Effects of dietary iron supplementation on growth performance, iron utilization, and intestinal health in juvenile American eels (Anguilla rostrata)

Abstract River herring (alewife, Alosa pseudoharengus and blueback herring, A . aestivalis ) can act as vectors of nutrient transfer between marine and freshwater environments. Adult river herring can transfer marine-derived nutrients to freshwater ecosystems during spawning migrations through excretions and direct consumption of adults, eggs, and early phase juveniles by freshwater scavengers and predators. Adults and juveniles can transfer nutrients derived during freshwater residency back to the marine environment following emigration and the same excretion and consumption mechanisms. Potential nutrient transfers were assessed by applying carbon, nitrogen, and sulfur stable isotope analyses to a collection of juvenile and adult alewives, baseline food web components, and freshwater fish predators in three ponds in southeastern Massachusetts USA. Out-migrating adult alewife tissue stable isotope values were lower for both carbon and sulfur but higher for nitrogen than in-migrating conspecifics reflecting incorporation of freshwater-derived nutrients. Most juvenile alewife stable isotope values were at steady state with freshwater food webs at the time of sampling reflecting additional export of freshwater-derived nutrients with their eventual emigration to the marine environment. Stable isotope-based detection of marine-derived nutrients in freshwater predators was limited to catadromous elvers ( Anguilla rostrata ), anadromous white perch ( Morone americana ), and semi-anadromous yellow perch ( Perca flavescens ) with no marine-derived nutrients detected in resident freshwater fish predators. However, juvenile alewives were observed in stomachs of smallmouth ( Micropterus dolomieu ) and largemouth bass ( M . nigricans ). Results further inform current alewife ecosystem-level interactions with the marine and freshwater environments that they occupy during different life history phases for the many coastal watersheds in New England.

Micro-minerals are essential for fish, but traditional inorganic micro-minerals (IMM) have low bioavailability. This study evaluated coated inorganic micro-minerals (CIMM) in juvenile American eels under commercial recirculating aquaculture system (RAS) conditions. Three experimental groups (n = 3 tanks per group, stocking density: 138 fish/m3) were fed basal diets supplemented for 56 days with: 1000 mg/kg IMM (IMM group, providing Cu 7, Fe 200, Mn 30, Zn 70, I 1.6, Se 0.4, and Co 1.2 mg/kg diet), 1000 mg/kg CIMM (CIMM group I), or 500 mg/kg CIMM (CIMM group II). Compared to the IMM group, the CIMM group I demonstrated significantly enhanced growth performance, with the specific growth rate increasing by approximately 31.14%, higher whole-body content and retention of minerals (Ca, P, Cu, Fe, Mn, Zn), and superior intestinal health, as reflected by significantly increased activities of digestive enzymes (amylase and lipase), enhanced antioxidant capacity (elevated SOD and CAT, reduced MDA), and improved morphology (villi length and muscular thickness), an altered intestinal microbiota (increased relative abundance of Firmicutes and reduced relative abundance of Proteobacteria), and significant metabolomic alterations in purine metabolism and linoleic acid metabolism. The CIMM group II maintained growth performance, with no significant difference in WGR and SGR compared to the IMM group, while still showing significant improvements in feed intake and mineral retention (P, Cu, Fe, Zn), and antioxidant capacity. Collectively, this study not only confirms the efficacy of CIMM in commercial RAS but also reveals that the supplementation level previously shown to be effective in the laboratory (50% CIMM) is insufficient under commercial farming conditions, implying that the dietary micro-mineral requirements for juvenile American eels in commercial RAS may be higher than those established in laboratory settings.

