Plasma non-esterified fatty acid profiles and 17β-oestradiol levels of juvenile immature and maturing adult American eels in the St Lawrence River

American eels Anguilla rostrata , collected from three distinct locations along the St Lawrence River (Lake St Lawrence, Quebec City and Kamouraska), were thought to consist entirely of out-migrating adults at Quebec City and Kamouraska and of both resident and migrants at Lake St Lawrence. The mean plasma levels of 17β-oestradiol closely paralleled the trend observed with gonadosomatic index (I_G) and oocyte development. The highest levels of 17βoestradiol and I_G were observed in Quebec City and Kamouraska, while the levels observed in the resident eels from Lake St Lawrence were very low. Generally, the concentration of total plasma non-esterified fatty acids (NEFA) appeared to follow the trend of 17β-oestradiol and I_G and increased with sexual maturity. However, the most sexually mature group, eels from Kamouraska, had the lowest total plasma NEFA levels. The reason for the drop is not clear at present. In addition, a number of shifts in the plasma NEFA profiles of American eels did occur during their migration in a downstream direction, with several fatty acids either increasing or decreasing in relative abundance. The ratio of n3: n6 fatty acids declined with increasing sexual maturity as most n3 fatty acids became less abundant. Although the correlation between gonadal development and the relative concentration of individual fatty acids was not always clear, both arachidonic acid (20: 4n6) and docosahexaenoic acid (22: 6n3) were significantly higher in the plasma of out-migrating adults than in resident yellow phase eels.     

Effectiveness of a strobe light eel exclusion scheme

In the laboratory and field, American eels (Anguilla rostrata) showed a strong avoidance of white strobe light. Eels avoided low light intensities for all strobe flash frequencies tested, and showed no behavioural adaptation to the light source over a prolonged time period. A strobe light barrier was 65–92% effective in repelling upstream migrating eels at Saunders GS on the St. Lawrence River. All size classes of eels were repelled, but effectiveness appeared to be reduced for smaller eels.     

Yellow-stage American eel movements determined by microtagging and acoustic telemetry in the St Jean River watershed, Gaspé, Quebec, Canada

The hypothesis that a part of the yellow American eel Anguilla rostrata sub-population of the St Jean River in eastern Quebec feeds in the brackish environment during summer and returns to the river to overwinter was tested. Three years of microtagging and the acoustic tagging and tracking of 40 American eels demonstrated that a part of the downstream migrants exploited the estuary as a summer feeding area. Upstream movement of some microtagged American eels provided support for the hypothesis that a part of those American eels returned to the river to overwinter. In addition to the demonstration of amphidromous behaviour of yellow eels, the study revealed that American eels in the estuary were active at night but homed to specific daytime resting sites.     

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.     

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.     

How does salinity influence habitat selection and growth in juvenile American eels Anguilla rostrata?

The influence of salinity on habitat selection and growth in juvenile American eels Anguilla rostrata captured in four rivers across eastern Canada was assessed in controlled experiments in 2011 and 2012. Glass eels were first categorized according to their salinity preferences towards fresh (FW), salt (SW) or brackish water (BW) and the growth rate of each group of elvers was subsequently monitored in controlled FW and BW environments for 7 months. Most glass eels (78–89%) did not make a choice, i.e. they remained in BW. Salinity preferences were not influenced by body condition, although a possible role of pigmentation could not be ruled out. Glass eels that did make a choice displayed a similar preference for FW (60–75%) regardless of their geographic origin but glass eels from the St Lawrence Estuary displayed a significantly higher locomotor activity than those from other regions. Neither the salinity preferences showed by glass eels in the first experiment nor the rearing salinities appeared to have much influence on growth during the experiments. Elvers from Nova Scotia, however, reached a significantly higher mass than those from the St Lawrence Estuary thus supporting the hypothesis of genetically (or epigenetically) based differences for growth between A. rostrata from different origins. These results provide important ecological knowledge for the sustained exploitation and conservation of this threatened species.     

