Wild Fish Populations Research Articles

Absence of Staphylococcus aureus in Wild Populations of Fish Supports a Spillover Hypothesis.

Staphylococcus aureus is a human commensal and opportunistic pathogen that also infects other animals. In humans and livestock, where S. aureus is most studied, strains are specialized for different host species. Recent studies have also found S. aureus in diverse wild animals. However, it remains unclear whether these isolates are also specialized for their hosts or whether their presence is due to repeated spillovers from source populations. This study focuses on S. aureus in fish, testing the spillover hypothesis in two ways. First, we examined 12 S. aureus isolates obtained from the internal and external organs of a farmed fish. While all isolates were from clonal complex 45, genomic diversity indicates repeated acquisition. The presence of a φSa3 prophage containing human immune evasion genes suggests that the source was originally human. Second, we tested for S. aureus in wild fish that were isolated from likely sources. In particular, we sampled 123 brown trout and their environment at 16 sites in the remote Scottish Highlands with variable levels of exposure to humans, birds, and livestock. This screen found no S. aureus infection in any of the wild populations or their environment. Together, these results support that the presence of S. aureus in fish and aquaculture is due to spillover from humans rather than specialization. Given the trends of increasing fish consumption, a better understanding of the dynamics of S. aureus spillover in aquaculture will mitigate future risks to fish and human health. IMPORTANCE Staphylococcus aureus is a human and livestock commensal but also an important pathogen responsible for high human mortality rates and economic losses in farming. Recent studies show that S. aureus is common in wild animals, including fish. However, we do not know whether these animals are part of the normal host range of S. aureus or whether infection is due to repeated spillover events from true S. aureus hosts. Answering this question has implications for public health and conservation. We find support for the spillover hypothesis by combining genome sequencing of S. aureus isolates from farmed fish and screens for S. aureus in isolated wild populations. The results imply that fish are unlikely to be a source of novel emergent S. aureus strains but highlight the prominence of the spillover of antibiotic-resistant bacteria from humans and livestock. This may affect both future fish disease potential and the risk of human food poisoning.

Spatial Patterns of Neutral and Functional Genetic Variations along Dendritic Networks of Riverscape in Brown Trout Populations

Understanding how environmental gradients shape the spatial patterns of intraspecific genetic diversity is a central issue in ecological and evolutionary sciences. In riverine ecosystems, there is generally an increase in neutral genetic diversity downstream, as well as an increase in genetic differentiation among upstream populations. However, selective pressures may vary markedly along the upstream–downstream gradient, which could modify these patterns, but this has rarely been tested empirically. Here, we investigated how environmental gradients in a river network could shape the spatial patterns of intraspecific genetic diversity and differentiation in both neutral SNP markers and functional genetic markers putatively under natural selection (candidate SNPs associated with physiological functions and immune Major Histocompatibility Complex (MHC) loci) in wild brown trout populations. First, we showed that both the distance from the confluence and the centrality on the river network could explain the variation in genetic diversity and differentiation. Second, we found that both neutral and functional markers followed a similar pattern, with a higher genetic diversity and a lower genetic differentiation among populations that were more central and/or near to the confluence. This study highlights the importance of considering both the spatial and hydrological factors of a river network to understand and predict the role of dendritic connectivity in the spatial patterns of genetic diversity and differentiation in wild fish populations.

Characterization of the productive articulation and tourism in the aquaculture farms of Toluca, State of Mexico, and Lake Tota in Boyacá, Colombia, first advance

