Occurrence Of Eels Research Articles

Asian swamp eels (Synbranchidae, Monopterus) in Florida: distribution, spread, and range of hydrologic tolerance over twenty-seven years (1997–2023)

Asian swamp eels (Monopterus albus/javanensis) were first reported as introduced to Florida waterbodies in 1997 near Tampa and Miami; a third population was recorded by 1999 in Homestead. Initial assessments, published soon after swamp eels in southern Florida were first recorded in wetlands beyond canals and ponds (in 2007), concluded there was little threat to Florida’s aquatic ecosystems. Long-term data now suggest they precipitated population crashes of crayfishes and small fishes in the eastern Everglades. We used records from continuous long-term monitoring programs, sporadic monitoring studies, and online databases to reconstruct swamp eel presence across Florida. Monitoring studies provided wetland hydrologic variables to assess limits for swamp eels. From 1997–2007, populations in southern Florida remained restricted to canals; initial spread from 2007–2017 across southern Everglades National Park proceeded slowly and the two populations covered ~1500 km2 of southern Florida. From 2017–2022, the rate of spread increased as they spread west and north (~5800 km2 range). Through 2014, the Tampa population occurred only along southern/eastern Tampa Bay (~60 km2) but has since spread south along the Gulf Coast, east into central Florida, and south along the Lake Wales Ridge (~11,000 km2). We found evidence of two potentially new introductions, in Palm Beach County and Orlando. There was no clear evidence of limitation of wetland drying on swamp eel occurrence in the Everglades; they were captured in marshes that dried for 1–5 months during the previous dry season, but short-hydroperiod wetlands may have slowed spread. In the Everglades, evidence suggests swamp eels may have been inadvertently spread into marshes from canals used to deliver water for flood control and hydrologic restoration. Swamp eels are currently spreading unchecked across Florida, and there should be great concern about continued spread in this region and their establishment and spread elsewhere.

When the Eel Meets Dams: Larger Dams’ Long-Term Impacts on Anguilla anguilla (L., 1758)

Diadromous fish, like the European eel Anguilla anguilla (L., 1758), are highly threatened by dams that disrupt river connectivity, consequently impeding fish movements to reach feeding and spawning habitats. In this study, variation in eel occurrence between a historical period (1940–1970) and recent data (2016–2020) was assessed throughout the Sardinian rivers’ network (more than 450 sites). Using Boosted Regression Trees (BRT) we investigated relationships between eel’s occurrence and a set of spatial and temporal environmental variables including a set of dams’ construction features for each period. An overall decrease by 65% of eel’s occurrence was noticed during the ca. 80-years period under scrutiny. Considering a subset (105 and 88 sites for the historical and the recent period, respectively) characterized by the presence of larger dams (height >15 m), eel’s occurrence dropped by 85%. Conversely, eel’s occurrence dropped only by ca. 44% in dam-free sites. During the historical period, eel’s occurrence was mostly affected by time since the initial habitat fragmentation, flow, distance to dams, connectivity, and dams’ height. In the most recent period, eel’s occurrence is mostly affected by dams’ building year, dam-to-sea distance, and, again, dams’ height. Results pinpoint that dams’ construction features and the time from their construction have significant negative effects on eel’s occurrence. Addition of future effective eel restoration practices, apart any other adverse environmental stressor, must consider dams’ removal, wherever socially sustainable or alternatively, the modification of construction features of dams (like excessive height) and the addition of fish ladders.

Environmental DNA analysis indicates that migration barriers are decreasing the occurrence of European eel (Anguillaanguilla) in distance from the sea

The European eel (Anguilla L.) is considered critically endangered by the IUCN Red List, and recruitment remains low. One of the challenges for the species today is migration barriers that limit their habitat. Along the Norwegian coast, like in other countries, the abundance of eels appears to decrease with distance from the sea. This pattern may be a result of factors like water temperature, water quality, competition, and habitat suitability and availability. This study aims to use environmental DNA (eDNA) analysis to investigate the potential relationship between migration barriers and the decreasing occurrence of eels in distance from the sea by the coast of southern Norway. Sixty locations with potential migration barriers are investigated by collecting water samples upstream and downstream from each construction before eDNA from each sample is isolated and analyzed by real-time PCR with specific primers and probes matching A. anguilla. The results reveal that the probability of detecting eel eDNA decreases significantly with number of hydroelectric power stations and their associated basins, even when the effect of distance to sea is accounted for. In addition, there is a clear border at which eel eDNA could no longer be detected upstream of the major watercourses. Therefore, it is likely that the migration of eels is prohibited by these constructions, which seem to constitute a much greater challenge than every other type of potential migration barrier investigated in this study.