Isolation and application of Bdellovibrio and like organisms (BALOs) for controlling Aeromonas hydrophila in the American eel (Anguilla rostrata) rearing water

Spawning events can provide resource pulses that can fuel food webs when other resources are limited, yet their utilization by fish communities is poorly understood. We combined traditional visual examinations and qPCR analysis of stomach content DNA (scDNA) to evaluate consumption of Striped Bass ( Morone saxatilis) eggs and larvae by 21 fish species collected before and during the Striped Bass spawning period in the Northwest Miramichi River estuary. Striped Bass was present throughout the study, with higher catches coinciding with peak eggs and larvae densities, which was a better predictor of scDNA compared to catches of Striped Bass. Visual inspection revealed eggs in only one American Eel ( Anguilla rostrata), whereas scDNA showed widespread increases in detections during spawning compared to the pre-spawning period, suggesting that Striped Bass reproduction generated a detectable community-level resource pulse. This study reveals hidden pathways of energy flow not detectable through traditional methods, emphasizing the value of molecular tools for resolving cryptic trophic interactions. These findings advance understanding of the ecological role of Striped Bass spawning for the fish community of the lower Northwest Miramichi River and provide novel insights into the detection and understanding of resource pulses associated with spawning events in aquatic ecosystems.

A dramatic decline in American eel Anguilla rostrata abundance led to the species being assessed as Endangered in Ontario (Canada) and the closure of fisheries. As part of efforts to recover populations, four million eels (glass eels and elvers) were batch marked and released over a five-year period (2006-2010) into the upper St. Lawrence River and Lake Ontario. Over 2019-2022, we repeated an earlier large-scale electrofishing survey of eastern Lake Ontario tributaries done to assess the distribution and survival of the released eels. A decade later, we collected two hundred and fifteen eels from 19 sites along four rivers near the release site. Eels were absent from three tributaries occupied in 2010-2011. Along the Trent River, navigational locks have likely permitted migration past seven large dams previously assessed as probably impassable. Growth of translocated eels was variable among rivers. As well, many eels were still smaller than yellow eels naturally migrating into Lake Ontario. Our study indicates high, long-term post-release survival in these rivers. However, timing of out-migration and potential for successful spawning migrations remain unknown.

Background: Aeromonas hydrophila is a common bacterial pathogen that causes hemorrhagic septicaemia in several farmed aquaculture species. Phage therapy is considered a promising and feasible alternative to antibiotic treatment. Methods: In this study, an A. hydrophila-infecting jumbo phage Z90 was isolated from an aquaculture pond. The biological characteristics, genomic features, and in vitro and in vivo experiments were investigated to evaluate its application potential. Results: Phage Z90 was a myovirus with distinctive curled tail fibers. Additionally, phylogenetic and genomic analyses found that the phage Z90 was a novel virus belonging to the genus Ferozepurvirus of the family Chimalliviridae. One-step growth curve analysis revealed that the phage Z90 was a lytic phage, exhibiting a short latency period of 20 min and a relatively large burst size of 270 ± 42 PFU/cell. The phage Z90 particles were stable at psychrotrophic and mesophilic temperatures (10–50 °C) and a wide range of pH (pH 3–12). Genomic analysis revealed that the phage Z90 did not contain any genes encoding toxins, virulence factors, or antibiotic resistance factors. In vivo analysis demonstrated that the phage Z90 protected American eels from A. hydrophila infection, greatly increasing eel survival rates and alleviating symptoms caused by bacterial infections. The comparison of different phage administration methods suggested that phage Z90 was better administered through intraperitoneal injection than immersion in aquaculture water. Moreover, the combination of phage Z90 and ampicillin improved the bactericidal effect and reduced the treatment dosage compared to antibiotics or phage alone. Conclusions: Altogether, the findings of this study indicate that the phage Z90 can serve as a promising biocontrol agent for the treatment of A. hydrophila infection in aquaculture.

The global decline of anguillid eels is well-documented across all continents where nineteen related species migrate. In North America, the population decline (and in some cases, extirpation) is related to numerous factors including industrial development. Eels experience violent mortality and migration barriers which have been linked to extractive infrastructure affiliated with settler colonial land occupation. The intricate migration pattern of Anguilla rostrata (American eels) is one of those species, an ecologically significant fish that has ancestral and persistent relevance to First Nations and tribal nations in Canada and the US, respectively. This paper draws from Anishinabe ontological grounding including intergenerational dodem gikendaasowin (clan or kinship knowledge) to suggest that humans are living in a world that includes an aquatic governance mediated by eels. A primary contribution is the suggestion that attention to such framing has applied relevance to intergenerational land-based healing, for extension of ongoing pursuits including Indigenous environmental justice, water governance strategies, and renewed interspecies relations. The application of these nascent concepts affects possibilities for current and future generations to exert reflective capacity and advocate for greater decision-making in matters of water governance. This paper suggests these opportunities be afforded to inheritors of ancestral Anishinabeg legacy dispersed throughout areas in Anishinabe-aki, where eels have resided and migrated and may do so again; to survive, eels benefit from informed policy and governance practices that facilitate physical assistance. New regimes may be built from human reflexivity and the desire to give back to life, an inherent principle of Anishinabe water governance and the application of Nibi Inaakonigewin (water laws).