Global aquaculture has had exponential growth in recent decades (FAO,2020), mainly due to the decrease of wild fish populations in the global oceans and inland water bodies. That is why, according to the projection of specialists, by 2050 these species will have to be produced in controlled conditions, creating, improving, and developing farms that favor the consumption of this food locally, nationally, and internationally. For this reason, the productive articulation that is carried out between the different actors, Company, government, university, and associations of producers, (quadruple hélix), favors the interrelation between them, thus achieving social and economic benefits for the participants. Specifically, this research Project within the framework of the Pacific Alliance Treat aims to analyze the trout farms in Toluca, State of México, and the Tota lagoon in Boyacá, Colombia, to determine in a first approach the characterization of these groups and their relationship with tourism, to propose alternatives that promote greater involvement of the participants and increase collective benefits. This field research, which is carried out in the aforementioned localities, with a qualitative, non-experimental approach based on content analysis, collecting existing data in statistics and official documents of both countries, and with structured interviews conducted with aquaculture managers of the federal and local government and owners of aquaculture farms. The article is preliminary to know the operating conditions of the farms, and the link with Tourism, which will be linked to subsequent publications once the process of obtaining the actors of the productive groupings in México and Colombia is concluded.

Size-selective harvesting alters biological traits of marine medaka (Oryzias melastigma)

Under long-term fishing pressure, the biological traits of many wild fish populations have changed. Such phenotypic changes include smaller in size and earlier age-at-maturation, strongly suggesting that fishing could induce rapid changes in the life-history traits of fish. However, whether extensive fishing will lead to genetic changes in fish, how they occur, and whether they are reversible still requires further research. In this study, we analyzed biological traits of marine medaka (Oryzias melastigma) subjected to 50% experimental harvest for 4 consecutive generations. This study adopted a factorial design with three size-selective harvesting strategies (large-, small-, and random-size selection) on groups either before or after maturation. We compared the growth, maturation, and early life stages of marine medaka for different harvesting strategies. The results showed that the mean lengths and mean weights in large-harvested populations significantly declined after 4 generations, whereas small-harvested populations did the opposite. Furthermore, selective harvesting of large individuals also led to earlier maturation, declining fecundity, declining hatching rate and increasing larval mortality. Importantly, compared to harvesting before maturation, harvesting after maturation could mitigate the effects of harvest on biological traits. This study provided experimental evidence for the impact of selective harvest on phenotypic evolution and contributed to the theoretical basis for sustainability in fisheries and fishery management.

Myxobolus shuifuensis sp. n. (Myxozoa: Myxobolidae) infecting the exotic mrigal Cirrhinus mrigala feral in China.

Myxobolus species represents a group of cosmopolitan metazoan parasites commonly harbored in the farmed and wild fish populations. Here, a species of Myxobolus is found in the kidney of an exotic mrigal Cirrhinus mrigala feral in the Yangtze River and utilized for an integrative characterization. Ellipsoidal myxospores are measured at 15.68±0.8 (13.93-17.11)×11.42±0.54 (10.34-12.3)×7.94±0.35 (7.58-8.5) μm in dimension. The polar capsules are pyriform, and unequal in size. The morphological and morphometric characteristics of the present isolates are distinct from those of other congeners. Molecularly, the pairwise comparison based on the SSU rDNA sequence indicates that the present amplicon does not match any sequences available in the GenBank database and shares the highest similarity of 92.12% to Myxobolus pavlovskii (MG520369). Accordingly, we propose a name Myxobolus shuifuensis sp. n. for the present isolates. Phylogenetical trees indicate an apparent host-associated phylogenetic pattern. M. shuifuensis sp. n. groups loosely with other Myxobolus species isolated from Cirrhinus fish. Insead, it forms a sister clade to some myxosporeans belonging to the Thelohanellus genus. This result underpins the species identification and provides evidence for challenging the taxonomic separation among both morphologically comparable genera.

Contrasting demographic responses to size‐selective harvesting among neighbouring wild fish populations

