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.     

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.     

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.     

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.     

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.     

Chemical marking of European glass eels Anguilla anguilla with alizarin red S and in combination with strontium: in situ evaluation of short‐term salinity effects on survival and efficient mass‐marking

This study presents a new chemical double‐marking technique for European glass eels Anguilla anguilla by combing alizarin red S (ARS) and strontium chloride hexahydrate (Sr). Marked eels (double marked with ARS and Sr, but also single marked with ARS) were exposed in situ to brackish water (15 g l^?1 artificial sea salt) for 14 days and did not exhibit increased mortalities compared with unmarked eels. Indeed, no mortality occurred in a marked group during the experiments. Moreover, an efficient mass‐marking approach with low handling effort for both single ARS and double ARS‐Sr techniques is described and was proven to be practicable for large‐scale stocking programmes.     

Behavioural responses of glass eels (Anguilla anguilla) towards amino acids

The behavioural responses of glass eels of Anguilla anguilla towards amino acids were investigated by binary choice experiments testing different concentrations of 14 L-amino acids in fresh water and salt water. Glass eels responded to solutions of individual amino acids down to a concentration of 10⁻⁸M, 10⁻⁹m. Media of different salinities influenced the responses. In freshwater, gln and thr were strongly attractive; asn, ala, met, glu and ile induced significant avoidance; gly elicited multimodal reactions depending on concentrations; his, lys, phe, val, leu and asp hardly influenced behaviour. In salt water, only gly, asn and lys significantly influenced the behaviour of the glass eels.     

Salinity-linked growth in anguillid eels and the paradox of temperate-zone catadromy

Temperate-zone anguillid eels use both saline (marine or brackish) and fresh waters during their continental phase, but use of fresh waters is paradoxical because on average these fishes grow more rapidly in saline than in fresh waters. Based on data from anguillid eels whose habitat-residency histories had been determined by Sr:Ca otolithometry, superiority of growth rates in saline water is much greater in American eels Anguilla rostrata in north-eastern North America (mean saline:fresh growth rate ratio 2·07) than in European Anguilla anguilla , Japanese Anguilla japonica and shortfinned Anguilla australis eels (range of mean ratios 1·12–1·14). Data from A. rostrata in the Hudson Estuary, U.S.A., and Prince Edward Island, Canada, were used to test adaptive explanations of catadromous migrations. The hypothesis that lower mortality in fresh water offsets faster growth in saline water was not supported because loss (mortality + emigration ) rates did not vary between saline and fresh zones of the Hudson Estuary. Hypotheses that anguillid eels move to fresh water to escape from larger anguillid eels in saline water or to evaluate habitat quality were not supported by size and age distributions. Catadromy in temperate-zone anguillid eels increases the diversity of occupied habitats and therefore lowers fitness variance caused by environmental fluctuations. Catadromy in temperate-zone anguillid eels could be due to natural selection for maximum geometric mean fitness which is sensitive to fitness variance. Temperate-zone catadromy might also be maladaptive, at least in local areas, due to shifts over time in selective pressures or to inability of panmictic genetic systems to adapt to local conditions.     

Time lag of the response on the otolith strontium/calcium ratios of the Japanese eel, <Emphasis Type="Italic">Anguilla japonica</Emphasis> to changes in strontium/calcium ratios of ambient water

We conducted a laboratory experiment to validate the relationship between the otolith strontium/calcium (Sr/Ca) ratio of Japanese eels (Anguilla japonica) and water Sr/Ca ratio when the ratio in water was changed. A linear and additive mixed modeling approach was used to assess otolith Sr/Ca ratio for elver-juvenile Japanese eels when ambient water was changed from seawater to freshwater. There was a significant difference between otolith Sr/Ca ratios of eels reared in freshwater and in seawater (freshwater: 1.3–2.3; seawater: 7.0–7.8 mmol/mol). The response of otolith Sr/Ca ratios of eels was not detected until after 10 d and models suggested that it might not be completed until at least 30–60 d. This study indicated the detailed ability of otolith Sr/Ca ratio to be used as a proxy for reconstructing the individual environmental history of Japanese eels. These findings can provide some assurances for future otolith Sr/Ca studies of eels in this system or in other areas that have similar environmental conditions.     

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.     

