Disentangling tropicalization and deborealization in marine ecosystems under climate change

Multidecadal changes in coastal benthic species composition and ecosystem functioning occur independently of temperature-driven community shifts

Rising global temperatures are often identified as the key driver impacting ecosystems and the services they provide by affecting biodiversity structure and function. A disproportionate amount of our understanding of biodiversity and function is from short-term experimental studies and static values of biodiversity indices, lacking the ability to monitor long-term trends and capture community dynamics. Here, we analyse a biennial dataset spanning 32 years of macroinvertebrate benthic communities and their functional response to increasing temperatures. We monitored changes in species' thermal affinities to examine warming-related shifts by selecting their mid-point global temperature distribution range and linking them to species' traits. We employed a novel weighted metric using Biological Trait Analysis (BTA) to gain better insights into the ecological potential of each species by incorporating species abundance and body size and selecting a subset of traits that represent five ecosystem functions: bioturbation activity, sediment stability, nutrient recycling and higher and lower trophic production. Using biodiversity indices (richness, Simpson's diversity and vulnerability) and functional indices (richness, Rao's Q and redundancy), the community structure showed no significant change over time with a narrow range of variation. However, we show shifts in species composition with warming and increases in the abundance of individuals, which altered ecosystem functioning positively and/or non-linearly. Yet, when higher taxonomic groupings than species were excluded from the analysis, there was only a weak increase in the measured change in community-weighted average thermal affinities, suggesting changes in ecosystem functions over time occur independently of temperature increase-related shifts in community composition. Other environmental factors driving species composition and abundance may be more important in these subtidal macrobenthic communities. This challenges the prevailing emphasis on temperature as the primary driver of ecological response to climate change and emphasises the necessity for a comprehensive understanding of the temporal dynamics of complex systems.

Future redistribution of fishery resources suggests biological and economic trade-offs according to the severity of the emission scenario

Climate change is anticipated to have long-term and pervasive effects on marine ecosystems, with cascading consequences to many ocean-reliant sectors. For the marine fisheries sector, these impacts can be further influenced by future socio-economic and political factors. This raises the need for robust projections to capture the range of potential biological and economic risks and opportunities posed by climate change to marine fisheries. Here, we project future changes in the abundance of eight commercially important fish and crab species in the eastern Bering Sea and Chukchi Sea under different CMIP6 Shared Socioeconomic Pathways (SSPs) leading to contrasting future (2021-2100) scenarios of warming, sea ice concentration, and net primary production. Our results revealed contrasting patterns of abundance and distribution changes across species, time periods and climate scenarios, highlighting potential winners and losers under future climate change. In particular, the least changes in future species abundance and distribution were observed under SSP126. However, under the extreme scenario (SSP585), projected Pacific cod and snow crab abundances increased and decreased, respectively, with concurrent zonal and meridional future shifts in their centers of gravity. Importantly, projected changes in species abundance suggest that fishing at the same distance from the current major port in the Bering Sea (i.e., Dutch Harbor) could yield declining catches for highly valuable fisheries (e.g., Pacific cod and snow crab) under SSP585. This is driven by strong decreases in future catches of highly valuable species despite minimal declines in maximum catch potential, which are dominated by less valuable taxa. Hence, our findings show that projected changes in abundance and shifting distributions could have important biological and economic impacts on the productivity of commercial and subsistence fisheries in the eastern Bering and Chukchi seas, with potential implications for the effective management of transboundary resources.

