Anguilla rostrata glass eel migration and recruitment in the estuary and Gulf of St Lawrence

This study describes catches of Anguilla rostrata glass eels and associated oceanographic conditions in the St Lawrence Estuary and Gulf. Ichthyoplankton survey data suggest that they enter the Gulf primarily in May, migrate at the surface at night, and disperse broadly once they have passed Cabot Strait. They arrive in estuaries beginning at about mid-June and through the month of July. Migration extends west up to Québec City, in the freshwater zone of the St Lawrence Estuary, 1000 km west of Cabot Strait. Anguilla rostrata glass eels travel between Cabot Strait and receiving estuaries at a straight-line ground speed of c. 10–15 km day⁻¹. Catches of fish per unit effort in estuaries in the St Lawrence system are much lower than those reported for the Atlantic coast of Canada. Low abundance of A. rostrata glass eels in the St Lawrence system may be due to cold surface temperatures during the migration period which decrease swimming capacity, long distances from the spawning ground to Cabot Strait and from Cabot Strait to the destination waters (especially the St Lawrence River), complex circulation patterns, and hypoxic conditions in bottom waters of the Laurentian Channel and the St Lawrence Estuary.     

Draft genome of the American Eel (Anguilla rostrata)

Freshwater eels (Anguilla sp.) have large economic, cultural, ecological and aesthetic importance worldwide, but they suffered more than 90% decline in global stocks over the past few decades. Proper genetic resources, such as sequenced, assembled and annotated genomes, are essential to help plan sustainable recoveries by identifying physiological, biochemical and genetic mechanisms that caused the declines or that may lead to recoveries. Here, we present the first sequenced genome of the American eel. This genome contained 305 043 contigs (N50 = 7397) and 79 209 scaffolds (N50 = 86 641) for a total size of 1.41 Gb, which is in the middle of the range of previous estimations for this species. In addition, protein‐coding regions, including introns and flanking regions, are very well represented in the genome, as 95.2% of the 458 core eukaryotic genes and 98.8% of the 248 ultra‐conserved subset were represented in the assembly and a total of 26 564 genes were annotated for future functional genomics studies. We performed a candidate gene analysis to compare three genes among all three freshwater eel species and, congruent with the phylogenetic relationships, Japanese eel (A. japanica) exhibited the most divergence. Overall, the sequenced genome presented in this study is a crucial addition to the presently available genetic tools to help guide future conservation efforts of freshwater eels.     

An Examination of Utilizing External Measures to Identify Sexually Maturing Female American Eels, Anguilla Rostrata, in the St. Lawrence River

It is well established that Anguillid eels undergo a complex suite of morphological and physiological changes during their transformation from resident, yellow-phase juveniles to actively migrating silver-phase eels. While it has been shown that some morphological measures can be used successfully to identify sexually maturing European eels, Anguilla anguilla, as well as Australian short fin, Anguilla australis, and long fin, Anguilla dieffenbachii eels, this relationship has never been quantitatively assessed for American eels, Anguilla rostrata. American eels of varying sexual development were collected from three locations on the St. Lawrence River: Lake St. Lawrence, Quebec City and Kamouraska. Sexual development of each eel was assessed with gonadosomatic index (GSI), oocyte diameter and degree of oocyte development. Morphological measures of total length, weight, head width, pectoral fin length and vertical and horizontal eye diameters were obtained from each fish. We used this data to test two hypotheses: (i) resident yellow phase eels, suspected migrants and known migrants are morphologically indistinguishable; and (ii) if differences exist, they cannot be used to reliably predict gonadal development or migratory status. Univariate analysis (ANOVA and ANCOVA) indicated that there were highly significant differences in all of the measured parameters and thus we were able to reject the first hypothesis. However, we failed to reject the second hypothesis as the high degree of overlap between groups eliminated the ability of any single measure to differentiate between resident and migratory eels. A multivariate discriminant model was developed that could classify only 72–80% of the eels correctly based on their morphological characters. While morphological measures may have some potential as a rapid, cost-effective method of pre-screening individual eels, morphological measures should not be considered a definitive indicator of sexual maturity or migratory status for female American eels in the Upper St. Lawrence River.     

