Yellow Eel Stage Research Articles

The expression profiles of cyp19a1, sf-1, esrs and gths in the brain-pituitary during gonadal sex differentiation in juvenile Japanese eels

Eels are gonochoristic species whose gonadal differentiation initiates at the yellow eel stage and is influenced by environmental factors. We revealed some sex-related genes were sex dimorphically expressed in gonads during gonadal sex differentiation of Japanese eel (Anguilla japonica); however, the expression of sex-related genes in the brain-pituitary during gonadal sex differentiation in eels is still unclear. This study aimed to investigate the sex-related gene expressions in the brain-pituitary and tried to clarify their roles in the brain and gonads during gonadal sex differentiation. Based on our previous histological study, the control eels developed as males, and estradiol-17β (E2) was used for feminization. Our results showed that during testicular differentiation, the brain cyp19a1 transcripts and aromatase proteins were increased significantly; moreover, the cyp19a1, sf-1, foxl2s, and esrs (except gperb) transcripts in the midbrain/pituitary also were increased significantly. Forebrain gnrh1 transcripts increased slightly during gonadal differentiation of both sexes, but the gnrhr1b and gnrhr2 transcripts in the midbrain/pituitary were stable during gonadal differentiation. The expression levels of gths and gh in the midbrain/pituitary were significantly increased during testicular differentiation and were much higher in males than in E2-feminized females. These results implied that endogenous estrogens might play essential roles in the brain/pituitary during testicular differentiation, sf-1, foxl2s, and esrs may have roles in cyp19a1 regulation in the midbrain/pituitary of Japanese eels. For the GnRH-GTH axis, gths, especially fshb, may be regulated by esrs and involved in regulating testicular differentiation and development in Japanese eels.

A Comment on: Migrating silver eels return from the sea to the river of origin after a false start (2021) by Tambets M et al.

A Comment on: Migrating silver eels return from the sea to the river of origin after a false start (2021) by Tambets M et al.

Ontogenetic Changes in Body Shape and the Scaling of Metabolic Rate in the American Eel (Anguilla rostrata).

AbstractThe body mass (M) scaling of resting metabolic rate (RMR) may vary significantly throughout ontogeny for multiple reasons that are not perfectly understood. To compare two major geometric theories of metabolic scaling, surface area (SA) theory and resource transport network (RTN) theory, we tested whether ontogenetic shifts in metabolic scaling relate to changes in body shape in the American eel (Anguilla rostrata). To do so, we compared the log-linear scaling exponents of RMR to M (bR) and M to body length (bL) in juvenile and subadult eels (glass and yellow eel life stages, respectively). Glass eels exhibited a and bR significantly <2/3, as predicted by SA theory. Yellow eels also had a , but their bR was not significantly different from 2/3 or 3/4. We hypothesize that two developmental changes contribute to bR being higher for yellow eels: (1) a greater reliance on branchial respiration than body-surface-dependent cutaneous respiration and (2) a lower rate of thickening during subadult growth. An ontogenetic decrease in the ratio of cutaneous to gill respiration may have increased the relative importance of the physical constraints of a single-pump, closed circulatory system on the body-size-dependent rate of resource supply to metabolizing tissues (as predicted by RTN theory) in subadult eels. Future research is needed to quantify these developmental changes and their potential mechanistic effects on metabolic scaling, especially in the elver, a critical life stage between the glass and yellow eel stages, that was not analyzed in this study.

Developmental features of Japanese eels, Anguilla japonica, from the late leptocephalus to the yellow eel stages: an early metamorphosis to the eel-like form and a prolonged transition to the juvenile.