Assessing the response of marine fish communities to climate change and fishing

Globally, marine fish communities are being altered by climate change and human disturbances. We examined data on global marine fish communities to assess changes in community-weighted mean temperature affinity (i.e., mean temperatures within geographic ranges), maximum length, and trophic levels, which, respectively, represent the physiological, morphological, and trophic characteristics of marine fish communities. Then, we explored the influence of climate change and fishing on these characteristics because of their long-term role in shaping fish communities, especially their interactive effects. We employed spatial linear mixed models to investigate their impacts on community-weighted mean trait values and on abundance of different fish lengths and trophic groups. Globally, we observed an initial increasing trend in the temperature affinity of marine fish communities, whereas the weighted mean length and trophic levels of fish communities showed a declining trend. However, these shift trends were not significant, likely due to the large variation in midlatitude communities. Fishing pressure increased fish communities' temperature affinity in regions experiencing climate warming. Furthermore, climate warming was associated with an increase in weighted mean length and trophic levels of fish communities. Low climate baseline temperature appeared to mitigate the effect of climate warming on temperature affinity and trophic levels. The effect of climate warming on the relative abundance of different trophic classes and size classes both exhibited a nonlinear pattern. The small and relatively large fish species may benefit from climate warming, whereas the medium and largest size groups may be disadvantaged. Our results highlight the urgency of establishing stepping-stone marine protected areas to facilitate the migration of fishes to habitats in a warming ocean. Moreover, reducing human disturbance is crucial to mitigate rapid tropicalization, particularly in vulnerable temperate regions.

Cross-basin and cross-taxa patterns of marine community tropicalization and deborealization in warming European seas

Ocean warming and acidification, decreases in dissolved oxygen concentrations, and changes in primary production are causing an unprecedented global redistribution of marine life. The identification of underlying ecological processes underpinning marine species turnover, particularly the prevalence of increases of warm-water species or declines of cold-water species, has been recently debated in the context of ocean warming. Here, we track changes in the mean thermal affinity of marine communities across European seas by calculating the Community Temperature Index for 65 biodiversity time series collected over four decades and containing 1,817 species from different communities (zooplankton, coastal benthos, pelagic and demersal invertebrates and fish). We show that most communities and sites have clearly responded to ongoing ocean warming via abundance increases of warm-water species (tropicalization, 54%) and decreases of cold-water species (deborealization, 18%). Tropicalization dominated Atlantic sites compared to semi-enclosed basins such as the Mediterranean and Baltic Seas, probably due to physical barrier constraints to connectivity and species colonization. Semi-enclosed basins appeared to be particularly vulnerable to ocean warming, experiencing the fastest rates of warming and biodiversity loss through deborealization.

Warming underpins community turnover in temperate freshwater and terrestrial communities

Rising temperatures are leading to increased prevalence of warm-affinity species in ecosystems, known as thermophilisation. However, factors influencing variation in thermophilisation rates among taxa and ecosystems, particularly freshwater communities with high diversity and high population decline, remain unclear. We analysed compositional change over time in 7123 freshwater and 6201 terrestrial, mostly temperate communities from multiple taxonomic groups. Overall, temperature change was positively linked to thermophilisation in both realms. Extirpated species had lower thermal affinities in terrestrial communities but higher affinities in freshwater communities compared to those persisting over time. Temperature change's impact on thermophilisation varied with community body size, thermal niche breadth, species richness and baseline temperature; these interactive effects were idiosyncratic in the direction and magnitude of their impacts on thermophilisation, both across realms and taxonomic groups. While our findings emphasise the challenges in predicting the consequences of temperature change across communities, conservation strategies should consider these variable responses when attempting to mitigate climate-induced biodiversity loss.

The ecological and evolutionary consequences of tropicalisation

Tropicalisation is a marine phenomenon arising from contemporary climate change, and is characterised by the range expansion of tropical/subtropical species and the retraction of temperate species. Tropicalisation occurs globally and can be detected in both tropical/temperate transition zones and temperate regions. The ecological consequences of tropicalisation range from single-species impacts (e.g., altered behaviour) to whole ecosystem changes (e.g., phase shifts in intertidal and subtidal habitats). Our understanding of the evolutionary consequences of tropicalisation is limited, but emerging evidence suggests that tropicalisation could induce phenotypic change as well as shifts in the genotypic composition of both expanding and retracting species. Given the rapid rate of contemporary climate change, research on tropicalisation focusing on shifts in ecosystem functioning, biodiversity change, and socioeconomic impacts is urgently needed.