Abstract Sustainable harvesting of wild populations relies on evidence‐based knowledge to predict harvesting outcomes for species and the ecosystems they inhabit. Although harvesting may elicit compensatory density‐dependence, it is generally size‐selective, which induces additional pressures that are challenging to forecast. Furthermore, responses to harvest may be population‐specific and whether generalizable patterns exist remains unclear. Taking advantage of Parks Canada's mandate to remove introduced brook trout Salvelinus fontinalis to restore alpine lakes in Canadian parks, we experimentally applied standardized size‐selective harvesting rates (the largest ~64% annually) for three consecutive summers in five populations with different initial size structures. Four unharvested populations were used as controls. At reduced densities, harvested and control populations exhibited similar density‐dependent increases in specific growth, juvenile survival and earlier maturation. However, size‐selective harvesting simultaneously induced changes to size and age structure that contrasted among harvested populations. Average body length decreased in three of five harvested populations, whereas it tended to increase in control populations over the 3 years. We also detected contrasting, population‐specific changes in body length variability and ultimately in length‐ and age‐at‐harvest in harvested populations but not controls. Overall, populations with smaller, more homogeneous body sizes, and living at high densities were most resilient to size‐selective harvesting, exhibiting the smallest change in size‐at‐age. In contrast, large‐bodied populations exhibited more substantial size‐structure changes following selective harvesting: large‐bodied populations experienced either stabilizing or disruptive pressures, when initial length variability was high or low, respectively. Synthesis and application. Our results show that within species, size‐selective harvesting inherently leads to more risk and uncertainty when harvesting populations with larger and more varied body sizes than smaller‐bodied populations with less range in body size. Our study supports prioritizing regulations that protect harvested populations with larger and more varied body sizes. Such a management strategy would reduce the likelihood of eliciting unpredictable or undesirable demographic changes to fish populations with these attributes.

Morphological variation of African snakehead (Parachanna obscura) populations along climate and habitat gradients in Côte d’Ivoire, West Africa

Intraspecific morphological variation may reflect phenotypic plasticity or adaptive divergence. While adaptive shape divergence may occur more likely among isolated populations with reduced gene flow, phenotypic plasticity may reflect morphological responses to heterogeneous environments, even in spatially connected populations. We evaluated both processes while examining morphological variations among seven wild populations of snakehead fish (Parachanna obscura) along climate and habitat gradients in Côte d’Ivoire, West Africa. Morphological variations were studied by multivariate canonical variate analysis (CVA) as based on geometric morphometrics of 15 fish body landmarks. Correlations between shape variations among populations and climate and habitat characteristics and between morphological and geographic distances were calculated. We found significant morphological variations among the seven populations. The variations in fish shape were concentrated on landmarks related to swimming and feeding, suggesting a contribution of environmental variation to morphological differentiation. However, we did not detect significant effects of climate and habitat variables on fish shape. The trend between geographical and morphological distances was likewise not significant. Therefore, a mechanistic understanding of the factors causing shape variation among P. obscura populations in West Africa could not yet be achieved.

Atypical flavobacteria recovered from diseased fish in the Western United States.

Flavobacterial diseases, caused by bacteria in the order Flavobacteriales, are responsible for devastating losses in farmed and wild fish populations worldwide. The genera Flavobacterium (Family Flavobacteriaceae) and Chryseobacterium (Weeksellaceae) encompass the most well-known agents of fish disease in the order, but the full extent of piscine-pathogenic species within these diverse groups is unresolved, and likely underappreciated. To identify emerging agents of flavobacterial disease in US aquaculture, 183 presumptive Flavobacterium and Chryseobacterium isolates were collected from clinically affected fish representing 19 host types, from across six western states. Isolates were characterized by 16S rRNA gene sequencing and phylogenetic analysis using the gyrB gene. Antimicrobial susceptibility profiles were compared between representatives from each major phylogenetic clade. Of the isolates, 52 were identified as Chryseobacterium species and 131 as Flavobacterium. The majority of Chryseobacterium isolates fell into six clades (A-F) consisting of ≥ 5 fish isolates with ≥ 70% bootstrap support, and Flavobacterium into nine (A-I). Phylogenetic clades showed distinct patterns in antimicrobial susceptibility. Two Chryseobacterium clades (F & G), and four Flavobacterium clades (B, G-I) had comparably high minimal inhibitory concentrations (MICs) for 11/18 antimicrobials tested. Multiple clades in both genera exhibited MICs surpassing the established F. psychrophilum breakpoints for oxytetracycline and florfenicol, indicating potential resistance to two of the three antimicrobials approved for use in finfish aquaculture. Further work to investigate the virulence and antigenic diversity of these genetic groups will improve our understanding of flavobacterial disease, with applications for treatment and vaccination strategies.