Organogenesis of Japanese eels (Anguilla japonica) was investigated histologically from the late leptocephalus to the yellow eel stages. Early organogenesis, such as the formation of inner ears and the appearance of round blood cells that might be larval erythrocytes, had already begun at the late leptocephalus stage. During the first developmental phase (M1-M3 stages) of metamorphosing into early glass eels (G1 stage), the formation of gills and lateral muscles progressed conspicuously with a drastic body shape change from leaf-like to eel-like. In contrast, obvious regression in oesophageal muscle and pancreas occurred during metamorphosis. Formation of lateral line canals advanced continuously until the yellow eel stage. When the second developmental phase was initiated at the G1 stage, cone photoreceptor cells appeared, and the formation of oesophageal, stomach and intestinal muscles was initiated. Differentiation of gastric glands began at 1 week after metamorphosis. Erythrocytes increased continuously in density in glass eels and elvers (G1-E2 stages), and the morphological features of cone cells and olfactory epidermal cells became clearer with stage progression. In early elvers (E1 stage), the swimbladder initiated inflation, the stomach fully expanded and the rectal longitudinal fold changed to a circle. Swimbladder gas glands appeared in late elvers (E2 stage). In the yellow eels (juvenile stage), almost all organ structures were formed. These observations indicate that the organogenesis of A. japonica is ongoing after metamorphosis into glass eels, and the M1-E2 stages are considered to be a homologous phase to first metamorphosis, which is a transformation from the larval to the juvenile stages in other teleosts. In comparison to conger eels, the completion of the body shape change to eel-like occurs at the G1 stage, when organogenesis is still in progress, being followed by a prolonged duration of the G1-E2 stages before reaching the yellow eel juvenile stage, which may be a unique characteristic that is related to the early migratory life history of A. japonica.

From yellow to silver: Transforming cranial morphology in European eel (Anguilla anguilla).

The European eel (Anguilla anguilla) has been extensively studied, especially because of its highly specialized migratory behaviour associated with substantial phenotypic transformations. During this migration, one of those transformations the eel undergoes is from yellow to silver eel, a process known as silvering. Although the cranial morphology during the earlier glass, elver and yellow eel stages are well studied, little is known about actual morphological changes during the transformation process from the yellow to the silver eel stage. Yet, literature suggests drastic changes in musculoskeletal anatomy. Here, we investigated the cranial musculoskeletal morphology of 11 male European eels at different stages during silvering, resulting both from natural and artificial maturation. Using 3D-reconstructed µCT data of the head, the skull and cranial muscles associated with jaw closing and respiration were studied. Eye size was used as a proxy for the silvering stage. Size-adjusted jaw muscle volumes increased during silvering, although insignificantly. Accordingly, a near-significant increase in bite force was observed. Respiratory muscles size did increase significantly during silvering, however. Considering the eel's long migration, which often includes deep and thus potentially oxygen-poor environments, having a better performing respiratory system may facilitate efficient migration. Both overall skull dimensions and specifically orbit size increased with eye index, suggesting they play a role in accommodating the enlarging eyes during silvering. Finally, artificially matured eels had a wider and taller skull, as well as larger jaw muscles than wild silver eels. This could be caused (a) by different conditions experienced during the yellow eel stage, which are maintained in the silver eel stage, (b) by side effects of hormonal injections or (c) be part of the maturation process as artificially induced silver eels had a higher eye index than the wild silver eels.

Salinity and odor preferences of Japanese eel during the first year of post-recruitment growth in saline water

Freshwater eels grow both as residents of estuaries, spending the yellow eel stage in nearshore saline waters, and as residents of freshwater in rivers. In order to understand how Japanese eels diverge between estuarine and river residence, we investigated their water preferences during the first year of growth in saline water after their recruitment into growth habitats by using Y-maze behavioral trials. In the water-choice trials between natural seawater and river water, glass eels and pigmented elvers showed a significant preference for river water, while small yellow eels showed a significant preference for seawater. Subsequent experiments examining odor and salinity preferences of the eels showed that a preference for riverine odor was maintained during the growth phase. However, as the eels developed into the yellow eel stage in natural seawater, their salinity preference shifted from no preference, or a preference for freshwater, to a preference for saline water. These results suggest that Japanese eels disperse over a wide range of salinities until the elver stage, as long as water has a rich river-originated odor. When individuals grow in saline water until reaching the yellow eel stage, they are likely to become estuary residents.