The thermal journey of macroalgae: Four decades of temperature-induced changes in the southeastern Bay of Biscay

Global warming is triggering significant shifts in temperate macroalgal communities worldwide, favoring small, warm-affinity species over large canopy-forming, cold-affinity species. The Cantabrian Sea, a region acutely impacted by climate change, is also witnessing this shift. This study delved into the impacts of increasing sea surface temperature on the subtidal macroalgal communities in the southeastern Bay of Biscay over the last four decades, by using data from the years 1982, 2007, 2014, and 2020. We found that temperature has shaped the community structure, with warm-affinity species steadily displacing their cold-affinity counterparts. Notably, new communities exhibited a profusion of smaller algal species, explaining the observed increased biodiversity within the area. In the last period investigated (2014-2020), we observed a partial recovery of the communities, coinciding with cooler sea surface temperatures. Shallow algal communities were more reactive to temperature variations than deeper communities, possibly associated with higher exposure to increased temperatures. Our study offered insights into the intricate relationship between the changes in ocean temperature and algal species in the southeastern Bay of Biscay, shedding light on the ongoing ecological shifts in this region.

Shipping traffic, salinity and temperature shape non-native fish richness in estuaries worldwide

Non-native species threaten biodiversity conservation and ecosystem functioning. Management at early-invasion stages can prevent ecological and socioeconomic impacts, but rely on the identification of drivers of non-native species occurrence at distinct scales. Here, we identify environmental and anthropogenic correlates of non-native fish richness across estuaries worldwide. We performed model selection using proxies of colonization pressure, habitat availability and connectivity, anthropogenic disturbance and climate, to assess the primary mechanisms underlying non-native species occurrence. Latitudinal and guild-related trends in non-native occurrence were also investigated using species thermal and salinity affinities. Data retrieved from a literature review revealed 147 non-native fish species in 147 estuaries worldwide. Shipping traffic, salinity (minimum and range values) and temperature (minimum value) were the main predictors of non-native fish richness. Hotspots of non-native species were under heavy levels of shipping traffic, had higher salinity (both minimum and range values) and colder waters. We also found evidence of thermal limits to species' geographic area of introduction. Latitude of invaded estuaries was negatively correlated with species' minimum, mean and maximum thermal affinities, and positively correlated with thermal affinity ranges. Most non-native species recorded in estuaries were freshwater, but their minimum salinity affinities ranged from 2 to 35 pss. Moreover, species within marine guilds were mostly stenohaline and showed affinity for minimum salinities around 20-30 pss, which may be related to the positive relationship between non-native richness and estuary's increased salinity. Our results indicate that colonization pressure, disturbance (as result of multiple shipping impacts) and habitat filtering are the primary mechanisms underlying non-native fish richness in estuaries, contributing to the development of management strategies targeting early-invasion stages. Matching climate between native and non-native ranges was particularly important for predicting introductions at the global scale, whereas local fluctuations in salinity likely drove non-native richness in response to increased habitat availability.

FISHGLOB_data: an integrated dataset of fish biodiversity sampled with scientific bottom-trawl surveys

Scientific bottom-trawl surveys are ecological observation programs conducted along continental shelves and slopes of seas and oceans that sample marine communities associated with the seafloor. These surveys report taxa occurrence, abundance and/or weight in space and time, and contribute to fisheries management as well as population and biodiversity research. Bottom-trawl surveys are conducted all over the world and represent a unique opportunity to understand ocean biogeography, macroecology, and global change. However, combining these data together for cross-ecosystem analyses remains challenging. Here, we present an integrated dataset of 29 publicly available bottom-trawl surveys conducted in national waters of 18 countries that are standardized and pre-processed, covering a total of 2,170 sampled fish taxa and 216,548 hauls collected from 1963 to 2021. We describe the processing steps to create the dataset, flags, and standardization methods that we developed to assist users in conducting spatio-temporal analyses with stable regional survey footprints. The aim of this dataset is to support research, marine conservation, and management in the context of global change.