Anthropogenic impacts on the distribution of wild and cultured Japanese eels in the Tone River watershed, Japan, from otolith oxygen and carbon stable isotopic composition

Effective management decisions are vital for the declining Japanese eel Anguilla japonica. Elucidating the distribution range of wild eels (i.e., naturally recruited individuals) represents the first step in resource management, although stocking of cultured eels throughout the waters of Japan masks the original distribution range of wild eels. Herein, we applied a recently developed discrimination model based on otolith oxygen and carbon stable isotopic ratios (δ18O and δ13C) to determine the distribution range of wild eels throughout the Tone River watershed, which is the largest fisheries ground for this species. The model indicated that eels naturally recruit up to 200 km in the catchment area from the river mouth. Although natural recruitment was confirmed at all sampling sites, the percentages of wild eel occurrence at each site ranged from 56 to 100% of sampled individuals. The presence of wild individuals at sites above river-crossing structures indicates that some individuals successfully navigate through these structures. The operation of sluice gates and fish passes installed on artificial structures might facilitate the recruitment of individuals in areas above the structures and further upstream in the watershed. Difference in the percentages of wild eels among sampling sites might be a product of major variation in the abundance of cultured eels stocked at respective sampling sites and nearby areas.

Occurrence and habitat use of European eel (Anguilla anguilla) in running waters: lessons for improved monitoring, habitat restoration and stocking

To improve the management of the European eel (Anguilla anguilla) in freshwater, it is essential to define important lotic habitats. Electrofishing data from 289 wadeable, hard-bottom sites in 69 Swedish coastal rivers and streams, originally surveyed for salmonid monitoring, were used to evaluate the effects of sampling- and habitat-related factors on eel occurrence. Probability of eel occurrence, as influenced by sampling procedure (sampled area, number of consecutive runs and ambient water temperature) and habitat characteristics (size of catchment, dominating bottom substrate, shade, water velocity, mean depth), was evaluated for small (total length ≤ 150 mm) and large (> 150 mm) yellow eels. Data were analysed in a mixed presence/absence generalized linear model with dispersal (distance to mouth from sampled site), habitat and sampling-related variables as covariates. The two models explained variation in occurrence to 81.5% for small eel and 76.2% for large eel. Probability of eel occurrence decreased with distance from the river mouth, and increased with sampled area, number of runs, water temperature, coarser substrate and size of river. We suggest that future eel habitat restoration should focus on lower reaches of larger rivers with suitable coarse bottom habitats. Stocking of young eel should be carried out in comparable accessible habitats in the upper reaches where eel densities are low. The results also strongly indicate that eel may be sampled together with young salmonids with DC electrofishing in wadeable habitats.

Coastal river connectivity and the distribution of ascending juvenile European eel (Anguilla anguilla L.): Implications for conservation strategies regarding fish‐passage solutions

Abstract Many diadromous fish populations are declining and at risk of collapse. Lack of river connectivity is a major contributor to these declines, as free migration routes between marine and freshwater habitats are crucial for life‐history completion. For the conservation and ultimately recovery of such species, it is imperative that remedial measures aimed at increasing connectivity are effective. This study investigated the distribution patterns of ascending juvenile European eel (Anguilla anguilla L.) in rivers in south‐western Sweden, with a focus on the effects of barriers and measures that aim to reduce the impact of barriers, i.e. fish‐passage solutions (FPSs). Eel occurrence data were spatially and temporally integrated with the national databases of dams and FPSs in a Geographic Information System (GIS) environment to evaluate their effect on ascending eel distribution. The types of barriers assessed were: (i) dams with nature‐like fishways; (ii) dams with eel ramps; (iii) dams with technical fishways; and (iv) dams without FPSs. Dams fitted with eel ramps or technical fishways, as well as dams without FPSs, produced a significant negative effect on the probability of eel occurrence upstream. This negative effect was not found for dams fitted with nature‐like fishways, indicating that these solutions may function better than the other FPS types in this study. The probability of eel occurrence decreased with distance from the sea and increased with area sampled, number of electrofishing runs, water temperature, and with the size of the bottom substrate. We suggest that future conservation strategies for improving the natural immigration of juvenile eels should include optimizing FPS function (e.g. placement and design), the continued maintenance of FPSs, the construction of nature‐like fishways, and preferably the removal of dams, which will also benefit the downstream migration of maturing eels as well as restoring other ecosystem services.