Spawning migration of American eel Anguilla rostrata from pristine (1843-1872) to contemporary (1963-1990) periods in the St Lawrence Estuary, Canada

A key for three putative species apparently found in three geographic areas, i.e. Coregonus clupeoides (in Scotland), Coregonus stigmaticus (in England), and Coregonus pennantii (in Wales) given in a recent review was tested quantitatively using 544 individuals from nine populations. The classification success of the key was very low (27%). It was concluded that there is currently no robust evidence for the recognition of the three putative species. Furthermore, the use of phenotypic characters alone to distinguish putative species in postglacial fish species such as those of the genus Coregonus that show homoplasy in many of these traits is questioned. In the absence of further evidence, it was concluded that a single highly variable species best describes the pattern of phenotypic variation in these U.K. populations. On this basis it is argued that taxonomic subdivision of U.K. European coregonids is inappropriate and that Coregonus lavaretus should prevail as the species name applicable to all populations.     

Furunculosis in cultured American eel Anguilla rostrata (Le Sueur)

Diseased American eels, Anguilla rostrata , were collected from an eel culture facility in South Carolina and examined. The disease was characterized by skin lesions located on the body. A bacterium identified as Aeromonas salmonicida was isolated from the lesions and shown to be the causative agent. The bacterium was also isolated from the kidney during later stages of the disease.     

Eustrongylides tubifex (Nitzsch) encystment in an American ell Anguilla rostrata (Le Sueur)

A diseased American ell, Anguilla rostrata (Le Sueur), was captured during a survey of Little Mosquito Creek, Accomac County, Virginia, U.S.A. Two skin lesions were present. One was caused by the dermal encystment of a nematode, Eustrongylides tubifex ; the other was of undetermined origin.     

Retinal response in the post metamorphic American eel (Anguilla rostrata) (Pisces,Teleostei)

Retinas of light and dark adapted post metamorphic American eelAnguilla rostrata were examined. The retinal epithelial pigment migrates vitreally in light and sclerally in darkness. Two layers of rods and a layer of single cones are present. Some cones elongate slightly in the dark and contract in the light. The cone synaptic ribbons show no difference between the light and dark adapted stages. It appears that this eel stage is capable of functioning in bright and dim environments.     

The genetic consequences of spatially varying selection in the panmictic American eel (Anguilla rostrata)

Our understanding of the genetic basis of local adaptation has recently benefited from the increased power to identify functional variants associated with environmental variables at the genome scale. However, it often remains challenging to determine whether locally adaptive alleles are actively maintained at intermediate frequencies by spatially varying selection. Here, we evaluate the extent to which this particular type of balancing selection explains the retention of adaptive genetic variation in the extreme situation of perfect panmixia, using the American eel (Anguilla rostrata) as a model. We first conducted a genome scan between two samples from opposite ends of a latitudinal environmental gradient using 454 sequencing of individually tagged cDNA libraries. Candidate SNPs were then genotyped in 992 individuals from 16 sampling sites at different life stages of the same cohort (including larvae from the Sargasso Sea, glass eels, and 1-year-old individuals) as well as in glass eels of the following cohort. Evidence for spatially varying selection was found at 13 loci showing correlations between allele frequencies and environmental variables across the entire species range. Simulations under a multiple-niche Levene's model using estimated relative fitness values among genotypes rarely predicted a stable polymorphic equilibrium at these loci. Our results suggest that some genetic-by-environment interactions detected in our study arise during the progress toward fixation of a globally advantageous allele with spatially variable effects on fitness.     

Origins of the freshwater attractant(s) of migrating elvers of the American eel,Anguilla rostrata

Synopsis Origins of the freshwater attractant(s) of migrating elvers of the American eel were investigated by assaying elvers' responses to rinses of plants, animals, and inanimate objects collected from a Rhode Island (U.S.A.) brook with a sizable elver run. Odor rinses were tested in a Y-maze at naturally occurring concentrations against both blank and brook water. Many items were attractive, several were repulsive, and some caused a reduction in elvers' rheotactic behavior, suggesting that elvers respond to a bouquet of odors. The odor of abundant decaying leaf detritus was highly attractive as were odors of the surfaces of aquatic plants, submerged stones, and migrating alewives. Conspecific odor was only weakly attractive. Because unattractive leaves became attractive when cultured with stream water, microorganisms responsible for detrital decomposition and present in/on most stream objects are thought to be the major source of the attractant(s). Decaying detritus and its associated microorganisms are abundant in most freshwater streams, where they often constitute the ecosystem's primary energy source; their odor could serve as an index of environmental suitability for migrating eels.     