Vibrio harveyi (Vh) and Vibrio vulnificus (Vv) are major bacterial pathogens affecting farmed eels, but their comparative virulence mechanisms remain poorly characterized. This study combined histopathology and transcriptomic profiling to investigate organ-specific damage and host responses in American eels (Anguilla rostrata, 20 g per fish, for a total of 60 fish) following experimental infection with LD50 doses of Vh (strain HA_1, 7.5 × 104 CFU/fish) and Vv (strain FJ_4, 5.0 × 105 CFU/fish). Tissue samples from liver, kidney, and spleen were collected at 0, 36, and 60 h post-infection (hpi). Histopathological analysis revealed distinct injury patterns: Vh induced severe hepatic edema and necrosis, whereas Vv caused vacuolar degeneration and vascular congestion in the liver. In the kidney, Vv triggered acute necrosis and vacuolization by 36 hpi, while Vh-induced renal damage was delayed until 60 hpi. Transcriptomic analysis of spleen tissue identified 4779 and 1215 differentially expressed genes (DEGs) in the Vh_36 vs. Vv_36 and Vh_60 vs. Vv_60 comparisons, respectively. Functional enrichment analysis associated these DEGs with 109 Gene Ontology (GO) terms—mainly catalytic activity, biological regulation, and binding—and 51 KEGG pathways, including “tuberculosis” and “pathways in cancer”. Differential alternative splicing (DAS) analysis further uncovered 1579 and 1214 DAS events originating from 12,482 and 12,316 splicing genes in the two comparisons. These were enriched in GO categories such as “binding”, “cellular process”, and “cell part”, as well as KEGG pathways related to “signal transduction”, “infectious diseases”, and “immune system.” Protein–protein interaction network analysis identified 119 cross-DAS-encoded proteins, including 8 that were predicted as key regulators of virulence differences. In summary, this work presents the first integrative study comparing the pathogenicity and host transcriptional dynamics of Vh and Vv in American eels, providing new molecular insights into their distinct virulence strategies.

Movement patterns and habitat use of yellow-phase American eels Anguilla rostrata in the Wolastoq/Saint John River were examined using acoustic telemetry. Tracking 72 tagged yellow-phase eels from late summer 2021 to autumn 2023 revealed overall restricted longitudinal movements (mean linear range ± standard deviation [SD] 6.4 ± 10.9 km) and high site fidelity. However, two eels exhibited small-scale (20-25 km) seasonal movements within freshwater habitats and one eel undertook large-scale (~83 km) seasonal movements between estuarine habitats in winter and freshwater habitats in summer. Tracking 24 yellow-phase eels equipped with depth sensors indicated that depth use correlated with diel period, season and fish length, with eels occupying shallower habitats at night, presumably for foraging, and deeper habitats during the day, likely for shelter. Sixteen tagged eels presumably matured to the silver phase and initiated out-migration in late summer. These eels moved rapidly through freshwater (mean ± SD 11.9 ± 8.8 km/day) but slowed considerably on reaching estuarine habitats (1.3 ± 1.3 km/day). These findings highlight the diversity of movement strategies in yellow-phase eels and provide new insights into the transition to the migratory silver phase.