Shark Predation on Migrating Adult American Eels (Anguilla rostrata) in the Gulf of St. Lawrence

In an attempt to document the migratory pathways and the environmental conditions encountered by American eels during their oceanic migration to the Sargasso Sea, we tagged eight silver eels with miniature satellite pop-up tags during their migration from the St. Lawrence River in Québec, Canada. Surprisingly, of the seven tags that successfully transmitted archived data, six were ingested by warm-gutted predators, as observed by a sudden increase in water temperature. Gut temperatures were in the range of 20 to 25°C—too cold for marine mammals but within the range of endothermic fish. In order to identify the eel predators, we compared their vertical migratory behavior with those of satellite-tagged porbeagle shark and bluefin tuna, the only endothermic fishes occurring non-marginally in the Gulf of St. Lawrence. We accurately distinguished between tuna and shark by using the behavioral criteria generated by comparing the diving behavior of these two species with those of our unknown predators. Depth profile characteristics of most eel predators more closely resembled those of sharks than those of tuna. During the first days following tagging, all eels remained in surface waters and did not exhibit diel vertical migrations. Three eels were eaten at this time. Two eels exhibited inverse diel vertical migrations (at surface during the day) during several days prior to predation. Four eels were eaten during daytime, whereas the two night-predation events occurred at full moon. Although tagging itself may contribute to increasing the eel''s susceptibility to predation, we discuss evidence suggesting that predation of silver-stage American eels by porbeagle sharks may represent a significant source of mortality inside the Gulf of St. Lawrence and raises the possibility that eels may represent a reliable, predictable food resource for porbeagle sharks.     

A phylogeny of freshwater eels inferred from mitochondrial genes

The genus Anguilla Shaw of Family Anguillidae consists entirely of freshwater eels, including 15 species and 2 subspecies. Conventionally, variegated markings and the length of the dorsal fin are the major morphological features used for reconstruction of phylogenetic relationships. The evolutionary history of these species remains unclear, especially for the Atlantic eels, whose habitats are far from the Metropolis in the Indo-Pacific region. This study reexamined the phylogenetic relationships of 12 Anguilla species by sequencing of the cytochrome b and 12S rRNA genes. In our analysis, species bearing similar coloration patterns or dorsal fin morphology are not necessarily clustered in the same clade, indicating that these morphological features might be unstable or might have occurred independently in different lineages during evolution. Combining our molecular data and geographical evidence, we speculate that (1) Anguilla first radiated about 20 million years ago, (2) the ancestors of Atlantic eels did not migrate by drifting through the Tethys Seaway at the leptocephali stage but instead trekked across the Central American Isthmus to the Sargasso Sea for spawning at the adult stage, and (3) multiple radiation events had occurred at the Metropolis during Anguilla evolution.     

Thermal preferenda and behavior of Atlantic eels (genus Anguilla) in relation to their spawning migration

Synopsis The final preferred temperatures (FPTs) of adult premigratory and migratory life-history phases of American eels, Anguilla rostrata, were determined by chronic tests in a horizontal thermal gradient. Mean FPTs were between 17 and 20°C and were not significantly different between life-history phases, acclimation temperatures, illumination regimes, photoperiods or sexual maturation states. Thermal behavior of eels was highly variable, both among individuals of the various test groups and among repeated tests of single individuals. Light inhibited behavioral thermoregulation by promoting shelter-seeking. The following inferences are drawn from the laboratory findings and observations of migrating A. rostrata and A. anguilla (European eels) in the North Atlantic: (1) decreasing temperatures may initiate downstream migration of silver eels, (2) eels may select temperatures close to their FPT in thermally stratified environments, but will tolerate higher and lower temperatures depending on illumination or other physical constraints, (3) the oceanic phase of the migration to the Sargasso Sea may take place at relatively shallow depths in the open ocean, probably within the upper 1000 meters. The strong eurythermality observed in eels may facilitate their occupation of and migration through thermally diverse and unpredictable habitats.     