Dispersal of yellow phase Japanese eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> after recruitment in the Kojima Bay-Asahi River system,Japan

The density, size and age distribution were investigated for 233 eels, Anguilla japonica, sampled in fresh and brackish water areas of the Kojima Bay-Asahi River system, Okayama, Japan, to evaluate the possible patterns of dispersal of eels that recruit to this area. Migratory histories of 183 eels were categorized into 5 types depending on the Sr and Ca concentrations in their otoliths: (1) brackish water residents (74 fish, 40.4%), which settled in saline water and remained until capture; (2) freshwater residents (46 fish, 25.1%), which settled in freshwater and remained until capture; (3) upstream shifters (3 fish, 1.6%), which settled in saline water and moved upstream into freshwater; (4) downstream shifters (53 fish, 29.0%), which settled in freshwater and moved downstream into saline water; (5) multiple habitat shifters (7 fish, 3.8%), which shifted their habitats between freshwater and saline water more than twice. For eels captured in the brackish water area, fish density decreased with distance in the downstream direction, while the size and age of eels increased. For eels captured in the freshwater area, size and age were greater than those in the upper-most brackish site. These observations suggest that eels in this system initially accumulate in the lower reaches of the river and then disperse in both upstream and downstream directions following their growth.     

Environmental cues and extended estuarine residence in seaward migrating eels (Anguilla australis)

相似文献   

Advanced sexual maturation before marine migration of Anguilla bicolor bicolor and Anguilla marmorata at Réunion Island

Maturing sub-adults of two species of anguillid eels (a female Anguilla bicolor bicolor and a male Anguilla marmorata ) were collected for the first time at Réunion Island, western Indian Ocean. Both were silver eels, i.e. maturing eels at the onset of their spawning migration, and characterized by advanced sexual maturation that has been only observed in Anguilla dieffenbachii from New Zealand.     

Discrimination of New Zealand stream waters by glass eels of Anguilla australis and Anguilla dieffenbachii

This study tested the hypotheses, using glass eels of longfin eels Anguilla dieffenbachii and shortfin eels Anguilla australis migrating into fresh waters, (1) that both species prefer water from their river of collection to well water, (2) that shortfin eels prefer lowland, pastoral stream water to mainstem river water, (3) that longfin eels are attracted to both waters but do not prefer one to the other, and (4) both prefer water scented with geosmin, a widely occurring metabolite of bacteria and algae, to well water. Glass eels of both species from a river on the west coast of South Island, New Zealand, and shortfin glass eels from an east coast river significantly preferred water from their river of capture to well water. Two to three times as many eels chose their own river water as chose well water. Longfin eels were rare in the east coast river. Shortfin glass eels from the two rivers chose lowland stream water to mainstem river water about two to one in three experiments with different pairs of waters to which they had no prior exposure. Longfin glass eels significantly chose mainstem river water over lowland water in one pair but showed no preference when presented with a different pair. Reactions to solutions of geosmin at concentrations of 10^–5-10^–7 mg 1^–1 were inconclusive, with geosmin being preferred significantly, by shortfin eels, in only one experiment. The interspecific differences in discrimination of natural waters demonstrated in this study, with shortfin eels preferring lowland waters and longfin eels more indifferent to water types, are in broad agreement with both the distribution of adults and observations on their habitat preferences.     

Variations in the migratory history of the tropical catadromous eels Anguilla bicolor bicolor and A. bicolor pacifica in south-east Asian waters

Fish movements between aquatic habitats of different salinity ranges (fresh, estuarine, marine) by the tropical catadromous eels Anguilla bicolor bicolor and A. bicolor pacifica were examined by analysing the otolith strontium and calcium concentrations of yellow (immature) and silver (mature) stage eels collected in south-east Asian (Indonesia, Malaysia and Vietnam) waters. The ratios suggest that all migratory-type eels, including freshwater, brackish water and marine residents, pass the river mouth. However, the habitat preference was different among the sites (countries). In Indonesia and Vietnam, most A. bicolor bicolor and A. bicolor pacifica were either marine or brackish water residents in this study. Alternatively, most A. bicolor bicolor were freshwater residents in Malaysia; such a typical catadromous migration pattern in these eels has not been found in previous studies. The wide range of otolith Sr:Ca in both subspecies indicates that the habitat use of these tropical eels was opportunistic among fresh, brackish and marine waters during their growth phases following recruitment to coastal areas. The geographical variability of migratory histories suggests that habitat use might be determined by the inter and intraspecific competition and environmental conditions at each site.     