Dimorphism throughout the European eel's life cycle: are ontogenetic changes in head shape related to dietary differences?

A well-known link exists between an organism's ecology and morphology. In the European eel, a dimorphic head has been linked to differences in feeding ecology, with broad-headed eels consuming harder prey items than narrow-headed ones. Consequently, we hypothesized that broad-heads should exhibit a cranial musculoskeletal system that increases bite force and facilitates the consumption of harder prey. Using 3D-reconstructions and a bite model, we tested this hypothesis in two life stages: the sub-adult yellow eel stage and its predecessor, the elver eel stage. This allowed us to test whether broad- and narrow-headed phenotypes show similar trait differences in both life stages and whether the dimorphism becomes more pronounced during ontogeny. We show that broad-headed eels in both stages have larger jaw muscles and a taller coronoid, which are associated with higher bite forces. This increased bite force together with the elongated upper and lower jaws in broad-headed eels can also improve grip during spinning behavior, which is used to manipulate hard prey. Head shape variation in European eel is therefore associated with musculoskeletal variation that can be linked to feeding ecology. However, although differences in muscle volume become more pronounced during ontogeny, this was not the case for skeletal features.

Head shape disparity impacts pollutant accumulation in European eel

Several aspects of the life cycle of the critically endangered European eel (Anguilla anguilla) remain poorly understood. One such aspect is the broad-versus narrow-head dimorphism, and how this impacts their overall performance at different stages of their life cycle. At the yellow eel stage, the phenotypes show a trophic divergence. We investigated whether pollutant accumulation is affected by this disparity. We show that broad-headed eels contained higher concentrations of mercury and several lipophilic organic pollutants, compared to narrow-headed ones, irrespective of their fat content. The hereby confirmed link between the phenotypic disparity, its associated feeding ecology and its impact on pollutant accumulation thus raises further concerns about their migratory and reproductive success. Considering that pollution is an important contributor to the European eel's decline, our results demonstrate that broad-headed eels are more vulnerable to detrimental pollutant accumulation. This compromises their successful contribution to their population's reproduction and its restoration.

Built to bite? Differences in cranial morphology and bite performance between narrow- and broad-headed European glass eels.

The presence of two phenotypes in a single species is a widespread phenomenon, also observed in European eel (Anguilla anguilla). This dimorphism has been related to dietary differences in the subadult elver and yellow eel stages, with broad-heads generally feeding on harder and/or larger-bodied prey items than narrow-heads. Nevertheless, both broad- and narrow-headed phenotypes can already be found among glass eels, the stage preceding the elver eel stage. As these glass eels are considered nonfeeding, we investigate here to what degree the observed variation in head width is reflected in variation in the musculoskeletal feeding system, as well as whether this reflects the same variation observed in the older, dimorphic yellow eels. Additionally, we investigate whether musculoskeletal differences between broad- and narrow-headed glass eels have implications on their feeding performance and could thus impact prey preference when eels start feeding. Therefore, we compared the cranial musculoskeletal system of five broad- and narrow-headed glass eels using 3D-reconstructions and simulated the glass eel's bite force using the data of the muscle reconstructions. We found that the variation in the musculoskeletal system of glass eels indeed reflects that of the yellow eels. Broader heads were related to larger jaw muscles, responsible for mouth closure. Accordingly, broad-heads could generate higher bite forces than narrow-headed glass eels. In addition, broader heads were associated with higher coronoid processes and shorter hyomandibulae, beneficial for dealing with higher mechanical loadings and consequently, harder prey. We, thus, show that head width variation in glass eels is related to musculoskeletal differences which, in turn, can affect feeding performance. As such, differences in prey preference can already take place the moment the eels start feeding, potentially leading to the dimorphism observed in the elver and yellow eel stage.

