Freshwater Fish Populations Research Articles

Effects of River Flow on Walleye (Sander vitreus) Recruitment in the Saskatchewan River Delta

Alteration of natural flow regimes are affecting freshwater fish populations. For example, the walleye (Sander vitreus) fishery in the Saskatchewan River Delta has declined since the mid-1990s, which may be related to changes to flow regimes due to upstream dams. To test this hypothesis, walleye age data obtained from otoliths collected through sustenance and commercial fishing were used in a generalized linear mixed model catch-curve analysis to test the relationship between discharge during pre-defined biologically significant periods and walleye recruitment. The best fit model identified that the fry growth period (weeks 30–42) had a positive relationship between river discharge and future recruits. Based on the estimated Bayesian posterior distribution, there was a very high probability (p > 0.99) that the effect was different from zero. This effect had an estimated 69% increase (28–105% credible interval) in recruitment with every 100 m3·s-1 increase in discharge over the fry growth period. These findings support previous work on walleye recruitment in another northern freshwater delta and will inform water resource management in these systems.

Morphology and Phylogenetic Analysis of Henneguya sp. Infecting the Orange-Spotted Snakehead (Channa Aurantimaculata) from Tamil Nadu, India.

Henneguya sp. is a crucial myxosporean parasite known to cause milky flesh or tapioca disease in the freshwater fish population, leading to heavy mortality. Studies to investigate its host range and to monitor their prevalence in wild and aquacultured fish are necessary. The infected orange-spotted snakehead (Channa aurantimaculata) fish samples showing clinical signs such as severe ulceration and open wounds on the mouth and operculum were collected from an ornamental fish-rearing unit in Chennai, Tamil Nadu. The sample was diagnosed with myxosporean infection by microscopic observation, morphological records and DNA sequencing followed by phylogenetic analysis. The Henneguya infection was identified in C. aurantimaculata for the first time. Necropsy of the infected fish revealed characteristic multifocal whitish-yellow, oval-shaped cysts measuring 3-5mm in length on the liver and spleen. The wet mount of cysts showed numerous mature Henneguya spores that were uninucleate, elongated, lenticular and ellipsoidal with the bluntly rounded anterior end gradually tapering off to the posterior with a caudal elongation. The spore body measured 12.1 ± 0.9μm (10.1-14.7μm) × 7.1 ± 0.6μm (5.3-8.5μm) with two slightly unequal pyriform polar capsules of 6.1 ± 0.5μm (5.1-7.4 )× 2.5 ± 0.4μm (1.6-3.9) (large); 6.0 ± 0.4μm (5.2-6.4) × 2.5 ± 0.3μm (1.7-3.1) (small) and a caudal elongation of 16.1 ± 1.2μm in length. The parasite was confirmed to be a Henneguya sp. by PCR amplification of SSrRNA followed by gene sequencing. The sequence generated was submitted in the GenBank under accession number PP852214.1 The maximum likelihood analysis revealed that the obtained sequence is a close relative of H. exilis with pairwise sequence variation of 3.3% and a homology of 99.04%. The findings highlight the potential impact of Henneguya sp. on ornamental fish health, underscoring the need for vigilant monitoring and management in India.

Identification of Potentially Novel Mycobacterium Spp. In Freshwater Ornamental Fish in Trinidad and Tobago.

Potentially zoonotic Mycobacterium spp. are impacting freshwater ornamental fish in Trinidad and Tobago. Clinical cases presented at the Aquatic Animal Health Unit of The University of the West Indies, School of Veterinary Medicine, from September 2011 to September 2018 indicated the presence of piscine mycobacteriosis in freshwater ornamental fish from locations throughout Trinidad and Tobago. Subsequently, an investigation was conducted from June 2018 to December 2021 to identify the specific Mycobacterium spp. involved. Nested polymerase chain reaction (PCR) was used to amplify and later sequence the 723 bp of the rpoB. Analysis of region V of the rpoB offers similar discriminatory power as concatenation or whole genome analysis of Mycobacterium spp., and serves as an acceptable method for differentiating and discriminating between species in this genus. With respect to tank-reared freshwater ornamental fish, this study identified M. fortuitum (98.8% identity), M. liflandii (100% identity), M. stomatepiae (97.2% identity), M. pseudoshottsii related (96.9% identity), two M. stomatepiae related (95.4% and 96.7% identity), M. immunogenum related (93.4% identity), and 17 M. insubricum related (91.4%-95.2% identity). Additionally, seven mycobacteria related to M. insubricum were identified in wild-caught guppies. These findings suggest that both known, and potentially novel Mycobacterium spp. are circulating, and adversely impacting the local populations of freshwater ornamental fish in Trinidad and Tobago.