On the actual recruitment of European eel (Anguilla anguilla) in the River Ems, Germany

AbstractData from the historical River Ems glass eel fisheries in the 20th Century have been used for the ICES recruitment series. Commercial catches ceased from 1980 and dropped to zero; thus, eel recruitment data are since lacking. The present study assessed the actual eel recruitment to the River Ems with regard to the local tidal conditions. Standardised measurements were performed on relative glass eel occurrence at the tidal weir from 2014 and quantitative sampling at the next weir upstream from 2013. Eel migration at the tidal weir was dominated by glass eels and increased 2–3 month earlier than at the upstream weir, where migration was dominated by pigmented eels. In 2016, glass eels were marked at the tidal weir and pigmented elvers were sampled at the upstream weir. The 2016 eel recruitment was estimated at about 1% of the mean historical recruitment. Eel recruitment at the two weirs was not directly linked. Many eels seem not to pass the upstream weir. Further investigations are needed to reveal whether the results represent a regular pattern. In the long run, the perpetuation of the recruitment series is also foreseen.

Fish and Benthic Macroinvertebrate Densities in Small Streams with and without American Eels

AbstractThe American Eel Anguilla rostrata has been excluded from many streams along the Atlantic slope of the USA because of movement barriers such as large dams. Loss of this formerly common top predator has the potential to influence many aspects of headwater streams. We used existing survey data for a landscape‐scale comparison of fish, benthic macroinvertebrate, and crayfish densities at 95 sites where American Eel access was excluded by dams and 74 sites containing eels. Benthic fish densities were significantly lower at eel‐containing sites, whereas benthic macroinvertebrate densities were significantly higher. Densities of nonbenthic fish, non‐eel top predators, and crayfish were not different between eel occurrence categories. While these initial results are correlative, we believe that American Eel may be capable of influencing lower trophic levels, and their presence may produce noticeable changes to headwater stream ecosystems.

Evaluation of relative distance as new descriptor of yellow European eel spatial distribution

Abstract – The spatial distribution of yellow European eel (Anguilla anguilla) smaller than 300 mm was analysed during the upstream colonisation process. A 9‐year electric‐fishing programme in the Gironde catchment (France) provided eel occurrence data in 256 sites and eel abundance data in 23 sites. Generalized linear models showed that small eel spatial distribution decreased with river slope, dam number and with downstream‐upstream distance, estimated using either the distance from the tidal limit, called ‘tidal distance’, or the ‘relative distance’, calculated as the fish’s position relative to the total distance between tidal limit and river source. This new descriptor should be considered in future eel distribution studies as it reveals fractal dimension in eel spatial distribution and may provide a standardised method to compare directly freshwater eel assessment between streams and catchments of different lengths. If the relevancy of this descriptor is subsequently confirmed, it may have important implications for the management of eel population conservation.

Phenotypic plasticity of habitat use by three temperate eel species, Anguilla anguilla, A. japonica and A. rostrata

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 308:231-241 (2006) - doi:10.3354/meps308231 Phenotypic plasticity of habitat use by three temperate eel species, Anguilla anguilla, A. japonica and A. rostrata Françoise Daverat1,*, Karin E. Limburg2, Isabel Thibault3, Jen-Chieh Shiao4, Julian J. Dodson3, François Caron6, Wann-Nian Tzeng7, Yoshiyuki Iizuka5, Håkan Wickström8 1Cemagref, 50 Avenue de Verdun, 33612 Cestas, France 2SUNY College of Environmental Science & Forestry, Syracuse, New York 13210, USA 3Département de Biologie, Université Laval, Ste-Foy, Québec G1K 7P4, Canada 4Institute of Cellular and Organismic Biology, and 5Institute of Earth Sciences, Academia Sinica, 128, Section 2, Academia Road, Nankang, Taipei, Taiwan 115, ROC 6Ministère des Ressources Naturelles et de la Faune du Québec, 675, boul. René-Lévesque Est, Québec (Québec) G1R 5V7, Canada 7Institute of Fisheries Science, College of Life Science, National Taiwan University, Taipei, Taiwan 106, ROC 8Swedish Board of Fisheries, Institute of Freshwater Research, SE-178 93, Drottningholm, Sweden *Email: francoise.daverat@cemagref.fr ABSTRACT: Habitat use patterns of 3 species of temperate eels, Anguilla anguilla, A. japonica and A. rostrata, were investigated using otolith strontium:calcium ratio life history transects. Published and unpublished data from 6 sites (Canada, United States, Sweden, France, Taiwan and Japan) sampled across the geographical range of each eel species were compiled. Sr:Ca patterns indicated that the 3 species displayed similar patterns of habitat use. In all sites, patterns of habitat use consisted of either residency in one habitat (fresh, brackish, or marine) or movements between habitats. One movement pattern consisted of either a single change or 2 changes of habitat from fresh to brackish waters, or from brackish water to freshwater. Seasonal movements between fresh and brackish waters were observed for all 3 species. When only a single habitat switch event was detected, it occurred between 3 and 5 yr of age. Occurrence of eels with no freshwater experience was demonstrated, but such eels accounted for a smaller proportion of the overall sample than eels with some (even brief) freshwater experience. Contrary to the common convention that these are obligate catadromous species, we must now consider them as facultative catadromous, with far more flexibility in habitat use. The most variable parameter among study sites was the relative proportion, rather than the diversity, of lifetime spent in the various habitat use patterns. Eels found at higher latitudes exhibited a greater probability of remaining in the lower reaches of watersheds in brackish water. Diversity of habitat use appears to be a common strategy of temperate eel species, and, as a life history tactic, is under environmental control. KEY WORDS: Habitat use · Anguilla spp · Otolith Sr:Ca Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 308. Online publication date: February 16, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Status and Diet of the Otter <i>Lutra lutra</i> in Northern Ireland