Validation of daily otolith increments in glass-phase American eels Anguilla rostrata (Lesueur) during estuarine residency

Prior to making inferences from otoliths about the residence time and growth rate of glass-phase anguillid eels Anguilla in estuaries, it is necessary to validate the deposition rate of microincrements in the otoliths. Glass-phase American eels Anguilla rostrata (Lesueur), which had been captured near the mouth of an estuary in Maine, USA, prior to freshwater exposure, deposited increments at a daily rate at ambient temperature and salinity in a field and laboratory study. The regression for glass eels not possessing a transition ring was: I=0.976(D-1)+0.434, where I is the number of otolith increments distal to a fluorescent mark placed on the otolith at the beginning of the experiment, and D is the number of days in the experiment, which ranged from 7 to 49. The slope was not significantly different than 1. Unexpectedly, many glass eels deposited the transition ring during the experiment, although this ring had previously been thought to mark entry into fresh water. The regression for these glass eels was: I=0.961(D-1)-3.880, and the slope was not significantly different than 1. The negative intercept suggests that approximately 4 days were lost from the otolith record during deposition of the ring. This study demonstrated daily deposition of increments prior to freshwater exposure and demonstrated that deposition of the transition ring is not linked to freshwater entry.     

Electrolyte changes of serum and muscle,and related mortalities in maturingAnguilla rostrata migrating down the St. Lawrence Estuary (Canada)

This study was made to investigate changes in serum and muscle ion concentrations and related mortalities in maturingAnguilla rostrata migrating down the St. Lawrence Estuary. Mortalities take place in the freshwater portion of the St. Lawrence. Electrolyte concentrations of moribund eels taken in freshwater were compared to those of freshwater and salt water controls. Moribund eels had a much lower serum osmolality (270 mOsm/kg) than the controls (328 and 358 mOsm/kg). This resulted from low sodium (125 mEq/l) and particularly low chloride (69 mEq/l) contents in the moribund eels compared to the freshwater controls (153 and 117 mEq/l) and the salt water controls (179 and 137 mEq/l). There was also a general decrease in muscle ion concentrations in moribund eels though the percentage water was similar to that of the freshwater controls (64.0 and 63.7%). The changes measured between the freshwater controls and the salt water controls in nature are similar to those measured onAnguilla anguilla in laboratory. These results suggest that mortalities are related to failure by some of the maturing eels to maintain their mineral balance in freshwater. Hypothesis is made that maturing eels migrating long distances in freshwater or retarded by physical or chemical barriers, start to excrete sodium and chloride under hormonal control before they have reached brackish water. In the conditions that prevail in the St. Lawrence Estuary, this results in mineral unbalance and possibly in mortalities.     

Movements and diving of grey seal females (Halichoerus grypus) in the Gulf of St. Lawrence, Canada

Five female grey seals were tracked with satellite-linked time-depth recorders during September to April 1993-1994. Seals remained in the northern Gulf of St. Lawrence (Gulf) for 1-2.5 months after capture. Four females dove primarily to depths <10 m and 20-70 m, while all dives of the fifth female, a blind animal, were <10 m. During October/November, all animals moved into the southern Gulf or onto the Scotian Shelf. This migration lasted 6-10 days, during which time animals covered 350-800 km. During that migration, all females, including the blind animal, dove up to 100 m, but the majority of dives were to depths of 40-70 m. Two seals stayed in the southern Gulf through the winter while the others left the Gulf in January. When in the southern Gulf and on the Scotian Shelf, dive depths and bathymetry information indicated that dives were to the bottom.     

The ultrastructure of the integument of the American eel,Anguilla rostrata

Summary The morphology and ultrastructure of the lateral body integument of the leptocephalus, glass eel, pigmented elver, and adult stages of the American eel, Anguilla rostrata, were examined with light and electron microscopy. The integument consists of an epidermis separated by a basal lamina from the underlying dermis. Three cell types are present in the epidermis in all stages. Filament-containing cells, which are the principal structural cell type, are increasingly numerous at each stage. Mucous cells, which secrete the mucous that compose the mucous surface coat, are also more numerous in each subsequent stage and are more numerous in the anterior lateral body epidermis than in the posterior lateral body epidermis of the adult. Club cells, whose function is unknown, are most numerous in the glass eel and pigmented elver. Chloride cells are common in the leptocephalus which is marine and infrequent in the glass eel. They are not present in the pigmented elver and adult which inhabit estuaries and fresh-water. Lymphocytes and melanocytes are also present in some stages. The dermis comprises two layers: a layer of collagenous lamellae, the stratum compactum, and an underlying layer of loose connective tissue, the stratum spongiosum.There is a progressive increase in epidermal thickness at each stage which is paralleled by an increase in the thickness of the stratum compactum. Rudimentary scales are present in the dermis of the adult. The increase in the number of epidermal filament-containing cells, epidermal thickness and stratum compactum thickness is correlated with an increased need for protection from abrasion and mechanical damage as the eel moves from a pelagic, oceanic habitat to a benthic, freshwater habitat. The increase in mucous cell numbers is likewise correlated with an increased need for the protective and anti-bacterial action of the mucous surface coat in the freshwater environment.This investigation was supported by NIH research grant NS-11276 from National Institute of Neurological Diseases and Stroke to Dr. J.D. McCleave and by N.S.F. Grant GD 38933 to the Bermuda Biological Station, St. Georges West, Bermuda. Bermuda Biological Station Contribution No. 668     