Northeast Atlantic elasmobranch community on the move: Functional reorganization in response to climate change

While spatial distribution shifts have been documented in many marine fishes under global change, the responses of elasmobranchs have rarely been studied, which may have led to an underestimation of their potential additional threats. Given their irreplaceable role in ecosystems and their high extinction risk, we used a 24-year time series (1997-2020) of scientific bottom trawl surveys to examine the effects of climate change on the spatial distribution of nine elasmobranch species within Northeast Atlantic waters. Using a hierarchical modeling of species communities, belonging to the joint species distribution models, we found that suitable habitats for four species increased on average by a factor of 1.6 and, for six species, shifted north-eastwards and/or to deeper waters over the past two decades. By integrating species traits, we showed changes in habitat suitability led to changes in the elasmobranchs trait composition. Moreover, communities shifted to deeper waters and their mean trophic level decreased. We also note an increase in the mean community size at maturity concurrent with a decrease in fecundity. Because skates and sharks are functionally unique and dangerously vulnerable to both climate change and fishing, we advocate for urgent considerations of species traits in management measures. Their use would make it better to identify species whose loss could have irreversible impacts in face of the myriad of anthropogenic threats.

Change in body size in a rapidly warming marine ecosystem: Consequences of tropicalization

Climate change is profoundly affecting the physical environment and biota of the Northeast U.S. Continental Shelf ecosystem. To understand adaptations to climate change, in particular warming temperatures, we used bottom trawl survey data to describe the size of individual fish and macroinvertebrates. Using species distribution models to estimate abundance and biomass, we determined body size in weight for all modeled species. We demonstrate a tendency for increased abundance and biomass and a concomitant decline in body size over time. An analysis of length frequency data supports this assertion. There was no trend in the combined anthropogenic removals from the ecosystem, i.e. catches, suggesting a limited role of fisheries in influencing these changes. The changes in the fish and macroinvertebrate communities are consistent with the hypothesis of a tropicalization of this ecosystem, where the ecosystem experiences a change in diversity, abundance, biomass, and the size of individuals consistent with lower latitudes. The changes in how productivity is expressed in the ecosystem factors into how human populations relate to it; in a practical sense, change in body size will likely influence the strategies and efficiencies of harvest procedures and the industries built to support them.

Boreal and Lusitanian species display trophic niche variation in temperate waters

Climate change has non-linear impacts on species distributions and abundance that have cascading effects on ecosystem structure and function. Among them are shifts in trophic interactions within communities. Sites found at the interface between two or more biogeographical regions, where species with diverse thermal preferenda are assembled, are areas of strong interest to study the impact of climate change on communities' interactions. This study examined variation in trophic structure in the Celtic Sea, a temperate environment that hosts a mixture of cold-affiliated Boreal species and warm-affiliated Lusitanian species. Using carbon and nitrogen stable isotope ratios, trophic niche area, width, and position were investigated for 10 abundant and commercially important demersal fish species across space and time. In general, the niches of Boreal species appear to be contracting while those of Lusitanian species expand, although there are some fluctuations among species. These results provide evidence that trophic niches can undergo rapid modifications over short time periods (study duration: 2014-2021) and that this process may be conditioned by species thermal preferenda. Boreal species displayed spatial variation in trophic niche width and seem to be facing increased competition with Lusitanian species for food resources. These findings underscore the need to utilize indicators related to species trophic ecology to track the ecosystem alterations induced by climate change. Such indicators could reveal that the vulnerability of temperate ecosystems is currently being underestimated.