Insights into the circulating microbiome of Atlantic and Greenland halibut populations: the role of species-specific and environmental factors

Establishing long-term microbiome-based monitoring programs is critical for managing and conserving wild fish populations in response to climate change. In most cases, these studies have been conducted on gut and, to a lesser extent, skin (mucus) microbiomes. Here, we exploited the concept of liquid biopsy to study the circulating bacterial microbiome of two Northern halibut species of economic and ecological importance. Amplification and sequencing of the 16S rRNA gene were achieved using a single drop of blood fixed on FTA cards to identify the core blood microbiome of Atlantic and Greenland halibut populations inhabiting the Gulf of St. Lawrence, Canada. We provide evidence that the circulating microbiome DNA (cmDNA) is driven by genetic and environmental factors. More specifically, we found that the circulating microbiome signatures are species-specific and vary according to sex, size, temperature, condition factor, and geographical localization. Overall, our study provides a novel approach for detecting dysbiosis signatures and the risk of disease in wild fish populations for fisheries management, most notably in the context of climate change.

Emergence of the Fungal Rosette Agent in the World: Current Risk to Fish Biodiversity and Aquaculture.

The emergence of pathogenic fungi is a major and rapidly growing problem (7% increase) that affects human and animal health, ecosystems, food security, and the economy worldwide. The Dermocystida group in particular has emerged relatively recently and includes species that affect both humans and animals. Within this group, one species in particular, Sphareothecum destruens, also known as the rosette agent, represents a major risk to global aquatic biodiversity and aquaculture, and has caused severe declines in wild fish populations in Europe and large losses in salmon farms in the USA. It is a species that has been associated with a healthy carrier for millions of years, but in recent decades, the host has managed to invade parts of Southeast Asia, Central Asia, Europe, and North Africa. In order to better understand the emergence of this new disease, for the first time, we have synthesized current knowledge on the distribution, detection, and prevalence of S. destruens, as well as the associated mortality curves, and the potential economic impact in countries where the healthy carrier has been introduced. Finally, we propose solutions and perspectives to manage and mitigate the emergence of this fungus in countries where it has been introduced.

Microplastics in freshwater wild and farmed fish species of Bangladesh.

In today's world, microplastic contamination of aquatic systems is a subject of major concern. In this study, the incidence of microplastics in freshwater fish from both farmed and wild sources was evaluated. Wild catla (Gibelion catla), stinging catfish (Heteropneustes fossilis), rui (Labeo rohita), tengara catfish (Mystus tengara), and pabdah catfish (Ompok pabda) were collected from Padma River, Turag River, and Bhairab River. Farmed fishes of same species were collected from Mymensingh and Chandpur fish markets. A total of 68 (43.6%) microparticles out of 156 were identified as microplastics using ATR-FTIR. About 66.67% (20 out of 30) of farmed fish samples had microplastics in their digestive system, while 88.4% (23 of 26) of wild fish samples presented microplastics. There was a significant difference (p < 0.05) in average microplastic count between wild and farmed fish. Average microplastic count also showed a significant difference among five fish species (p < 0.01). A variety of polymers were observed, with polyethylene being the most abundant, followed by polyester, polypropylene, polyamide-6, and polyurethane. This study suggests that microplastic contamination is ubiquitous and contamination is common in all of the fish species investigated, independent of their origin (wild or farmed). However, wild fish population is at a greater risk of ingesting microplastics. This study, for the first time ever, reports plastic pollution in freshwater fishes of Bangladesh from wild and farmed sources.