We assess the status and diet of otters in Northern Ireland in 2002. Signs of otter presence were noted at 65% of 441 sites surveyed. This figure suggests a decline in signs of otters since the 1980s. Highest occurrence of signs of otters was around lakes, with coastal sites having the lowest occurrence. Percentage occurrence of signs of otter varied considerably with respect to catchment. The highest levels were in Lough Melvin, County Fermanagh, and around the Foyle catchment, County Londonderry; fewest signs occurred along the Antrim coast. Signs of otter occurrence were higher in larger rivers and at unpolluted sites (68%) as compared to polluted sites (57%). Over 50% of otter spraints were composed of stickleback (Gasterosteus aculeatus), salmonids and cyprinids, with stickleback constituting the most frequently occurring prey category. The frequency of occurrence of eels (Anguilla anguilla) in otter diet was found to be consistently higher in spraints collected from smaller streams across all land class groups. The frequency of occurrence of most prey items differed significantly with catchment with the exception of salmonids, which occurred consistently in spraints collected throughout all catchments in Northern Ireland. The current investigation confirms that otters remain widespread in Northern Ireland. However, otters may be feeding on less profitable prey items. urther investigations into the relationship between water quality, prey availability and prey selection are required to fully understand the factors affecting otter diet in Northern Ireland.

STATUS AND DIET OF THE OTTER LUTRA LUTRA IN NORTHERN IRELAND

We assess the status and diet of otters in Northern Ireland in 2002. Signs of otter presence were noted at 65% of 441 sites surveyed. This figure suggests a decline in signs of otters since the 1980s. Highest occurrence of signs of otters was around lakes, with coastal sites having the lowest occurrence. Percentage occurrence of signs of otter varied considerably with respect to catchment. The highest levels were in Lough Melvin, County Fermanagh, and around the Foyle catchment, County Londonderry; fewest signs occurred along the Antrim coast. Signs of otter occurrence were higher in larger rivers and at unpolluted sites (68%) as compared to polluted sites (57%). Over 50% of otter spraints were composed of stickleback (Gasterosteus aculeatus), salmonids and cyprinids, with stickleback constituting the most frequently occurring prey category. The frequency of occurrence of eels (Anguilla anguilla) in otter diet was found to be consistently higher in spraints collected from smaller streams across all land class groups. The frequency of occurrence of most prey items differed significantly with catchment with the exception of salmonids, which occurred consistently in spraints collected throughout all catchments in Northern Ireland. The current investigation confirms that otters remain widespread in Northern Ireland. However, otters may be feeding on less profitable prey items. Further investigations into the relationship between water quality, prey availability and prey selection are required to fully understand the factors affecting otter diet in Northern Ireland.

A model to predict the presence of longfin eels in some New Zealand streams, with particular reference to riparian vegetation and elevation

Two logistic regression models were developed from a database of 27 biotic and physicochemical variables for 99 sites in the Taieri River, New Zealand, to predict the probability of occurrence of longfin eels. Average depth was associated positively with eels while woody debris and oxygen concentration were negatively associated. At a macro‐scale probability of eel occurrence declined with increasing elevation and, for a given elevation, was higher in tussock and pasture catchments and lower in pine and native forest settings. Using a separate fish database for the Taieri River this macro‐scale model predicted eel presence 95·4% in agreement with observation. A map was generated from the model showing areas of predicted high, moderate and low probabilities of eel occurrence. The model also estimated the minimum total number of eels present in the Taieri River catchment (excluding lakes, and streams below 100 m and above 1000 m) as 20 865 (95% CL: 10 560‐36 350).