Historical biogeography supports Point Conception as the site of turnover between temperate East Pacific ichthyofaunas

The cold temperate and subtropical marine faunas of the Northeastern Pacific meet within California as part of one of the few eastern boundary upwelling ecosystems in the world. Traditionally, it is believed that Point Conception is the precise site of turnover between these two faunas due to sharp changes in oceanographic conditions. However, evidence from intraspecific phylogeography and species range terminals do not support this view, finding stronger biogeographic breaks elsewhere along the coast. Here I develop a new application of historical biogeographic approaches to uncover sites of transition between faunas without needing an a priori hypothesis of where these occur. I used this approach to determine whether the point of transition between northern and southern temperate faunas occurs at Point Conception or elsewhere within California. I also examined expert-vetted latitudinal range data of California fish species from the 1970s and the 2020s to assess how biogeography could change with the backdrop of climate change. The site of turnover was found to occur near Point Conception, in concordance with the traditional view. I suggest that recent species- and population-level processes could be expected to give signals of different events from historical biogeography, possibly explaining the discrepancy across studies. Species richness of California has increased since the 1970s, mostly due to species's ranges expanding northward from Baja California (Mexico). Range shifts under warming conditions seem to be increasing the disparity between northern and southern faunas of California, creating a more divergent biogeography.

Marine heatwaves are not a dominant driver of change in demersal fishes

Marine heatwaves have been linked to negative ecological effects in recent decades1,2. If marine heatwaves regularly induce community reorganization and biomass collapses in fishes, the consequences could be catastrophic for ecosystems, fisheries and human communities3,4. However, the extent to which marine heatwaves have negative impacts on fish biomass or community composition, or even whether their effects can be distinguished from natural and sampling variability, remains unclear. We investigated the effects of 248 sea-bottom heatwaves from 1993 to 2019 on marine fishes by analysing 82,322 hauls (samples) from long-term scientific surveys of continental shelf ecosystems in North America and Europe spanning the subtropics to the Arctic. Here we show that the effects of marine heatwaves on fish biomass were often minimal and could not be distinguished from natural and sampling variability. Furthermore, marine heatwaves were not consistently associated with tropicalization (gain of warm-affiliated species) or deborealization (loss of cold-affiliated species) in these ecosystems. Although steep declines in biomass occasionally occurred after marine heatwaves, these were the exception, not the rule. Against the highly variable backdrop of ocean ecosystems, marine heatwaves have not driven biomass change or community turnover in fish communities that support many of the world's largest and most productive fisheries.

Short-term response of macroalgal communities to ocean warming in the Southern Bay of Biscay

Climate change is causing significant shifts in biological communities worldwide, including the degradation of marine communities. Previous research has predicted that southern Bay of Biscay canopy-forming subtidal macroalgal communities will shift into turf-forming Mediterranean-like communities by the end of the century. These predictions were based on a community-environment relationship model that used macroalgal abundance data and IPCC environmental projections. We have tested the short-term accuracy of that model by resampling the same communities and locations four years later and found the short-term predictions to be consistent with the observed communities. Changes in sea surface temperature were positively correlated with changes in the Community Temperature Index, suggesting that macroalgal communities had responded quickly to global warming. The changes over four years were significant, but canopy-forming macroalgae were more resilient in local sites with favourable temperature conditions. Our study demonstrated that updating predictive models with new data has the potential to yield reliable predictions and inform effective conservation strategies.

Rapid tropicalization evidence of subtidal seaweed assemblages along a coastal transitional zone

Anthropogenic climate change, particularly seawater warming, is expected to drive quick shifts in marine species distribution transforming coastal communities. These shifts in distribution will be particularly noticeable in biogeographical transition zones. The continental Portuguese coast stretches from north to south along 900 km. Despite this short spatial scale, the strong physical gradient intensified by the Iberian upwelling creates a transition zone where seaweed species from boreal and Lusitanian-Mediterranean origin coexist. On the northern coast, kelp marine forests thrive in the cold, nutrient-rich oceanic waters. In the south, communities resemble Mediterranean-type seaweed assemblages and are dominated by turfs. Recent evidence suggests that in these coastal areas, marine intertidal species are shifting their distribution edges as a result of rising seawater temperatures. Taking advantage of previous abundance data collected in 2012 from subtidal seaweed communities, a new sampling program was carried out in the same regions in 2018 to assess recent changes. The results confirmed the latitudinal gradient in macroalgal assemblages. More importantly we found significant structural and functional changes in a short period of six years, with regional increases of abundance of warm-affinity species, small seaweeds like turfs. Species richness, diversity, and biomass increase, all accompanied by an increase of community temperature index (CTI). Our findings suggest that subtidal seaweed communities in this transitional area have undergone major changes within a few years. Evidence of "fast tropicalization" of the subtidal communities of the Portuguese coast are strong indication of the effects of anthropic climate change over coastal assemblages.