Skin and Blood Microbial Signatures of Sedentary and Migratory Trout (Salmo trutta) of the Kerguelen Islands

Our understanding of how microbiome signatures are modulated in wild fish populations remains poorly developed and has, until now, mostly been inferred from studies in commercial and farmed fish populations. Here, for the first time, we have studied changes in the skin and blood microbiomes of the Salmo trutta population of the volcanic Kerguelen archipelago located at the northern limit of the Antarctic Ocean. The Kerguelen Islands present a natural framework of population expansion and reveal a likely situation representing further climate change in distribution areas. Our results showed that S. trutta of the Kerguelen Islands has a microbiome signature distinct from those of salmonids of the Northern Hemisphere. Our study also revealed that the skin and blood microbiomes differ between sedentary and migratory S. trutta. While 18 phyla were shared between both groups of trout, independent of the compartment, 6 phyla were unique to migratory trout. Further analyses showed that microbiome signatures undergo significant site-specific variations that correlate, in some cases, with the peculiarity of specific ecosystems. Our study also revealed the presence of potential pathogens at particular sites and the impact of abiotic factors on the microbiome, most notably due to the volcanic nature of the environment. This study contributes to a better understanding of the factors that modulate the microbiome signatures of migratory and sedentary fish populations. It will also help to better monitor the impacts of climate change on the colonization process in the sub-Antarctic region.

Application of a Lipopolysaccharide (LPS)-Stimulated Mitogenesis Assay in Smallmouth Bass (Micropterus dolomieu) to Augment Wild Fish Health Studies

The utility of a functional immune assay for smallmouth bass (Micropterus dolomieu) lymphocyte mitogenesis was evaluated. Wild populations in the Potomac River have faced disease and mortality with immunosuppression from exposure to chemical contaminants a suspected component. However, a validated set of immune parameters to screen for immunosuppression in wild fish populations is not available. Prior to use in ecotoxicology studies, ancillary factors influencing the mitogenic response need to be understood. The assay was field-tested with fish collected from three sites in West Virginia as part of health assessments occurring in spring (pre-spawn; April–May) and fall (recrudescence; October–November). Anterior kidney leukocytes were exposed to lipopolysaccharide (LPS) from E.coli O111:B4 or mitogen-free media and proliferation was measured using imaging flow cytometry with advanced machine learning to distinguish lymphocytes. An anti-smallmouth bass IgM monoclonal antibody was used to identify IgM+ lymphocytes. Lymphocyte mitogenesis, or proliferative responses, varied by site and season and positively and negatively correlated with factors such as sex, age, tissue parasites, and macrophage aggregates. Background proliferation of IgM− lymphocytes was negatively correlated to LPS-induced proliferation in both seasons at all sites, but only in spring for IgM+ lymphocytes. The results demonstrate that many factors, in addition to chemical contaminants, may influence lymphocyte proliferation.

Trophic level and proteobacteria abundance drive antibiotic resistance levels in fish from coastal New England

BackgroundThe natural marine environment represents a vast reservoir of antimicrobial resistant bacteria. The wildlife that inhabits this environment plays an important role as the host to these bacteria and in the dissemination of resistance. The relationship between host diet, phylogeny, and trophic level and the microbiome/resistome in marine fish is not fully understood. To further explore this relationship, we utilize shotgun metagenomic sequencing to define the gastrointestinal tract microbiomes of seven different marine vertebrates collected in coastal New England waters.ResultsWe identify inter and intraspecies differences in the gut microbiota of these wild marine fish populations. Furthermore, we find an association between antibiotic resistance genes and host dietary guild, which suggests that higher trophic level organisms have a greater abundance of resistance genes. Additionally, we demonstrate that antibiotic resistance gene burden is positively correlated with Proteobacteria abundance in the microbiome. Lastly, we identify dietary signatures within the gut of these fish and find evidence of possible dietary selection for bacteria with specific carbohydrate utilization potential.ConclusionsThis work establishes a link between host lifestyle/dietary guild, and microbiome composition and the abundance of antibiotic resistance genes within the gastrointestinal tract of marine organisms. We expand the current understanding of marine organism-associated microbial communities and their role as reservoirs of antimicrobial resistance genes.