Population demographics of American eels Anguilla rostrata in two Arkansas,U.S.A., catchments that drain into the Gulf of Mexico

The goal of this study was to compare American eel Anguilla rostrata life history in two inland river systems in Arkansas, U.S.A., that ultimately discharge into the Gulf of Mexico via the Mississippi River and the Red‐Atchafalaya catchments. From 21 June 2011 to 24 April 2014, 238 yellow‐phase A. rostrata were captured in the middle Ouachita River and tributaries using boat electrofishing and 39 in the lower White River using multiple sampling gears. Most of them were caught downstream of dams in both basins (61%). Medium‐sized A. rostrata ranging from 225 to 350 mm total length (L_T) were the most abundant size group in the Ouachita River basin, but they were absent from the White River. Mean L_T at age 4 years (i.e. youngest shared age) was 150 mm greater for the White River than the Ouachita River basin. Anguilla rostrata appeared to have a greater initial L_T (i.e. minimum size upon arrival) in the White River that allowed them to reach a gonado‐somatic index (I_G) of 1·5 up to 4 years earlier, and downstream migration appeared to occur 5 years earlier at 100 mm greater L_T; these differences may be related to increased river fragmentation by dams in the Ouachita River basin. Growth and maturation of A. rostrata in this study were more similar to southern populations along the Atlantic coast than other inland populations. Adult swimbladder nematodes Anguillicoloides crassus were not present in any of the 214 swimbladders inspected. Gulf of Mexico catchments may be valuable production areas for A. rostrata and data from these systems should be considered as range‐wide protection and management plans are being developed.     

Evidence for Environmental Sex Determination in the American eel, Anguilla rostrata

Males have predominated among migrating silver eels in the Annaquatucket River, Rhode Island, for at least two decades, with no significant variation in mean total length in either sex. Because the species is panmictic (random breeding), this consistency suggests environmental sex determination (ESD). Most yellow (feeding phase) eels <300mm total length in the Annaquatucket are sexually undifferentiated, and in contrast to all other published sex ratios, males greatly outnumber females (3:1) among differentiated yellow eels. Estimates of yellow eel population densities are 4–10 times greater than published values for other habitats. We propose that this crowding results in a long period of undifferentiation and the suppression of femaleness. Published field and experimental evidence indicates that high population density results in high proportions of males in Atlantic Anguilla, and that low population density results in the predominance of females. This ESD may be adaptive, resulting in vast numbers of small males in coastal habitats, relatively close to the spawing area, and much larger and more fecund females that occupy most of the available eel habitat.     

Habitat use of American eel (Anguilla rostrata) in a tributary of the Hudson River,New York

American eel Anguilla rostrata populations are declining over much of their native range. Since American eels spend extended periods in freshwater, understanding their habitat requirements while freshwater residents is important for the management and conservation of this species. As there is little information on American eel habitat use in streams, the ontogenetic, diel, and seasonal habitat use as well as habitat selectivity of three size groups (i.e. ≤199 mm total length, 200–399 mm, ≥400 mm) of eel were examined in a tributary of the Hudson River. American eels in Hannacroix Creek exhibited ontogenetic, diel, and seasonal variation in habitat use as well as habitat selection. During both summer and autumn all sizes of American eels used larger substrate and more cover during the day. American eels ≤199 mm exhibited the strongest habitat selection, whereas eels 200–399 mm exhibited the least. During the autumn all sizes of American eels occupied slower depositional areas where deciduous leaf litter accumulated and provided cover. This may have important implications for in‐stream and riparian habitat management of lotic systems used by American eel.