Habitat-use of the vulnerable Atlantic Nurse Shark: a review

Human activities have led to the loss of critical habitats for aquatic species at such an accelerated rate that habitat modification is considered a leading threat to biodiversity. Sharks and rays are considered the second most threatened group of vertebrates that have also suffered from habitat loss, especially in nursery grounds and reef-associated species. In this sense, actions toward the conservation of critical grounds for species survival are urgently needed, especially for those threatened with extinction. This study aimed to gather and provide information on the worldwide distribution and habitat association of the 'vulnerable' Atlantic Nurse Shark Ginglymostoma cirratum through a literature review performed at the Dimensions research database. A total of 30 studies published between 1950 and 2021 were retained since they defined at least the type of habitat in which G. cirratum was associated. Most studies covered the Floridian ecoregion, where G. cirratum is more common and abundant. Reefs, seagrass, sandy, rocky, mangrove, and macroalgae accounted for the majority of habitat associations, with a higher diversity of habitats detected within marine protected areas (MPAs). Ginglymostoma cirratum was recorded at a maximum depth of 75 m, temperatures ranging from 25 °C to 34 °C, and salinities between 31 and 38 ppt. Neonates were associated with shallower habitats (<20 m), mostly reefs, rocks, macroalgae, sandy shores, and seagrass, in an average temperature of 26 °C and salinity of 36 ppt. Breeding events and habitats were reported by 11 studies, 72.7% of them in shallow waters, mostly inside MPAs (90.9%). Our findings highlighted the key role played by MPAs in protecting essential grounds for threatened species, such as the Atlantic Nurse Shark. Major ecoregions (e.g., the Eastern Atlantic) are still underrepresented in the scientific literature as long as studies aim specifically to assess G. cirratum habitat association. Thus, further insights into the essential habitats needed to conserve the Atlantic Nurse Shark can still emerge from future studies. Considering the recent IUCN extinction risk status change in G. cirratum (i.e., Data Deficient to 'Vulnerable'), new conservation measures that integrate habitat protection and management are urgently needed and should consider the data collected herein.

Resident species, not immigrants, drive reorganization of estuarine fish assemblages in response to warming

Climate change is reshaping biological communities, as species track environmental temperature. Assemblage reorganization is underpinned by shifts in species abundance and distribution, but studies often focus on documenting compositional turnover. As a consequence, phenomena such as the tropicalization of temperate communities have been widely associated with increased occupancy of warm-affinity species. Abundance-weighted change in thermal affinity can be tracked with the Community Temperature Index (CTI), and decomposed into four processes: tropicalization (increasing warm-affinity), borealization (increasing cold-affinity), deborealization (decreasing cold-affinity), and detropicalization (decreasing warm-affinity). Further evaluation of these processes according to species persistence (i.e., immigrant, emigrant, and resident) may provide insights on whether novel communities emerge primarily from local shifts in species abundance or distribution. Using long-term data on fish assemblages undergoing climate change's effects across 19 temperate estuaries surveyed for at least 20 years, we hypothesized (1) deborealization is the main process reshaping communities under climate change, and (2) the contribution of resident species to processes reshaping communities surpass the ones from immigrants and emigrants. Community dissimilarity was calculated through the Temporal Beta Index (TBI), which was further decomposed into species and individual losses and gains. These values were then used as effect sizes in the meta-analyses performed to detect systematic trends in assemblage reorganization in response to climate change. We also calculated CTI and the strength of temperature-related processes for resident, immigrant and emigrant species. Species and individual gains outweighed losses in estuaries. Temperature was correlated with changes in species abundance, but not occurrence. Fish abundance decreased with warming, and initially cooler estuaries gained more fish than warmer ones. Novel communities were shaped by a variety of processes, but mainly tropicalization. Assemblage reorganization was primarily driven by shifts in abundance of resident species with distinct thermal affinities, while contributions of arriving and exiting species played a secondary role. These findings reveal that novel communities are drawn primarily from the local species pool, due to changes in climate-related drivers that favor distinct resident species.