Are Lithuanian eels fat enough to reach the spawning grounds?

Stocks of the European eel Anguilla anguilla have been in a steep decline since the 1980s. Stocking of water bodies with juvenile eels captured in the wild to establish or enhance local populations has been a common practise in Europe for many decades. However, the degree of contribution by stocked eels to natural spawning capacity is poorly known and extensively debated. There have been suggestions that eels derived from stocking are less likely to contribute to the spawning stock due to a lack of navigational capability and lower fitness related to insufficiency of energetic resources. Results of the current study indicated that eels translocated long distances from the point of capture and released into inland waters in Lithuania are successfully undergoing the silvering process. A proportion of 23.7% (N = 27) among all migrating eels were described to be at the yellow (SI, SFII or SFIII) eel stage and downstream movements of these eels should be attributed to local movements, rather than spawning migration; 76.3% were assigned to the silver eel stage. This study suggests that 36.8% (N = 32) of downstream migrating silver eels of stocked origin had accumulated sufficient energetic resources for spawning migration and gonadal development and should be able to traverse the 7900-km distance to the presumptive spawning grounds in the Sargasso Sea. The rest of migrating silver eels (63.2%, N = 55) had insufficient energetic resources; the average potential swimming range of these eels was estimated to be 6135 ± 683 km.

Differential gene expression in narrow- and broad-headed European glass eels (Anguilla anguilla) points to a transcriptomic link of head shape dimorphism with growth rate and chemotaxis.

One of the major challenges in evolutionary biology is to understand the mechanisms underlying morphological dimorphism and plasticity, including the genomic basis of traits and links to ecology. At the yellow eel stage of the European eel (Anguilla anguilla), two morphotypes are found: broad- and narrow-heads. This dimorphism has been linked to dietary differences, with broad-heads feeding on harder, larger prey than narrow-heads. However, recent research showed that both morphotypes could be distinguished at the glass eel stage, the nonfeeding predecessor of the yellow eel stage, implying that nondietary factors play a role in the development of this head shape dimorphism. Here, we used transcriptome profiling (RNAseq) to identify differentially expressed genes between broad- and narrow-headed glass eels. We found 260 significantly differentially expressed genes between the morphotypes, of which most were related to defence and immune responses. Interestingly, two genes involved in growth (soma and igf2) were significantly upregulated in narrow-heads, while nine genes involved in chemotaxis showed significant differential expression. Thus, we found support for the observation that head shape is associated with somatic growth, with fast-growing eels developing a narrower head. Additionally, observations in the wild have shown that slow-growers prefer freshwater, while fast-growers prefer brackish water. The differential expression of genes involved in chemotaxis seems to indicate that glass eel growth rate and habitat choice are linked. We hypothesize that two levels of segregation could take place in the European eel: first according to habitat choice and second according to feeding preference.

Assessment of the Entering Stock, Migration Dynamics and Fish Pass Fidelity of European Eel in the Belgian Meuse River

AbstractMigration dynamics of incoming eels in Belgium via Lixhe in the Meuse River were investigated using two fish passes with different configurations—net traps and automatic detection stations—as tools to distinguish resident and migrating eels. From April to September 2013, 435 eels (P50 length, 403 mm; range, 196–836 mm) were caught (daily maxima catch, 90 eels per day), 90% between 13 June and 1 August (50 days) and P50 on 19 July. Eels migrated mostly at 19–26 °C (P50, 24.4 °C), river discharge 65–314 m3 s−1 (P50, 84 m3 s−1), during the dark at 00:00–05:00 h and during both the waxing and waning phases of moonlight. From 396 eels tagged and released 0.3 km downstream of the Lixhe dam, 6.8% of them were recaptured, and 37.4% were detected. Migration flux was estimated at 7184 eels (0.863 t) using the mark‐recapture method and decreased to 1156 eels (0.139 t) using automatic transponder detection. Most eels probably migrated through a sluice located downstream of Lixhe to reach the upper Meuse via the Albert Canal. Eels moved almost independently to the configuration of the fish passes and their location, but most eels displayed fidelity to the fish pass where they were captured. Migrant eels showed a wide range of size and life stages, with a higher proportion of eels (80%) belonging to the yellow eel stage. A lower proportion of eels (6%) had a larger size and presented an advanced continental silvering process corresponding to the migrating stage before their transatlantic migration. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Head shape dimorphism in European glass eels (Anguilla anguilla)