Otolith Sr:Ca and Ba:Ca may give inconsistent indications of estuarine habitat use for American eels (<Emphasis Type="Italic">Anguilla rostrata</Emphasis>)

Temporal patterns in otolith Sr:Ca and Ba:Ca ratio values of American eels Anguilla rostrata from two sites in western Newfoundland gave insight into the use of freshwater and saline habitats. Mean Sr:Ca and Ba:Ca values at the core zone did not differ between sites, indicative of a common oceanic origin. At the otolith edge, representing continental life, both Sr:Ca and Ba:Ca values varied between sites consistent with ambient element:Ca ratio values and salinity, with typically higher Sr:Ca and lower Ba:Ca values in saline than in fresh waters. Most eels (73%) from Muddy Hole, an estuarine site, were evaluated as estuarine residents while most (70%) eels from Castors River, a freshwater site, were evaluated as freshwater residents, with the remaining eels from each site evaluated as inter-habitat migrants. An otolith element:Ca critical value appropriate for distinguishing between fresh and saline water residence is fundamental for estimating the proportion of eel residence in freshwater and their subsequent classification into habitat residence groups. Such classification is moderately robust to the critical value selected. For inter-habitat migrants, moderate otolith Sr:Ca values between the elver check and otolith edge suggestive of estuarine residence may coincide with Ba:Ca values suggestive of freshwater residence. No general critical value for separating fresh and estuarine habitats was found for otolith Ba:Ca. Otolith Ba:Ca temporal patterns may assist the use of Sr:Ca in the evaluation of historical habitat residence and inter-habitat movement but the use of otolith Ba:Ca values should be applied cautiously for American eels and perhaps of other estuarine/freshwater migratory fishes.     

Unravelling the life-history patterns and habitat preferences of the Japanese eel (Anguilla japonica) in the Pearl River,China

Eels have fascinated biologists for centuries due to their amazing long-distance migrations between freshwater habitats and very distant ocean spawning areas. The migratory life histories of the Japanese eel, Anguilla japonica, in the waters of south China are not very clear despite its ecological importance, and the need for fishery regulation and management. In this study, strontium (Sr) and calcium (Ca) microchemical profiles of the otoliths of silver eels were measured by X-ray electron probe microanalysis based on data collected from different habitats (including freshwater and brackish habitats), in the large subtropical Pearl River. The corresponding habitat preference characteristics were further analysed using redundancy analysis (RDA). A total of 195 Japanese eels were collected over 6 years. The collected individuals ranged from 180 to 771 mm in total length and from 8 to 612 g in body weight. Two-dimensional pictures of the Sr:Ca concentrations in otoliths revealed that the A. japonica in the Pearl River are almost entirely river eels, spending the majority of their lives in fresh water without exposure to salt water, while the catadromous migration time has delayed about 1 month in the Pearl River estuary in the past 20 years. RDA analysis further indicated that juveniles and adults preferred water with high salinity and high tide levels. Youth preferred habitats with high river fractals. Our findings contribute to a growing body of evidence showing that the eels are extremely scarce currently and conservation measures against them are imminent, including the protection of brackish and freshwater areas where they live in south China.     

Interspecific and sexual differences in riverine distribution of tropical eels Anguilla spp.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%). Anguilla bicolor pacifica and Anguilla interioris were rare (1.5 and 0.4%, respectively). Otolith Sr:Ca ratio electron‐probe micro analysis (EPMA) for individual migratory histories revealed that 15 A. celebesensis caught in Poso Lake and its inlet rivers were categorized into 14 river eels (Sr:Ca < 2·5) showing upstream migration seemingly at their elver stage and only one sea eel (Sr:Ca ≥ 6·0) that stayed in the marine habitat for the majority of its life after recruiting to Sulawesi Island before its late upstream migration. In A. marmorata, 19 examined eels from Poso Lake and its inlet rivers were all river eels, while 17 eels from the lower reaches of Poso River were two river eels, six sea eels and nine estuarine eels (2·5 ≤ Sr:Ca < 6·0) that mostly lived in the brackish water. The sex ratio of A. celebesensis was highly skewed towards a dominance of females (99%). In A. marmorata, females were predominant in Poso Lake (95·2%), its inlet rivers (94·7%) and Laa River (100%), while males were more frequent in the lower reaches of Poso River (76·5%) and small inlet rivers of Tomini Bay (94·1%). These results indicate that the riverine distribution pattern of tropical eels differs among species and between sexes.     