Heterogeneity within the native range: population genetic analyses of sympatric invasive and noninvasive clades of the freshwater invading copepod Eurytemora affinis

Invasive species are often composed of highly differentiated populations or sibling species distributed across their native ranges. This study analysed patterns of distribution and the evolutionary and demographic histories of populations within the native range of the copepod species complex Eurytemora affinis. Genetic structure was analysed for samples from 17 locations from both the invaded and native ranges in the St Lawrence River drainage basin, using 652 base pairs of the mitochondrial cytochrome oxidase subunit I gene. This study revealed a high degree of heterogeneity in genetic structure and habitat type in the native range, as well as a bias in the sources of invasive populations. Two genetically distinct clades showed a pattern of niche partitioning within the St Lawrence basin. The noninvasive North Atlantic clade primarily occupied the central portion of the St Lawrence Middle Estuary, whereas the invasive Atlantic clade was more prevalent along the margins, in the upstream reaches of the estuary and downstream salt marshes. Habitat partitioning and genetic subdivision was also present within the Atlantic clade. The freshwater populations were genetically more proximate to the Atlantic clade populations in the estuary than to those in the salt marsh, suggesting the estuary as the source of the invasive populations. The freshwater invading populations showed evidence of a modest population bottleneck. Populations from both clades showed genetic signatures of demographic population expansions that preceded the timing of the last glacial maximum, supporting the St Lawrence as a secondary contact zone between the two clades. Additional analyses on physiological and evolutionary properties of populations in the native range, along with analysis of the selection regime within native habitats, might yield insights into the evolutionary potential to invade.     

Evidence of silver eels contamination by microcystin‐LR at the onset of their seaward migration: what consequences for breeding potential?

Thirty migrating silver eels Anguilla anguilla were collected in a river system where algal blooms occurred yearly. Fifty per cent of eel livers were contaminated by microcystin-LR (mean ± s . d . toxin level: 28·1 ± 22·4 ng g⁻¹). Contaminated silver ( v. healthy) eels had lower fish condition. Consequences of this impact for the breeding potential of these migrating eels are discussed.     

Diving activity of migrating silver eel with and without Anguillicola crassus infection

Infection with the swim bladder nematode Anguillicola crassus has been hypothesised to threaten the spawning migration success of the endangered European eel (Anguilla anguilla). To examine this assumption, we compared the swimming behaviour of one Anguillicola crassus infested eel in the North Sea and three parasite‐free eels in the Baltic using data recovered from data storage tags attached to migrating silver eels. In both areas, eel activity was characterized by frequent diving behaviour throughout the water column during the night, with reduced activity during the day. Despite substantial damage of the swim bladder, the behaviour of the infested eel from the North Sea was within the same range of migrating and diving activity parameters as the three parasite‐free eels from the Baltic Sea. All eels had a similar frequency distribution of descent or ascent speeds and a similar average horizontal migration speed. The diving speeds and dive ranges exclude the possibility that the eels were in continuous hydrostatic equilibrium during their migrations and suggests therefore that the role of the swim bladder in vertical migration is likely to be more complex than currently thought. Our results suggest that eels infested by Anguillicola crassus are capable of diving in a similar manner to uninfested eels during the first stretch of their spawning migration.     

Ecological Aspects of the Downstream Migration of Introduced European Eels in the Uono River, Japan

We examined the species composition, timing of downstream migration, and biological characteristics of eels using catches at three commercial weirs from 1996 to 1998 in the Uono River, Niigata Prefecture, Japan, which is located farther north in the Japan Sea than where most Japanese eels, Anguilla japonica, recruit. Analyses of a sub-sample of the 292 eels caught in the weirs found that 93.6% were introduced European eels, Anguilla anguilla, that were sexually maturing silver phase eels. Their average age based on otolith annuli was 10.2 years, indicating a relatively high average growth rate of 6.3 cm year^–1. Catch records in 1996 and 1997 indicated that downstream migration occurred sporadically from the middle of August to the end of November and that catches generally coincided with abrupt increases in water discharge and drops in water temperature. The highest catches in both years occurred between the last quarter and new moon. These findings were similar to studies on this species in Europe and indicate that A. anguilla can grow rapidly, begin maturation, and start downstream migration far from its native range. This discovery of introduced eels initiating their spawning migration at the same time as A. japonica raises concerns about the potential impact of interbreeding between species and the possible effects on the fishery resources of A. japonica.