Responses of fish to nationwide improvements in the water quality of a densely populated and heavily modified country over four decades.

Globally, fish have been severely affected by the widespread, chronic degradation of fresh waters, with a substantial proportion of species declining in abundance or range in recent decades. This has especially been the case in densely populated countries with an industrial heritage and intensive agriculture, where the majority of river catchments have been affected by deteriorations in water quality and changes in land use. This study used a spatially and temporally extensive dataset, encompassing 16,124 surveys at 1180 sites representing a wide range of river typologies and pressures, to examine changes in the fish populations of England's rivers over four decades (1980s-2010s). The analyses revealed gradual, nationwide increases in mean fish species richness and diversity across the range of pressure gradients. In the majority of cases, increases were most pronounced in the 1980s, since when any further changes have been comparatively minor, but there were no declining trends across the full time series. There were also temporal, nationwide changes in fish assemblage structure, driven largely by variations in the densities of brown trout Salmo trutta or roach Rutilus rutilus, but no consistent increases in the abundance of sensitive, pollution-intolerant species in response to improvements in wastewater treatment and, consequently, water quality. Although the increases in fish species richness and diversity over the last four decades are encouraging, subtle and contrasting changes in the abundance of a range of species require further investigation, and causal relationships between fish assemblage structure and putative drivers should be modelled at a national scale. This study is the first to examine long-term, nationwide trends in the freshwater fish populations of England, and significantly advances our understanding of the ecological health of rivers in densely populated and heavily modified countries.

Fish reproductive phenology shifts with increasing temperature and year.

Temperate fishes often spawn in response to environmental cues, such as temperature, thereby facilitating larval emergence concurrent with suitable biotic and abiotic conditions, such as plankton blooms. Climatic changes may alter the reproductive phenology of spring- and autumn-spawning freshwater fish populations. Such effects may depend on the sensitivity of reproductive phenology to ambient temperatures. We applied a meta-analysis approach to test whether annual temperature and year affected fish reproductive phenology. Based on preliminary tests in walleye (Sander vitreus) and Lake Constance whitefish (Coregonus arenicolus), we hypothesized that increasing temperature would promote earlier spring-spawning and later autumn-spawning. We found spawning was significantly earlier in the spring and later in the autumn. We found that migration of autumn-spawning species occurred earlier with warmer temperatures, implying that with increasing temperatures, migrating autumn-spawning species will increase residence time in tributaries. We also found that spring-spawning fishes reproduced earlier in more recent years, while we observed no significant effect in autumn-spawners. Spring- and autumn-spawning fishes displayed interannual variation in spawning dates (mean range of 34.4 and 27.0 days over 33.9 years, respectively), with spring-spawning fishes displaying a significantly broader range in spawning dates.

A fish-friendly axial flow pump turns out to be eel safe, roach unfriendly and bream unsafe

Additional and refurbished pumping stations are required to mitigate the intensifying occurrence of droughts and floodings. These installations negatively impact threatened freshwater fish populations due to the increased risk of injury and mortality when fish pass through them. Fish-friendly pumping installations have been proposed as a potential solution to reduce these risks. However, published assessments of these new types of pumps remains lacking, and the few available studies do not enable a cross-comparison with conventional pump types. The promising, yet understudied, Fairbank Nijhuis ‘fish-friendly’ axial flow pump has been assessed in previous works, however the results remain ambiguous due to low recapture rates, unconsidered parameters, fixed operating conditions, and the inability to identify the likely sources of injury and mortality. In this study, we address the limitations of previous works by implementing a standardized protocol for live fish in conjunction with passive barotrauma detection sensors. The major finding of this work is that safe passage of eel (100% survival) is confirmed, but that bream and roach had a much lower survival probability (24% and 70% survival respectively) than expected, albeit higher than for a conventional axial flow pump (roach survival: 13%). Furthermore, roach and bream passing at higher rpm suffered significantly higher mortalities. The impact of the impeller was found to be the most common source of severe injury for both pumps. These results are significant because they conclusively show that fish-friendly pumps may be considered safe for eel, but not for other endemic European fish species such as roach and bream.