American eels produce and release bile acid profiles that vary across life stage

The American eel (Anguilla rostrata) is an imperilled fish hypothesized to use conspecific cues, in part, to coordinate long-distance migration during their multistage life history. Here, holding water and tissue from multiple American eel life stages was collected and analysed for the presence, profile and concentration of bile acids. Distinct bile acid profiles were identified in glass, elver, yellow eel and silver eel holding waters using ultraperformance liquid chromatography high-resolution mass spectrometry and principal component analysis. Taurochenodeoxycholic acid, taurodeoxycholic acid, cholic acid, deoxycholic acid, taurolithocholic acid and taurocholic acid were detected in whole tissue of American glass eels and elvers, and in liver, intestine and gallbladder samples of late-stage yellow eels. Bile acids were not a major component of silver eel washings or tissue. This study is novel because little was previously known about bile acids produced and emitted into the environment by American eels. Future behavioural studies could evaluate whether any bile acids produced by American eels influence conspecific migratory behaviour.     

Development of a key using morphological characters to distinguish south-western Indian Ocean anguillid glass eels

Two morphological characters were examined in 4099 anguillid glass eels sampled in four south-western Indian Ocean islands, and resulting identifications were tested using genetic analysis. Distance between the origin of the dorsal and anal fins as related to total length and tail and caudal fin pigmentation enabled formulation of a useful field identification key, which was able to discriminate 4036 glass eels of the regional species Anguilla bicolor bicolor , Anguilla marmorata and Anguilla mossambica , and excluded the occurrence of Anguilla nebulosa labiata in the sample.     

Age,growth, mortality,and abundance of lake sturgeon in the Grasse River,New York,USA

An increased understanding of lake sturgeon (Acipenser fulvescens) population dynamics is a key requirement for successful management efforts. Little is known regarding the Grasse River population of lake sturgeon except that it is one of a few populations in New York State where spawning has been documented. Thus our purpose was to assess the current status of lake sturgeon in the Grasse River system, including age, growth, mortality, and abundance. Age was determined for 196 of 211 lake sturgeon by examination of sectioned pectoral fin rays. Ages ranged from 0 to 32 years and the annual mortality rate for fish between ages 7 and 14 was 16.8%. The weight (W, g) to total length (TL, mm) relationship was W = 1.281 × 10^?6TL^3.202. The von Bertalanffy growth equation was TL = 1913(1?e^{?0.0294(t+9.5691)}). While the range of observed ages was similar to that of nearby St. Lawrence River populations, mean weight at age for an individual at 1000 mm TL was lower than that observed for lake sturgeon within Lake St. Francis of the St. Lawrence River. Predicted growth based on von Bertalanffy parameters was similar to that observed for the nearby Lake St. Francis. An open population estimator using the POPAN sub‐module in the Program MARK produced an abundance estimate of 793 lake sturgeon (95% CI = 337–1249).     

Isolation and characterization of two different 5S rDNA in Anguilla anguilla and in Anguilla rostrata: possible markers of evolutionary divergence

We cloned and sequenced the HaeIII 350‐bp 5S ribosomal DNA (rDNA) band of Anguilla rostrata and designed specific primers from this sequence. Polymerase chain reaction performed with these primers is able to distinguish DNA samples obtained from European (Anguilla anguilla) and American (Anguilla rostrata) eels. Two amplicons of 1200 bp and 600 bp were obtained, respectively, from A. rostrata and A. anguilla, and the whole 5S rDNA repeated unit from these eels was cloned and sequenced. Southern blot experiments, using four different restriction enzymes and the 5S nontranscribed spacers regions as probe, are able to point out specific diversity in these eels.     