The life cycle of the European eel (Anguilla anguilla) remained a mystery until the 20th century, when Schmidt discovered that the Sargasso Sea was its spawning area. However, many aspects of the eel's life cycle remain poorly understood. Among these is the bimodal distribution in head shape, with broad- and narrowheaded phenotypes reported in the yellow eel stage. Although this has been linked to dietary preferences of the yellow eels, very little is known about why, how and when this dimorphism arises during their ontogeny. To determine whether this dimorphism indeed appears in relation to trophic niche segregation, we examined head shape variation at an earlier ontogenetic stage, the glass eel stage, as at this stage eels are considered to be non-feeding. Head shape was studied in a large dataset, containing glass eels captured from the Yser river mouth, the Leopold Canal (Belgium) and from the rivers Severn, Trent and Parret (UK), by both taking measurements (head width/head length) and using an outline analysis. Our results show that there is already considerable variation in broadness and bluntness of the head at the glass eel stage. In most cases, equal support for a unimodal and bimodal head shape distribution is found, whereas some cases support head shape bimodality in glass eels, suggesting that glass eel head shape might be shifting from a unimodal to a bimodal distribution. This, in combination with the observation that variation in head width/head length ratios in non-feeding glass eels shows a similar range as in feeding yellow eels, indicates that head shape in European eel might be at least partially determined through other mechanisms than trophic segregation.

Gonadal transcriptome analysis of wild contaminated female European eels during artificial gonad maturation

Since the early 1980s, the population of European eels (Anguilla anguilla) has dramatically declined. Nowadays, the European eel is listed on the red list of threatened species (IUCN Red List) and is considered as critically endangered of extinction. Pollution is one of the putative causes for the collapse of this species. Among their possible effects, contaminants gradually accumulated in eels during their somatic growth phase (yellow eel stage) would be remobilized during their reproductive migration leading to potential toxic events in gonads. The aim of this study was to investigate the effects of organic and inorganic contaminants on the gonad development of wild female silver eels. Female silver eels from two sites with differing contamination levels were artificially matured. Transcriptomic analyses by means of a 1000 candidate gene cDNA microarray were performed on gonads after 11weeks of maturation to get insight into the mechanisms of toxicity of contaminants. The transcription levels of several genes, that were associated to the gonadosomatic index (GSI), were involved in mitotic cell division but also in gametogenesis. Genes associated to contaminants were mainly involved in the mechanisms of protection against oxidative stress, in DNA repair, in the purinergic signaling pathway and in steroidogenesis, suggesting an impairment of gonad development in eels from the polluted site. This was in agreement with the fact that eels from the reference site showed a higher gonad growth in comparison to contaminated fish.

Occurrence of larval and adult types of ion-secreting ionocytes in Japanese eel Anguilla japonica