An Overview of Atlantic Bluefin Tuna Farming Sustainability in the Mediterranean with Special Regards to the Republic of Croatia

Atlantic bluefin tuna (Thunnus thynnus) is the most important tuna species in Mediterranean tuna fishery and a valuable commodity on the global fish market. Croatia is a pioneer in tuna farming in the Mediterranean and the only country that has the exclusive right to farm wild-caught juvenile tuna (8 to 30 kg). This paper identifies key challenges to the sustainability of current farming and fattening practices, primarily economic and environmental, and possible solutions to overcome these challenges. This paper analyses data on tuna catch and aquaculture production (FAO FishStatJ and EU-Eurostat database) and updates the latest literature on farming practices, production challenges related to biotechnical, economic and environmental issues, the market and current legislation in Croatia, as well as fattening in other Mediterranean countries. Tuna capture-based aquaculture is attractive to investors because it promises high returns, but the sustainability of intensive tuna farming and fattening is questionable and raises many ethical issues. Tuna farming and fattening relies on wild fish for stocking and feeding, and further expansion of tuna farming and fattening is limited by the size of wild tuna and small fish populations. To meet the growing global demand for tuna and to conserve wild tuna stocks, further investments are needed. The knowledge gained in Croatian tuna farming is valuable for future sustainable close-cycled tuna farming in the Mediterranean. Due to its good environmental status, the availability of small pelagic fish, the availability of a highly qualified and well-organised labour force, the good cooperation between producers and researchers, and the application of modern farming technologies, ABFT farmed in Croatia have high quality and a good reputation on the market. The main weakness of Croatian tuna farming is that the entire industry is dependent on the Japanese market, but this can be overcome by the possibility of product diversification for new markets, including the tourism industry.

A contrastive learning approach for individual re-identification in a wild fish population

In both terrestrial and marine ecology, physical tagging is a frequently used method to study population dynamics and behavior. However, such tagging techniques are increasingly being replaced by individual re-identification using image analysis. This paper introduces a contrastive learning-based model for identifying individuals. The model uses the first parts of the Inception v3 network, supported by a projection head, and we use contrastive learning to find similar or dissimilar image pairs from a collection of uniform photographs. We apply this technique for corkwing wrasse, Symphodus melops, an ecologically and commercially important fish species. Photos are taken during repeated catches of the same individuals from a wild population, where the intervals between individual sightings might range from a few days to several years. Our model achieves a one-shot accuracy of 0.35, a 5-shot accuracy of 0.56, and a 100-shot accuracy of 0.88, on our dataset.

Seasonal, environmental and individual effects on hypoxia tolerance of eastern sand darter (Ammocrypta pellucida).

Metabolic rate and hypoxia tolerance are highly variable among individual fish in a stable environment. Understanding the variability of these measures in wild fish populations is critical for assessing adaptive potential and determining local extinction risks as a result of climate-induced fluctuations in temperature and hypoxic conditions. We assessed the field metabolic rate (FMR) and two hypoxia tolerance metrics, oxygen pressure at loss of equilibrium (PO2 at LOE) and critical oxygen tolerance (Pcrit) of wild-captured eastern sand darter (Ammocrypta pellucida), a threatened species in Canada, using field trials (June to October) that encompassed ambient water temperatures and oxygen conditions typically experienced by the species. Temperature was significantly and positively related to hypoxia tolerance but not FMR. Temperature alone explained 1%, 31% and 7% of the variability observed in FMR, LOE, and Pcrit, respectively. Environmental and fish-specific factors such as reproductive season and condition explained much of the residual variation. Reproductive season significantly affected FMR by increasing it by 159-176% over the tested temperature range. Further understanding the impact of reproductive season on metabolic rate over a temperature range is crucial for understanding how climate change could impact species fitness. Among-individual variation in FMR significantly increased with temperature while among-individual variation in both hypoxia tolerance metrics did not. A large degree of variation in FMR in the summer might allow for evolutionary rescue with increasing mean and variance of global temperatures. Findings suggest that temperature may be a weak predictor in a field setting where biotic and abiotic factors can act concurrently on variables that affect physiological tolerance.