Validation of the occurrence of the American eel Anguilla rostrata (Le Sueur, 1817) in free-draining European inland waters

The screening of 2,735 eels from European waters and aquaculture farms was conducted using mitochondrial Cytochrome b and 16S rRNA gene fragments amplified by polymerase chain reaction. Reaction products were either sequenced directly or subjected to analysis using restriction fragment length polymorphism which resulted in species-specific restriction patterns. Beside the expected European eel, Anguilla anguilla (Linnaeus, 1758), the American eel, Anguilla rostrata (Le Sueur, 1817), was also identified in samples from both aquaculture (N = 40 out of 1,025) and from natural waters (N = 44 out of 1,710). The life stages of American eels identified from several German waters draining to either the Baltic Sea and the North Sea ranged from elver to silver eels. This indicates that stocking with glass eels or elvers must have occurred several times most likely in the period from 1998 to 2002. The application of a fast and precise method for species identification and genetic monitoring of eels delivered for stocking is therefore essential for maintaining the autochthonous species composition in future. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Age and growth of migrating tropical eels,Anguilla celebesensis and Anguilla marmorata

The age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata from central Sulawesi, Indonesia, were examined. Migrating eels (63 A. celebesensis and 38 A. marmorata ) were obtained from weirs near the Poso Lake outlet and non‐migrating eels (35 A. celebesensis and 119 A. marmorata ) were captured by baited hooks, eel pots, scoop net and electro‐fishing in the Poso River system, Laa River system, Baluga River, Tongku River and Padapu River from February 2009 to October 2010. In both species, the proportion of eels with opaque otolith edges showed a single peak in July, suggesting that one annulus (a pair of translucent and opaque zones) was formed each year in their otoliths. Mean ± s.d . and range of total length (L _T) and age was 785·2 ± 114·9 (585–1083) mm and 7·5 ± 1·6 (5–11) years in migrating female A. celebesensis and 1132·2 ± 173·7 (800–1630) mm and 11·6 ± 3·3 (7–23) years in A. marmorata . The age of migrating female eels was negatively correlated with annual growth rate, 100·7 ± 17·2 (68·1–145·0) mm year^?1 in A. celebesensis and 97·9 ± 19·3 (66·6–131·6) mm year^?1 in A. marmorata , but there was no significant correlation between the L _T and annual growth rate in either species. The annual growth rates of these female tropical eels were typically higher than those of temperate anguillid species, suggesting a latitudinal cline in growth rate in the genus Anguilla reflecting the environmental conditions of their growth habitat.     

Dynamics of reproductive hormones during downstream migration in females of the Japanese eel, Anguilla japonica

The profiles of sex steroids (estradiol-17β, testosterone and 11-ketotestosterone) and the mRNA levels of gonadotropins (luteinizing hormone and follicle-stimulating hormone) were investigated before and after downstream migration in females of the Japanese eel species Anguilla japonica, which were collected in the brackish Hamana Lake and its inlet freshwater rivers. Eels were separated into three groups using otolith microchemistry: 'migrants' that grew in the inlet rivers and then made a downstream migration to Hamana Lake mainly in October and November; 'non-migrant' yellow eels caught in rivers during the same season; and 'residents,' which were yellow eels caught in rivers in August. Sex steroid levels, especially those of testosterone and 11-ketotestosterone, were higher in migrants than in non-migrants and residents. Real-time quantitative PCR analysis indicated that mRNA levels of luteinizing hormone (LH) β-subunits were significantly higher in migrants than in other groups, whereas those of follicle-stimulating hormone β-subunits did not show significant changes during downstream migration. The high levels of these hormones during downstream migration raise the question about if they also play a role in motivating the migratory behavior of eels.