The anguillid eels are catadromous fishes that migrate between marine and freshwater habitats. The long migration of eel larvae, called leptocephali, as long as thousands of kilometers in the ocean is important to determine their recruitment successes. The leptocephali in the ocean have a pelagic lifestyle totally different from the benthic one of glass eels and yellow eels in rivers. It is known that eel leptocephali have ionocytes on the body surface that may maintain ionic and osmotic status in the internal environment; however, detailed morphology and function of ionocytes in leptocephali are still unknown. In the present study, we aimed 1) to clarify the morphological features of the epidermis in Japanese eel Anguilla japonica leptocephali cultured in hyper-osmotic condition, and 2) to examine the ion-transporting functions of ionocytes of both leptocephali and yellow eels. Na+/K+-ATPase-immunoreactive ionocytes were distributed all over the body surface of leptocephali. Ionocytes were in contact with external environments through their apical membrane, which was located at the boundary of pavement cells. Na+/K+-ATPase-immunopositive cells were not observed in the skin of seawater-acclimated yellow eels. In ionocytes of the larval skin, the apical membrane appeared as a slightly projecting disk with a microvilli-like structure. Meanwhile, the apical membrane of gill ionocytes of yellow eels formed a concave surface. In ionocytes of leptocephali, mitochondria were enlarged and the tubular system was well developed, as compared with those of the gill of yellow eels. Ionocytes of leptocephali showed CFTR immunoreaction in their apical region and NKCC1 immunoreaction in their basolateral region, suggesting that the skin ionocytes are involved in salt secretion. These results support the notion that Japanese eel maintain their ion balance through skin ionocytes during early life stages, and that the skin ionocytes of leptocephali disappear in yellow eel stages after the formation of functional gills and gill ionocytes.

Growth, Female Size, and Sex Ratio Variability in American Eel of Different Origins in Both Controlled Conditions and the Wild: Implications for Stocking Programs

AbstractFreshwater eels Anguilla spp. are declining worldwide, and a major challenge is understanding why these panmictic species show contrasting patterns of intraspecific phenotypic variation and recruitment. We present results on studies of the American Eel A. rostrata to understand and discriminate the effects of origin and plasticity on growth and sex determination. We considered two separate growth and one length‐at‐age data sets. The first growth data set originated from a 34‐month rearing experiment starting from the glass eel life stage to test the effects of origin, salinity, and density on growth and sex determination. The second growth data set originated from a shorter rearing experiment of 18 months starting at the yellow eel stage (around 3 years old) and compared transplanted individuals in Lake Ontario (LO) with natural migrants to the LO area. The third data set compared transplanted individuals in LO sampled by electrofishing with naturally migrating individuals. Sex ratios were identical for all origins and treatments in the long‐term growth experiment (34–35% females). While male size distribution had little variability, certain female groups had a large variation in growth and presented fast‐ and slow‐growing clusters. On the other hand, both cases of natural migrants to the LO area were consistent with being only slow‐growing females. We found that wild individuals rearing in the LO area were nearly exclusively transplanted individuals and that males, as well as fast‐growing females, were present. Even though the entire species is panmictic, these results support a role for spatially varying selection in explaining the phenotypic variation observed among regions and among individuals of the same region, and such factors must be considered for any successful management strategies of American Eel.Received December 17, 2013; accepted September 29, 2014

Migratory history of African longfinned eel Anguilla mossambica from Maningory River, Madagascar: discovery of a unique pattern in otolith Sr:Ca ratios

The migratory life histories were investigated for 30 African longfinned eels (Anguilla mossambica) of the yellow eel stage from the Maningory River, East Madagascar using otolith Sr:Ca ratios and parasite composition. The Sr:Ca ratio patterns of nine eels resembled those of A. mossambica in South Africa and of other anguillids and were classified as “typical” Sr:Ca ratio pattern eels. Typical pattern eels were further classified into freshwater residents (five eels) and habitat shifters (four eels) depending on whether they spent their first several years in saltwater. An unique Sr:Ca ratio pattern was found in 21 eels whereby the Sr:Ca ratios in the yellow eel stage increased drastically to extremely high values of 10 to 30 × 10−3 which have not been observed for any Elopomorpha species. These individuals were categorized as “elevated” Sr:Ca ratio pattern eels. The elevated pattern eels were characterized by significantly older ages, lower somatic growth rates, a much higher intensity and abundance of the gastrointestinal parasite Heliconema africanum and a lower abundance of the swim bladder parasite Anguillicoloides papernai. The extremely elevated Sr:Ca ratios of the yellow eel stage result most likely from inhabiting freshwater areas characterized by a high strontium concentration because the ratios far exceeded the known values for Elopomorpha species. Cyclical lows in the otolith Sr:Ca ratios of the elevated type eels corresponded to annuli, revealing either an overwintering movement pattern between habitats with different Sr:Ca ratios, or a seasonal change in the ambient environment.