Monitoring Algal Blooms with Complementary Sensors on Multiple Spatial and Temporal Scales

Climate change, and other human-induced impacts, are severely increasing the intensity and occurrences of algal blooms in coastal regions (IPCC, 2022). Ocean warming, marine heatwaves, and eutrophication promote suitable conditions for rapid phytoplankton growth and biomass accumulation. An increase in such primary producers provides food for marine organisms, and phytoplankton play an important global role in fixing atmospheric carbon dioxide and producing much of the oxygen we breathe. But harmful algal blooms (HABs) can also form, and they may adversely affect the ecosystem by reducing oxygen availability in the water, releasing toxic substances, clogging fish gills, and diminishing biodiversity. Understanding, forecasting, and ultimately mitigating HAB events could reduce their impact on wild fish populations, help aquaculture producers avoid losses, and facilitate a healthy ocean.

Freshwater protected areas can preserve high-performance phenotypes in populations of a popular sportfish

Recreational fishing has the potential to cause evolutionary change in fish populations; a phenomenon referred to as fisheries-induced evolution. However, detecting and quantifying the magnitude of recreational fisheries selection in the wild is inherently difficult, largely owing to the challenges associated with variation in environmental factors and, in most cases, the absence of pre-selection or baseline data against which comparisons can be made. However, exploration of recreational fisheries selection in wild populations may be possible in systems where fisheries exclusion zones exist. Lakes that possess intra-lake freshwater protected areas (FPAs) can provide investigative opportunities to evaluate the evolutionary impact(s) of differing fisheries management strategies within the same waterbody. To address this possibility, we evaluated how two physiological characteristics (metabolic phenotype and stress responsiveness) as well as a proxy for angling vulnerability, catch-per-unit-effort (CPUE), differed between populations of largemouth bass (Micropterus salmoides) inhabiting long-standing (>70years active) intra-lake FPAs and adjacent, open access, main-lake areas. Fish from FPA populations had significantly higher aerobic scope (AS) capacity (13%) and CPUE rates compared with fish inhabiting the adjacent main-lake areas. These findings are consistent with theory and empirical evidence linking exploitation with reduced metabolic performance, supporting the hypothesis that recreational fishing may be altering the metabolic phenotype of wild fish populations. Reductions in AS are concerning because they suggest a reduced scope for carrying out essential life-history activities, which may result in fitness level implications. Furthermore, these results highlight the potential for unexploited FPA populations to serve as benchmarks to further investigate the evolutionary consequences of recreational fishing on wild fish and to preserve high-performance phenotypes.

Elevated estuary water temperature drives fish gut dysbiosis and increased loads of pathogenic vibrionaceae

Marine water temperatures are increasing globally, with eastern Australian estuaries warming faster than predicted. There is growing evidence that this rapid warming of coastal waters is increasing the abundance and virulence of pathogenic members of the Vibrionaceae, posing a significant health risk to both humans and aquatic organisms. Fish disease, notably outbreaks of emerging pathogens in response to environmental perturbations such as heatwaves, have been recognised in aquaculture settings. Considerably less is known about how rising sea surface temperatures will impact the microbiology of wild fish populations, particularly those within estuarine systems that are more vulnerable to warming. We used a combination of Vibrio-specific quantitative PCR and amplicon sequencing of the 16S rRNA and hsp60 genes to examine seawater and fish (Pelates sexlineatus) gut microbial communities across a quasi-natural experimental system, where thermal pollution from coal-fired power stations creates a temperature gradient of up to 6 °C, compatible with future predicted temperature increases. At the warmest site, fish hindgut microbial communities were in a state of dysbiosis characterised by shifts in beta diversity and a proliferation (71.5% relative abundance) of the potential fish pathogen Photobacterium damselae subsp. damselae. Comparable patterns were not identified in the surrounding seawater, indicating opportunistic proliferation within estuarine fish guts under thermal stress. A subsequent evaluation of predicted future warming-related risk due to pathogenic Vibrionaceae in temperate estuarine fish demonstrated that warming is likely to drive opportunistic pathogen increases in the upper latitudinal range of this estuarine fish, potentially impacting adaptations to future warming. These findings represent a breakthrough in our understanding of the dynamics of emerging pathogens in populations of wild aquatic organisms within environments likely to experience rapid warming under future climate change.