Fishy Business—Assessing the Efficacy of Active and Passive eDNA to Describe the Fish Assemblage of a River in Southwestern Western Australia to Support Effective Monitoring

ABSTRACTWorldwide, freshwater vertebrate populations are declining with increasing pressure on rivers due to numerous environmental and climatic threats. Environmental DNA (eDNA) could potentially provide a more efficient and non‐invasive mechanism to monitor freshwater systems, either as a complement or in replacement to traditional methods to accurately assess species' distributions. Here, we utilize a hierarchical multispecies N‐mixture model to compare three fish sampling methods: traditional fyke netting and active and passive environmental DNA sampling along a 30 km stretch of the Canning River in Western Australia. We used the fitted model to compare capture probabilities among sampling methods and reveal the sampling effort required to describe the species assemblage. Results indicated that while all methods could detect fish, combined eDNA methodologies detected one more fish species than those caught by fyke netting. In addition, active eDNA sampling produced the highest capture probabilities and more consistently described the entire fish assemblage at any given site. Fyke netting and passive eDNA did not show significant differences in their average capture probabilities, and both methods had lower abilities to capture individual species than active eDNA. Active eDNA also required fewer replicate samples to detect the expected observed richness, and fyke netting required the most replicates. Additionally, a hierarchical multispecies abundance model showed that active environmental DNA (eDNA) sampling is the most effective method for monitoring freshwater fish populations. This study contributes to our understanding of eDNA in aquatic systems and demonstrates that, at least under current conditions, active sampling is still the preferred method in freshwater systems with low flow compared to both passive sampling and fyke netting.

Length in assessing status of freshwater fish populations: A review

AbstractObjectiveEffective policy formulation regarding the conservation of freshwater fish necessitates an understanding of water‐specific prevailing conditions and trends. Assessing fish populations in inland waters is difficult and expensive because there are many independent systems that need to be evaluated. Therefore, numerous freshwater systems are beset by insufficient data and the lack of systematic assessments of their status. To alleviate this deficiency, the objective of this study was to review length‐based metrics that may have utility in evaluating the well‐being of freshwater fish populations.MethodsLength measurements can serve as proxies for a range of ecological and population dynamics attributes that are essential for the effective management of fish and associated fisheries. A review of the historical development of length measurements in fish conservation is provided, along with an examination of the potential biases that may arise from the use of lengths in practical contexts. In addition, we examine techniques that enable the spatial and temporal visualization of length data sets, as well as a range of indices and metrics that can be computed using length measurements.ResultBuilding populations assessments around length may be a cost‐effective strategy that allows a first cut at managing a large number of waters. Length‐based assessments can signal if management intervention is necessary, if management policies are yielding the intended outcome, or if surveys beyond mere length are necessary.ConclusionOur review indicates that length offers a straightforward and efficient approach to evaluate the status of fish populations in inland systems. We encourage pursuing additional study and to this end propose specific areas for investigation.

Immediate Genetic Augmentation and Enhanced Habitat Connectivity Are Required to Secure the Future of an Iconic Endangered Freshwater Fish Population.

Genetic diversity is rapidly lost from small, isolated populations by genetic drift. Measuring the level of genetic drift using effective population size (N e) is highly useful for management. Single-cohort genetic N e estimators approximate the number of breeders in one season (N b): a value < 100 signals likely inbreeding depression. Per-generation N e < 1000 estimated from multiple cohort signals reduced adaptive potential. Natural populations rarely meet assumptions of N e-estimation, so interpreting estimates is challenging. Macquarie perch is an endangered Australian freshwater fish threatened by severely reduced range, habitat loss, and fragmentation. To counteract low N e, augmented gene flow is being implemented in several populations. In the Murrumbidgee River, unknown effects of water management on among-site connectivity impede the design of effective interventions. Using DArT SNPs for 328 Murrumbidgee individuals sampled across several sites and years with different flow conditions, we assessed population structure, site isolation, heterozygosity, inbreeding, and N e. We tested for inbreeding depression, assessed genetic diversity and dispersal, and evaluated whether individuals translocated from Cataract Reservoir to the Murrumbidgee River bred, and interbred with local fish. We found strong genetic structure, indicating complete or partial isolation of river fragments. This structure violates assumptions of N e estimation, resulting in strongly downwardly biased N b estimates unless assessed per-site, highlighting the necessity to account for population structure while estimating N e. Inbreeding depression was not detected, but with low N b at each site, inbreeding and inbreeding depression are likely. These results flagged the necessity to address within-river population connectivity through flow management and genetic mixing through translocations among sites and from other populations. Three detected genetically diverse offspring of a translocated Cataract fish and a local parent indicated that genetic mixing is in progress. Including admixed individuals in estimates yielded lower N e but higher heterozygosity, suggesting heterozygosity is a preferable indicator of genetic augmentation.

