habitat changes, highlighting the vulnerability of cold-water species and potential advantages for warm-water species in the future. Moreover, fish communities exhibited a spatial gradient distribution with southern regions characterized by shorter-lived and earlier maturity, contrasting with longer-lived and later maturity species in the north. Secondly, to assess the impact of climate change on marine biodiversity, we explored the taxonomic, phylogenetic, and functional diversity under contemporary and future scenarios, revealing higher indices in the East China Sea (ECS) and the coastal sea of Japan, with the Taiwan Strait emerging as a contemporary biodiversity hotspot. In future scenarios, the three biodiversity indices would decline in the Yellow Sea and ECS, but increase in the sea beyond the continental shelf, coastal sea of Hokkaido, and Sea of Okhotsk. Lastly, we explored processes and mechanisms in the change of community composition. By quantifying β-diversity, we identified species loss (nestedness) as the primary driver of fish community change by 2040–2050, with species replacement (turnover) predicted to become dominant in the far future. Our results explore the potential changes in multiple facets of fish biodiversity, providing crucial insights for policymakers aiming to protect fish resources and biodiversity.

Freshwater habitat invaders: Impacts on artificial flow refuge use by native fish under pulsed flows

Flow regulation and the spread of invasive species are significant threats to freshwater biodiversity, including native fish communities in rivers across the globe. The bleak (Alburnus alburnus), an invasive fish currently widespread throughout the main rivers of the Iberian Peninsula, threatens native fish biodiversity, mostly via trophic competition and hybridisation. In this study, we assessed the impacts of bleak presence and rapid flow fluctuations (both main and interactive effects) on the use of artificial habitats (a flow refuge) and the stress physiology of barbels. We examined how bleak's presence influenced the selection of flow refuges by the native Iberian barbel (Luciobarbus bocagei) under base (continuous 7 L s−1 flow stimulus) and pulsed (7 L s−1 to a 60 L s−1 pulse) flows in an indoor flume. We quantified habitat use within three specific regions of two installed flow refuges and two physiological responses (glucose and lactate) associated with the stress axis. The results indicated that the presence of bleak significantly reduced flow refuge use by barbels and this effect was more prominent during pulsed flows. Additionally, barbels exhibited higher levels of glucose and lactate in the presence of bleaks. These results show that bleaks compete with barbels for habitat under fluctuating flows and induce physiological adjustments that may reduce the ability of barbels to find low-velocity areas to shelter. Overall, the bleaks had a competitive advantage under high flow variability. This underscores the need for further studies and effective management strategies to protect native freshwater fish communities in the context of biological invasions and anthropogenic flow variability.

Interbasin trade worsens the state of freshwater fish biodiversity in China

Interbasin trade worsens the state of freshwater fish biodiversity in China

Ichthyofaunal diversity of Ratapani wildlife sanctuary, Raisen and Sehore district of Madhya Pradesh

The present study has been conducted to assess the Ichthyofaunal biodiversity from different localities of Ratapani Wildlife Sanctuary, Raisen and Sehore district of Madhya Pradesh. The surveys were conducted different localities of the study areas during September 2022 to January 2024. The results of present investigation revealed the occurrence of 70 fish species belonging to 12 orders 27 families and 42 genera. The members of Order Cypriniformes were dominated by 35 species with five families followed by Siluriformes 14 species with six families, Anabantiformes 7 species in four families, Synbranchiformes 3 species in one family. The orders Perciformes, Osteoglossiformes and Cichliformes 2 species each were recorded. The other orders Angullifromes, Beloniformes, Clupeiformes, Cypronidontiformes, Gobiiformes 1 species each were also recorded from the study area. The results of this study are promising; it sheds light on the unknown fish biodiversity of Ratapani Wildlife Sanctuary, which needs to be strengthened through comprehensive future surveys.