Tracking widespread climate-driven change on temperate and tropical reefs

Warming seas, marine heatwaves, and habitat degradation are increasingly widespread phenomena affecting marine biodiversity, yet our understanding of their broader impacts is largely derived from collective insights from independent localized studies. Insufficient systematic broadscale monitoring limits our understanding of the true extent of these impacts and our capacity to track these at scales relevant to national policies and international agreements. Using an extensive time series of co-located reef fish community structure and habitat data spanning 12 years and the entire Australian continent, we found that reef fish community responses to changing temperatures and habitats are dynamic and widespread but regionally patchy. Shifts in composition and abundance of the fish community often occurred within 2 years of environmental or habitat change, although the relative importance of these two mechanisms of climate impact tended to differ between tropical and temperate zones. The clearest of these changes on temperate and subtropical reefs were temperature related, with responses measured by the reef fish thermal index indicating reshuffling according to the thermal affinities of species present. On low latitude coral reefs, the community generalization index indicated shifting dominance of habitat generalist fishes through time, concurrent with changing coral cover. Our results emphasize the importance of maintaining local ecological detail when scaling up datasets to inform national policies and global biodiversity targets. Scaled-up ecological monitoring is needed to discriminate among increasingly diverse drivers of large-scale biodiversity change and better connect presently disjointed systems of biodiversity observation, indicator research, and governance.

Fishing for fish environmental DNA: Ecological applications, methodological considerations, surveying designs, and ways forward

Vast global declines of freshwater and marine fish diversity and population abundance pose serious threats to both ecosystem sustainability and human livelihoods. Environmental DNA (eDNA)-based biomonitoring provides robust, efficient, and cost-effective assessment of species occurrences and population trends in diverse aquatic environments. Thus, it holds great potential for improving conventional surveillance frameworks to facilitate fish conservation and fisheries management. However, the many technical considerations and rapid developments underway in the eDNA arena can overwhelm researchers and practitioners new to the field. Here, we systematically analysed 416 fish eDNA studies to summarize research trends in terms of investigated targets, research aims, and study systems, and reviewed the applications, rationales, methodological considerations, and limitations of eDNA methods with an emphasis on fish and fisheries research. We highlighted how eDNA technology may advance our knowledge of fish behaviour, species distributions, population genetics, community structures, and ecological interactions. We also synthesized the current knowledge of several important methodological concerns, including the qualitative and quantitative power eDNA has to recover fish biodiversity and abundance, and the spatial and temporal representations of eDNA with respect to its sources. To facilitate ecological applications implementing fish eDNA techniques, recent literature was summarized to generate guidelines for effective sampling in lentic, lotic, and marine habitats. Finally, we identified current gaps and limitations, and pointed out newly emerging research avenues for fish eDNA. As methodological optimization and standardization improve, eDNA technology should revolutionize fish monitoring and promote biodiversity conservation and fisheries management that transcends geographic and temporal boundaries.

Climate change: Large-scale abundance shifts in fishes

Warming seas are driving a mass-scale restructuring of marine life, with observed responses expanding beyond species' range shifts. New evidence highlights large regions where ecological change has been dominated by the declining abundance of species that prefer cooler waters.