ABSTRACT The upstream migration of juvenile American eels ( Anguilla rostrata ) is frequently obstructed by dams and other in‐stream barriers, leading to habitat loss and fragmentation, factors that are believed to contribute to the species' population decline. While fishways can help restore access to upstream habitats, their effective design and implementation require reliable estimates of the number of eels arriving at migration barriers. The Mactaquac Dam and Generating Station (672 MW) is the lowermost barrier on the Wolastoq|Saint John River in Canada and currently does not provide an eel‐specific fish passage. To inform the design and required capacity of a potential future eel passage at this site, this study provides mark‐recapture‐based estimates of the number of American eels arriving in the tailrace during three consecutive upstream migration seasons (2021–2023). During the peak migration period from late June to early August, a total of 30,144 juvenile eels were captured, tagged with visible implant elastomer tags, and released in the spillway bays of the dam over multiple sampling occasions spanning 30 to 42 days. Data were analyzed using POPAN Jolly‐Seber models and resulted in population estimates ranging from approximately 150,000 individuals in 2021 and 2022 to approximately 950,000 in 2023. Although confidence intervals were relatively wide, these results highlight the importance of an upstream passage system for eels at this dam, provide guidance for future research and management, and demonstrate the potential for long‐term monitoring at a future eel ladder to support local stock assessment.

Abstract American eel (Anguilla rostrata) life history strategies and food web interactions are highly variable in temperate climates but not well understood in sub-tropical hydrologic systems. Intraspecific variability in isotopic niche breadth and overlap can have major implications for species of conservation concern by revealing dietary plasticity of populations across varied habitats. Muscle tissue stable isotope ratios of carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) were analyzed to assess geographic variance in recent trophic histories within and among American eels collected in tributaries of the northwestern Gulf of Mexico and to quantify isotopic niche breadths and overlaps of capture groups. A total of eight specimen groupings were designated according to the riverine and coastal basins where they were collected, with one group comprised of only individuals caught within a wastewater treatment plant. Univariate analyses revealed highly variable isotopic ratios within and between groups. The probability of groups being found in the niche region of another group was low, with only six instances in which the probability of overlap was over 10%. The Colorado (C) group showed the highest probability of overlap of any group, being found in 86.57% of the Brazos-San Jacinto (B-SJ) group’s niche region. Collectively, these results illustrate the high degree of geographic variation in isotope ratios within and between regional habitats in an understudied portion of the species’ range.

The abundance of the American eel is declining, in part due to the fragmentation of riverine habitat by human-made barriers like hydroelectric dams. Despite the importance of successful passage at barriers during upstream migration, little is known about the movement behavior of immature eels at hydroelectric dams and the potential effects of artificial environmental conditions. Here, we investigate the movement behavior of migrating yellow eel below the Carillon dam, the first barrier during their upstream migration in the Ottawa River (Canada). Using acoustic telemetry, we quantify individual variability in movement behavior, timing of arrival at the dam, retention near the structure, return rates downstream, and whether eels find the only passage route to upstream habitats under fluctuating environmental conditions. Our results highlight intraspecific variability of exploratory behavior downstream of the Carillon dam and effects of atmospheric pressure, water flow, and temperature. On average, eels spent 17.7 days near the structure, but none successfully passed the dam, despite some individuals (16%) attempting passage multiple times. Thus, the Carillon dam likely represents an important barrier to immature eels during upstream migration that potentially confounds environmental migratory cues, induces migratory delay, and ultimately reduces access to headwater habitat.

Dietary chlorogenic acid supplementation to alleviate intestinal damage in juvenile American eel (Anguilla rostrata) under histamine stress

Fisheries resources depend on natural ecosystems, yet their sustainable management is often limited by uneven regional capacities and the pressures of international trade. High demand from certain regions can lead to overexploitation in others, highlighting the need to understand global consumption patterns of key aquatic species. This study introduces an integrated approach that combines DNA barcoding of freshwater eel (Anguilla spp.) products collected from end markets in 11 countries/regions with global production and trade statistics. We estimate that over 99% of eels consumed worldwide belong to three IUCN-listed threatened species: the American eel, Japanese eel, and European eel. Consumption was heavily concentrated in East Asia—particularly China, Japan, and South Korea—where supply volumes far exceed those of other regions. Our approach yields the most comprehensive quantitative global estimate to date of eel species composition in consumption, offering essential insights for the conservation and sustainable management of this highly exploited group.