The role of protected areas in the Balkan freshwater biodiversity conservation: The case of Blidinje Nature Park (Southwestern Bosnia-Herzegovina)

Freshwaters of the Balkan Peninsula show high levels of endemism and relict species, which are still little studied and are threatened by numerous anthropogenic stressors. Protected areas could give special opportunities to improve knowledge of their biodiversity and develop proper conservation measures for the most at-extinction risk species. The aims of this research were to: i) fill the knowledge gap regarding fish and crustaceans occurring in the main water bodies of the Blidinje Nature Park (Southwestern Bosnia-Herzegovina); ii) assess the population abundance and growth, for the species of conservational interest; iii) identify fish biodiversity hotspots in Bosnia-Herzegovina; iv) identify anthropogenic pressures and suggest proper conservation measures. Fish and crayfish data were collected in four sampling sites located on the main water bodies, using electrofishing and gillnets, in three sampling phases over the period July 2022-August 2023. The presence of three species at high extinction risk has been detected: Squalius tenellus, Aulopyge huegelii and Astacus astacus. Their populations were abundant, with balanced age structure and good growth performance. A spatial analysis allowed to quantify the fractions of the endemic species range covered by the Blidinje Nature Park, which were 6.1% and 2.3% for S. tenellus and A. huegelii, respectively. The presence of invasive Atlantic trout and overharvesting of crayfish seem to represent the main anthropogenic pressures on freshwater biota. The obtained information allowed to expand the knowledge on the distribution and population status of threatened species and to highlight diversity hotspots as valuable tools for their conservation and management.

Reef fish biodiversity and occurrence of endangered sharks within a small marine protected area off Sint Maarten, Dutch Caribbean

AbstractMarine protected areas (MPAs) are common conservation tools supporting the protection of threatened marine fishes, such as sharks. However, the creation of shark MPAs has been less common in the Greater Caribbean region despite a growing need and opportunity. In this study, we evaluated the occurrence of shark and reef fish biodiversity off Sint Maarten, Dutch Caribbean, with a particular emphasis on endangered shark presence within the Man of War Shoal Marine Protected Area (MPA). We utilized baited remote underwater video systems (BRUVs) to gather non-invasive data on the abundance and diversity of reef fish and shark species inside and outside the local MPA. Generalized linear models (GLMs) revealed no significant effect of protection status on the presence of the endangered Caribbean reef shark (Carcharhinus perezi). However, we found a significant influence of depth on shark occurrence, as well as an effect of habitat type on shark and reef fish biodiversity, with reef habitats showing the greatest significance. These results suggest that the effect of small coastal MPAs on bolstering local endangered species conservation in the Dutch Caribbean is likely to vary according to habitat features and how those species utilize those habitats across various life stages. These findings have implications for adaptive MPA management, which should be informed by the ecology and habitat preferences of target species to achieve maximum benefits for biodiversity conservation.

Managing nutrition-biodiversity trade-offs on coral reefs

Coral reefs support an incredible abundance and diversity of fish species, with reef-associated fisheries providing important sources of income, food, and dietary micronutrients to millions of people across the tropics. However, the rapid degradation of the world's coral reefs and the decline in their biodiversity may limit their capacity to supply nutritious and affordable seafood while meeting conservation goals for sustainability. Here, we conduct a global-scale analysis of how the nutritional quality of reef fish assemblages (nutritional contribution to the recommended daily intake of calcium, iron, and zinc contained in an average 100g fish on the reef) relates to key environmental, socioeconomic, and ecological conditions, including two key metrics of fish biodiversity. Our global analysis of more than 1,600 tropical reefs reveals that fish trophic composition is a more important driver of micronutrient concentrations than socioeconomic and environmental conditions. Specifically, micronutrient density increases as the relative biomass of herbivores and detritivores increases at lower overall biomass or under high human pressure. This suggests that the provision of essential micronutrients can be maintained or even increase where fish biomass decreases, reinforcing the need for policies that ensure sustainable fishing, and that these micronutrients are retained locally for nutrition. Furthermore, we found a negative association between micronutrient density and two metrics of fish biodiversity, revealing an important nutrition-biodiversity trade-off. Protecting reefs with high levels of biodiversity maintains key ecosystem functions, whereas sustainable fisheries management in locations with high micronutrient density could sustain the essential supply of micronutrients to coastal human communities.