Individual variability in the movement ecology of Northern pike Esox lucius in a highly connected wetland system

Maintaining hydrological connectivity is important for sustaining freshwater fish populations as the high habitat connectivity supports large-scale fish movements, enabling individuals to express their natural behaviours and spatial ecology. Northern pike Esox lucius is a freshwater apex predator that requires access to a wide range of functional habitats across its lifecycle, including spatially discrete foraging and spawning areas. Here, pike movement ecology was assessed using acoustic telemetry and stable isotope analysis in the River Bure wetland system, eastern England, comprising of the Bure mainstem, the River Ant and Thurne tributaries, plus laterally connected lentic habitats, and a system of dykes and ditches. Of 44 tagged pike, 30 were tracked for over 100 days, with the majority of detections being in the laterally connected lentic habitats and dykes and ditches, but with similar numbers of pike detected across all macrohabitats. The movement metrics of these pike indicated high individual variability, with total ranges to over 26 km, total movements to over 1182 km and mean daily movements to over 2.9 km. Pike in the Thurne tributary were more vagile than those in the Ant and Bure, and with larger Thurne pike also having relatively high proportions of large-bodied and highly vagile common bream Abramis brama in their diet, suggesting the pike movements were potentially related to bream movements. These results indicate the high individual variability in pike movements, which was facilitated here by their access to a wide range of connected macrohabitats due to high hydrological connectivity.

Multi-method survey rediscovers critically endangered species and strengthens Madagascar's freshwater fish conservation

Freshwater ecosystems are crucial for global biodiversity through supporting plant and animal species and providing essential resources. These ecosystems are under significant threat, particularly in island environments such as Madagascar. Our study focuses on the Amboaboa River basin, home to the rare and endemic fish species Rheocles derhami, last recorded in 2013. To assess the status of this and other threatened fish species including Ptychochromis insolitus and Paretroplus gymnopreopercularis, and to understand freshwater fish population dynamics in this biodiversity hotspot, we conducted a comprehensive survey using both environmental DNA (eDNA) and traditional fishing methods. While traditional methods effectively captured a diverse range of species, including several invasive aliens and the critically endangered endemic species that were the focus of this study, the eDNA approach detected only a fraction of these introduced species and struggled to identify some critically endangered endemics at the species level. This highlights the value of combining methods to enhance species detection. We also investigated the trade-offs associated with multi-primer assessments in eDNA analysis, focusing on three different primer combinations targeting the 12S mitochondrial gene: MiFish, Tele02, and Riaz. Additionally, we provided 12S reference barcodes for 10 species across 9 genera of fishes from the region to increase the coverage of the public reference databases. Overall, our study elucidates the current state of freshwater biodiversity in the Amboaboa River basin and underscores the value of employing multiple methods for effective conservation strategies.

Planning and implementing genetic rescue of an endangered freshwater fish population in a regulated river, where low flow reduces breeding opportunities and may trigger inbreeding depression.