ABSTRACT Catadromous American eel ( Anguilla rostrata ) are native to Maine's Penobscot River watershed and historically have migrated through many of its tributaries prior to extensive damming. Recent restoration efforts, including dam removals, have improved connectivity in the lower reaches of the Penobscot River. Characterizing the extent of the American eel's distribution is important to inform restoration and identify extant barriers to migrations within the watershed. In the summer of 2023, we conducted eDNA surveys throughout the Penobscot River watershed to estimate the current distribution of the American eel and identify barriers to inland waters. Water samples were collected from 70 sites representing 37 rivers and streams; the presence or absence of American eel genetic markers within those samples was assessed using qPCR. We have shown that American eel are present in virtually the full extent of the area surveyed (68/70 sites). The results suggest that the majority of the main‐stem dams may be passed by American eels at some level, with eel DNA being confirmed upstream of six dams. We confirmed the presence of American eels throughout the lower watershed with just 1 week of eDNA sampling and have highlighted this method for determining the species' access to habitat upstream of dams. The use of eDNA to sample locally (or regionally) for American eel may provide cost‐effective information in data deficient areas and help assess the permeability of dam structures to diadromous species.

Molecular characterization and expression analysis of interleukin-1 beta in Japanese eel (Anguilla japonica).

Distribution of American eels (Anguilla rostrata) and the influence of barriers in the Lake Champlain basin

ABSTRACT This study is the first to document glass American Eels Anguilla rostrata in Texas, and it provides critical information about recruitment timing along the northern Gulf Coast. The American Eel is a facultative catadromous fish. Data are lacking related to juvenile (glass eel and elver) recruitment into the bays and estuaries of the Gulf of Mexico. Objective The goal of the study was to document glass and elver recruitment timing and distribution for American Eels in Texas. Methods The study implemented year-round monitoring in rivers and streams using eel ramps and environmental DNA (eDNA) to increase the likelihood of detecting even highly sporadic recruitment events. Thirteen sites were identified based on their connectivity to coastal estuarine waters and the ability to support a gravity-fed eel ramp. Ramps were deployed during June–July 2022 and were checked weekly for 1 year. A subset of nine ramp sites was also monitored for eDNA during each ramp check. Results American Eels were caught in one of the 13 eel ramps. In total, 26 juvenile eels (19 glass and 7 elvers) were captured. Captures of glass eels began in January, and captures of elvers ended in May. American Eel DNA was detected at eight of the nine sites that were monitored for eDNA. Environmental DNA detection rates were highest for the only ramp site where eels were captured. Conclusions Positive eDNA samples were temporally associated with the recruitment window observed through captures in the ramps. Continued deployment of eel ramps during the estimated recruitment window are planned for 2024 and 2025, with more frequent checks. Continued monitoring will be conducted with the goal of assisting natural resource agencies in determining the conservation and management needs of American Eels in Texas.

ABSTRACT Objective Estuarine fishes experience significant diel and seasonal variations in their environments, with climate change introducing additional stressors, including altered salinity, temperatures, and water levels. American Eels Anguilla rostrata are present in Atlantic estuaries from Venezuela to Greenland. Despite their wide distribution and shrinking population, American Eels are understudied, in part because of the research challenges posed by their unusual catadromous life history. This study examines the spatial effects of changing estuarine water quality variables (water temperature, dissolved oxygen, and salinity) on the American Eel population in the Hudson River estuary (HRE). Methods The Hudson River Biological Monitoring Program, conducted from 1974 to 2017, consists of a suite of surveys recording fish abundance data and water quality variables. As the largest component of the Hudson River Biological Monitoring Program, the Long River Ichthyoplankton Survey contains 44 years of data on American Eels in the HRE. Using LRS catch data and Hudson River Biological Monitoring Program water quality measurements, we developed statistical models of American Eel population centers in the HRE. Results The young-of-year and yearling-or-older population centers shifted downstream over the course of the Long River Ichthyoplankton Survey at average rates of approximately 1.1 and 0.41 km per year, respectively, despite higher temperatures and lower dissolved oxygen conditions closer to the estuary’s mouth. Mean water temperature and dissolved oxygen for the entire estuary have significant relationships with the population centers of both age-classes, although the eels were not apparently tracking stable conductivity or water temperature conditions; nor were the young of year tracking stable dissolved oxygen levels. Conclusions The downstream shift in HRE American Eel population centers over several decades and the relationship between this shift and changing environmental conditions indicate the need for improved understanding of the population dynamics of the globally distributed and declining species of the genus Anguilla. This knowledge is critical in the face of rapidly changing ecosystems.