High diversity and abundance of fish and coral in an artificial breakwater in Colombo, Sri Lanka

The diversity of reef fishes and the diversity and abundance of corals in artificial breakwaters were studied between June and August 2023 in Port City, Colombo in the Western Province of Sri Lanka. Four methods were used: visual census using video, photographs, stationary point count and environmental DNA (eDNA). A total of 137 fish species, 74 coral species and 175 other species were recorded using the four methods. There were similarities and differences between the methods for recording fish, corals and other species. Only three fish species: Moon wrasse (Thalassoma lunare); Yellowtail fusilier (Caesio cuning) and Diamond fish (Monodactylus argenteus) were recorded by all methods. The highest number of fish species was recorded by video, followed by photographs, eDNA and stationary point count in the sequence. Coral cover ranged from 9 to 50.8% and mean live coral varied from 24.4% to 34.5 ± 5.2%. The dominant coral species was Pocillopora damicornis. Rare observations of coral bleaching (n=1) and frequent observations of marine debris (n=20) and coral damage (n=25) were recorded. Species detection using eDNA is a biomonitoring tool that did not add significantly to the list of fish and corals, but included many microscopic groups such as bacteria, diatoms and ciliates that were not recorded using other methods, and provided new ecological insights into marine systems. This is the first use of multiple monitoring tools to identify and compare methods for the biodiversity of marine fish, coral and other species in Sri Lankan marine waters.

Three-dimensional conservation planning of fish biodiversity metrics to achieve the deep-sea 30×30 conservation target.

Accelerating rate of human impact and environmental change severely affects marine biodiversity and increases the urgency to implement the Convention on Biological Diversity (CBD) 30×30 plan for conserving 30% of sea areas by 2030. However, area-based conservation targets are complex to identify in a 3-dimensional (3D) ocean where deep-sea features such as seamounts have been seldom studied mostly due to challenging methodologies to implement at great depths. Yet, the use of emerging technologies, such as environmental DNA combined with modern modeling frameworks, could help address the problem. We collected environmental DNA, echosounder acoustic, and video data at 15 seamounts and deep island slopes across the Coral Sea. We modeled 7 fish community metrics and the abundances of 45 individual species and molecular operational taxonomic units (MOTUs) in benthic and pelagic waters (down to 600-m deep) with boosted regression trees and generalized joint attribute models to describe biodiversity on seamounts and deep slopes and identify 3D protection solutions for achieving the CBD area target in New Caledonia (1.4millionkm2). We prioritized the identified conservation units in a 3D space, based on various biodiversity targets, to meet the goal of protecting at least 30% of the spatial domain, with a focus on areas with high biodiversity. The relationship between biodiversity protection targets and the spatial area protected by the solution was linear. The scenario protecting 30% of each biodiversity metric preserved almost 30% of the considered spatial domain and accounted for the 3D distribution of biodiversity. Our study paves the way for the use of combined data collection methodologies to improve biodiversity estimates in 3D structured marine environments for the selection of conservation areas and for the use of biodiversity targets to achieve area-based international targets.

Deep-Pelagic Fishes Are Anything But Similar: A Global Synthesis.

Deep-pelagic fishes are among the most abundant vertebrates on Earth. They play a critical role in sequestering carbon, providing prey for harvestable fishing stocks and linking oceanic layers and trophic levels. However, knowledge of these fishes is scarce and fragmented, hampering the ability of both the scientific community and stakeholders to address them effectively. While modelling approaches incorporating these organisms have advanced, they often oversimplify their functional and ecological diversity, potentially leading to misconceptions. To address these gaps, this synthesis examines the biodiversity and ecology of global deep-pelagic fishes. We review pelagic ecosystem classifications and propose a new semantic framework for deep-pelagic fishes. We evaluate different sampling methods, detailing their strengths, limitations and complementarities. We provide an assessment of the world's deep-pelagic fishes comprising 1554 species, highlighting major groups and discussing regional variability. By describing their morphological, behavioural and ecological diversity, we show that these organisms are far from homogeneous. Building on this, we call for a more realistic approach to the ecology of deep-pelagic fishes transitioning between very different ecological niches during diel vertical migrations. To facilitate this, we introduce the concept of 'diel-modulated realised niche' and propose a conceptual model synthesising the multiple drivers responsible for such transitions.