Augmenting depleted genetic diversity can improve the fitness and evolutionary potential of wildlife populations, but developing effective management approaches requires genetically monitored test cases. One such case is the small, isolated and inbred Cotter River population of an endangered Australian freshwater fish, the Macquarie perch Macquaria australasica, which over 3 years (2017-2019) received 71 translocated migrants from a closely related, genetically more diverse population. We used genetic monitoring to test whether immigrants bred, interbred with local fish and augmented population genetic diversity. We also investigated whether levels of river flow affected recruitment, inbreeding depression and juvenile dispersal. Fish length was used to estimate the age, birth year cohort and growth of 524 individuals born between 2016 and 2020 under variable flow conditions. DArT genome-wide genotypes were used to assess individual ancestry, heterozygosity, short-term effective population size and identify parent-offspring and full-sibling families. Of 442 individuals born after translocations commenced, only two (0.45%) were of mixed ancestry; these were half-sibs with one translocated parent in common. Numbers of breeders and genetic diversity for five birth year cohorts of the Cotter River fish were low, especially in low-flow years. Additionally, individuals born in the year of lowest flow evidently suffered from inbreeding depression for juvenile growth. The year of highest flow was associated with the largest number of breeders, lowest inbreeding in the offspring and greatest juvenile dispersal distances. Genetic diversity decreased in the upstream direction, flagging restricted access of breeders to the most upstream breeding sites, exacerbated by low river flow. Our results suggest that the effectiveness of translocations could be increased by focussing on upstream sites and moving more individuals per year; using riverine sources should be considered. Our results indicate that river flow sufficient to facilitate fish movement through the system would increase the number of breeders, promote individuals' growth, reduce inbreeding depression and promote genetic rescue.

Fish Species in Area Conservation (Lubuk Larangan) on the Bangko River, Solok Selatan District, West Sumatra Province, Indonesia

The conservation Area (lubuk larangan) is one of the fish resource conservation areas. However, recently, the types of fish that live in this area have become increasingly threatened due to environmental changes and non-selective fishing. Therefore, it is important to study the diversity of freshwater fish in the conservation area (Lubuk Larangan) located along the Bangko River, Luak Kapau Village, South Solok Regency, West Sumatra Province, Indonesia. This data will be valuable in determining which fish species should be prioritized for domestication as potential candidates for fish cultivation. The process of collecting fish is carried out between June and July 2020 by involving the assistance of local fishermen who use various types of fishing gear, including gill nets, spread nets, trap nets, and fishing rods. The findings from this study revealed the existence of 13 fish species belonging to three orders, six families, and thirteen genera. Among the species collected, the family Cyprinidae was the most dominant, accounting for 61.53% of the total fish population collected, followed by the families Cobitidae, Bangridae, Cichlidae, Mastacembelidae, and Gobiidae, each consisting of 7.69%. of the total. The species Acanopsis octoactinotus was identified as vulnerable, while Sicyopterus aeinsis was classified as near threatened. The level of functioning of the conservation area for fish preservation is 84.35%, which is included in the very well functioning category. Priority should be given to breeding programs and restoration initiatives targeting future populations of endangered freshwater fish.

Characterization of pure and admixed brown trout (Salmo trutta) populations of high conservation value in the upper Danubian contact zone using ddRADseq genotyping

Increasing rates of hybridization and introgression in managed populations of freshwater fish are a major threat to the long-term viability of native species. The conservation challenge begins with identifying native gene pools. For brown trout (Salmo trutta Linnaeus, 1758) in the Upper Danube drainage, this task is complicated by the presence of both naturally and anthropogenically induced admixture of highly divergent lineages (Atlantic and Danubian). Herein, a ddRADseq protocol was used to type 377 individuals from 24 populations in the Upper Danube in Austria and Germany, and from reference populations from adjacent drainages and commercial hatcheries. High genetic differentiation at small geographic scales was found among pure Danubian-lineage populations, especially in the Kalkalpen National Park (Austria). In the Upper Danube drainage of Germany, as well as in the Rhine and Elbe drainages, brown trout populations were predominantly of Atlantic-lineage origin – as were those of all commercial hatcheries. Most populations, however, showed various degrees of admixture between Danubian and Atlantic lineages, hypothesized to be the result of both natural and anthropogenic processes. We highlight the conservation value of pure Danubian-lineage populations, and the challenges promoting conservation of naturally admixed populations, while discouraging continued stocking and admixture via management activities.