Evaluation of the pigmentation stages and body proportions from the glass eel to yellow eel in Anguilla japonica

The pigmentation stages of Anguilla japonica were classified from the glass eel to yellow eel stage using 412 wild-caught eels from Hamana Lake and detailed developmental observations of 10 laboratory-reared eels. The sequential appearance of pigment was similar to that in A. anguilla, except for delayed pigment formation on the nerve cord of A. japonica. The general classification of pigmentation stages (VA–VIB) in A. anguilla was applicable to A. japonica, but the VB stage could be separated into two stages (VB1, VB2) in A. japonica. The completion of guanine deposition on the intra-abdominal membrane may be a trait to discriminate the yellow eel stage (VII) from the VIB stage. In natural conditions, wild-caught 0-age eels (n = 3,298) did not increase in size and retained a slender body form until VIA4, and then became thicker during VIB. The transition between body forms and growth may correspond to their ecological change from using passive transport in glass eels with tidal flow into rivers, to their settlement and further movements of elvers upstream. Considering this behavioral transition and pigmentation progression, it is proposed that the terms "glass eel" and "elver" could be defined as stages VA–VIA4 and VIB, respectively.

Regional variation in otolith Sr:Ca ratios of African longfinned eel Anguilla mossambica and mottled eel Anguilla marmorata: a challenge to the classic tool for reconstructing migratory histories of fishes

Otolith Sr:Ca ratios of the African longfinned eel Anguilla mossambica and giant mottled eel Anguilla marmorata from nine freshwater sites in four rivers of South Africa were analysed to reconstruct their migratory life histories between freshwater and saltwater habitats. For A. mossambica, the Sr:Ca ratios in the otolith edge differed significantly among rivers and had large effect sizes, but did not differ among sites within a river. Otolith Sr:Ca ratios did not differ among rivers for A. marmorata. When rivers were pooled, the edge Sr:Ca ratios of A. mossambica were not significantly different from those of A. marmorata. According to the river-specific critical Sr:Ca ratio distinguishing freshwater from saltwater residence, most A. mossambica and A. marmorata had saltwater habitat experience after settlement in fresh water. This was primarily during their elver stage or early in the yellow eel stage. During the middle and late yellow eel stage, freshwater residency was preferred and only sporadic visits were made to saltwater habitats. The data also suggest that regional variations in otolith Sr:Ca ratios affect the critical Sr:Ca value and are a challenge for the reconstruction of migratory life histories that should be explicitly considered to avoid bias and uncertainty.

Do otolith annular structures correspond to the first freshwater entry for yellow European eels Anguilla anguilla in the Baltic countries?

To examine the relationship between freshwater entry and otolith annular structures, a total of 113 naturally recruited European eels Anguilla anguilla from Lithuania and Latvia that entered fresh water at least once were collected. In some individuals (8.3-11.3%), the first freshwater entry coincided with a dark check that was distinctly different from neighbouring annuli. In most individuals (81.7-84.9%), the first freshwater entry occurred on rings and increments indistinguishable from other annuli. For the remaining individuals (3.8-10%), the first freshwater entry did not correspond to any otolith ring, band or annulus. According to recent evidence, the observed high correspondence between the first freshwater entry and otolith annuli was more likely due to the movement into fresh water during winter when the annulus was deposited, rather than stress resulting from habitat change. Consequently, the age estimation based on otoliths might be less influenced by this habitat change during the yellow eel stage.