Threats of dams to the persistence of the world's freshwater fishes

AbstractHabitat fragmentation due to dams is a well‐known threat to freshwater fish. Yet, the global consequences of fragmentation for the viability of freshwater fish populations are unknown. Here, we provide the first global assessment of the threats of dams to the persistence of freshwater fish species. We developed a global macroecological relationship between freshwater fish range size and body size and used this relationship to assess whether isolated range fragments are too small to support a species. Our assessment includes 7369 freshwater fish species and considers the effects of 31,780 dams globally. Furthermore, we performed a more detailed analysis of the threats of dams in Brazil, the greater Mekong region and the United States, using complementary national and regional data sets. Globally, more than half of the species analysed face extirpation in a part of their geographic range, with an average potential range loss of 3.3% (95%‐range: 0%–31.8%) across all species analysed. For 74 fish species, occurring in Brazil, China, India, the Mekong basin, the United States, South‐Africa and the East Adriatic Coast, more than 50% of their range is potentially lost. This includes 18 species threatened with extinction across their whole range. Our complementary regional analysis revealed that the potentially lost range increases by a factor of 2–4 when considering both large and small dams compared with considering only large dams (≥15 m), highlighting the need to establish more comprehensive global dam inventories. Our novel approach and global analysis identifies species at risk of extirpation as well as geographic hotspots of extirpation threat by dams, which can aid in establishing more effective strategies for global hydropower development and barrier removal efforts to optimise the trade‐offs between biodiversity conservation and the socio‐economic benefits of dams.

Constraints of digestion on swimming performance and stress tolerance vary with habitat in freshwater fish species.

Limited aerobic scope (AS) during digestion might be the main constraint on the performance of bodily functions in water-breathing animals. Thus, investigating the postprandial changes in various physiological functions and determining the existence of a shared common pattern because of possible dependence on residual AS during digestion in freshwater fish species are very important in conservation physiology. All species from slow-flow habitats showed impaired swimming speed while digesting, whereas all species from fast-flow habitats showed strong swimming performance, which was unchanged while digesting. Only two species from slow-flow habitats showed impaired heat tolerance during digestion, suggesting that whether oxygen limitation is involved in the heat tolerance process is species-specific. Three species from slow- or intermediate-flow habitats showed impaired hypoxia tolerance during digestion because feeding metabolism cannot cease completely under hypoxia. Overall, there was no common pattern in postprandial changes in different physiological functions because: (1) the digestion process was suppressed under oxygen-limiting conditions, (2) the residual AS decreased during digestion, and (3) performance was related to residual AS, while digestion was context-dependent and species-specific. However, digestion generally showed a stronger effect on bodily functions in species from slow-flow habitats, whereas it showed no impairment in fishes from fast-flow habitats. Nevertheless, the postprandial change in physiological functions varies with habitat, possibly due to divergent selective pressure on such functions. More importantly, the present study suggests that a precise prediction of how freshwater fish populations will respond to global climate change needs to incorporate data from postprandial fishes.

Divergence of the freshwater sleeper, Neodontobutishainanensis (Chen, 1985) (Teleostei, Odontobutidae), in the Pearl River basin and on Hainan Island of southern China.

Study of divergence of freshwater fish populations between island and adjacent mainland areas can shed light on the phylogeographical relationships of these regions. Neodontobutishainanensis is a freshwater fish species restricted to Hainan Island and in Guangdong and Guangxi provinces in the southern mainland China. We examine the phylogenetic relationship and population structure of N.hainanensis based on 3,176 nuclear loci using a gene-capture method. STRUCTURE analysis and principal coordinate analyses (PCA) indicate that populations from Guangdong, Guangxi and Hainan are each distinct, except that some individuals of the Guangdong population share minor genetic components with individuals of the Guangxi population. In the concatenated gene tree, the Hainan population is grouped with the Guangdong population, but the coalescent tree groups the Hainan population as the sister to the Guangxi population. Finally, coalescent simulations confirmed the divergence pattern supported by the coalescent tree and revealed a one-way introgression from the Guangxi population to the Guangdong population, which can explain the discordant results supported by the concatenated and coalescent phylogenetic analyses. Due to recent decline of N.hainanensis populations and the genetic patterns in this species, as revealed in this study, the populations in the three areas should be treated as separate conservation units.

The Presence of a Parasite in the Head Tissues of a Threatened Fish (Bidyanus bidyanus, Terapontidae) from South-Eastern Australia.

The silver perch, Bidyanus bidyanus (Mitchell) (Terapontidae) is a freshwater fish, endemic to the Murray-Darling river system in south-eastern Australia. Population declines have led to the fish being listed as critically endangered by the Australian Government. Knowledge about parasites and diseases of wild populations of freshwater fish are limited in Australia. During an examination of wild-caught silver perch, digenean mesocercaria were observed in the head tissues. A total of five of the 11 silver perch collected from the Wakool River, New South Wales, were infected with mesocercaria. All mesocercaria were found in the head tissues; no mesocercaria were found encysted in the eye lens. The mesocercaria were found to belong to the family Strigeidae based on the sequences of their internal transcribed spacer (ITS) region. The lack of comparable sequences of strigeid digeneans from Australian hosts precludes being able to determine if the mesocercaria found in this study are a new species or representatives of an already described species. However, genetic results confirm that this is a different species to other digeneans previously described from silver perch, thus increasing the number of digeneans reported from B. bidyanus to three species. The presence of digenean mesocercaria in the head tissues of a wild population of silver perch, as found in the present study, is of potential conservation significance. Given the critically endangered conservation status of B. bidyanus, and previous evidence of strigeid infection altering fish behaviour, ecology, and predation mortality, further research on the potential impacts of infection on wild populations is warranted.

Density-dependent population regulation in freshwater fishes and small mammals: A literature review and insights for Ecological Risk Assessment.

The regulation of populations through density dependence (DD) has long been a central tenet of studies of ecological systems. As an important factor in regulating populations, DD is also crucial for understanding risks to populations from stressors, including its incorporation into population models applied for this purpose. However, study of density-dependent regulation is challenging because it can occur through various mechanisms, and their identification in the field, as well as the quantification of the consequences on individuals and populations, can be difficult. We conducted a targeted literature review specifically focusing on empirical laboratory or field studies addressing negative DD in freshwater fish and small rodent populations, two vertebrate groups considered in pesticide Ecological Risk Assessment (ERA). We found that the most commonly recognized causes of negative DD were food (63% of 19 reviewed fish studies, 40% of 25 mammal studies) or space limitations (32% of mammal studies). In addition, trophic interactions were reported as causes of population regulation, with predation shaping mostly small mammal populations (36% of the mammal studies) and cannibalism impacting freshwater fish (26%). In the case of freshwater fish, 63% of the studies were experimental (i.e., with a length of weeks or months). They generally focused on the individual-level causes and effects of DD, and had a short duration. Moreover, DD affected mostly juvenile growth and survival of fish (68%). On the other hand, studies on small mammals were mainly based on time series analyzing field population properties over longer timespans (68%). Density dependence primarily affected survival in subadult and adult mammal stages and, to a lesser extent, reproduction (60% vs. 36%). Furthermore, delayed DD was often observed (56%). We conclude by making suggestions on future research paths, providing recommendations for including DD in population models developed for ERA, and making the best use of the available data. Integr Environ Assess Manag 2024;20:1225-1236. © 2023 Syngenta Crop Protection. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Ichthyofauna in the Lomami National Park and Its Hinterlands, Democratic Republic of the Congo

This investigation deals with freshwater fish populations in thirteen sampling rivers of Lomami National Park (LNP) and its hinterlands. The study aimed to inventory the ichthyofauna in freshwater bodies of LNP and its hinterlands; determine fish species with high economic value according to preference by riparian communities; and assess the conservation status of fish species. Intensive fish sampling was conducted thirteen rivers of LNP and its hinterlands using gill nets with stretched mesh sizes of 9.5, 10, 12, 15 and 20 mm in 2016. A survey questionnaire was administered to fishermen in order to determine fish species with high economic value. A total of 2985 fish were caught in thirteen rivers belonging to 129 species, 54 genera, 20 families and 8 orders. The family of Mormyridae dominated in the fish collection with 30 species followed by family of Alestidae with 23 species. Clarias spp., Schilbe spp., Chrysichthys spp., Auchenoglanis occidentalis, Polypterus spp., Hydrocynus spp., Alestes spp., Parachana spp. and species of Mormyridae were recorded as fish species with high economic value. The majority of species was least concerned, but one was reported as endangered species (Nannothrissa sterwarti) and another vulnerable (Labeo rectipinnis). The analysis displayed that all rivers were diverse with a minimum observed in the Onema River. The probability of catching two individuals of different species in the same river was high in all rivers, but it was higher in the Amboko, Lodja, Lomami and Yalo Rivers (96%); and it was slightly low in the Onema River (73%). The Kruskal–Wallis result showed statistical differences among sampling rivers (P = 0.0000).&#x0D; Our findings provided valuable information for the conservation and management of fish species in the freshwater of Lomami National Park and its hinterlands.