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.     

Evaluation of downward movements of Japanese eel Anguilla japonica inhabiting brackish water areas

This study evaluated the size and age distributions and otolith microchemistry of the Japanese eel Anguilla japonica in freshwater and brackish water areas in the Aki and Tsuchikawa rivers for 1 year, and in brackish water areas in the Asahi River for 3 years to understand the movements of Japanese eels between continental habitats of different salinity after recruitment (n = 759). For all three rivers, the total length (L_T) and age distributions were consistent; yellow eels captured in the upper brackish water (Aki River: 353.5 ± 77.4 mm and 3.0 ± 0.8 years; Tsuchikawa River: 287.7 ± 87.3 mm and 3.7 ± 1.3 years; Asahi River: 418.2 ± 112.1 mm and 4.2 ± 1.7 years) were smaller and younger than not only those in the fresh water of the two rivers but also those in the lowest brackish water sampling areas (Aki River: 436.0 ± 71.6 mm and 3.8 ± 1.1 years; Tsuchikawa River: 370.9 ± 121.7 mm and 4.9 ± 2.3 years; Asahi River: 558.5 ± 85.9 mm and 5.7 ± 1.7 years). In the Asahi River, these tendencies were found throughout the 3 years. Otolith analysis indicated that the majority of the eels captured in the lowest brackish water areas had moved down from upstream. These results suggest that Japanese eels inhabiting saline water generally move from the upper estuary as they grow. The upper estuary can be an important area for the management of this species because these eels spend their early continental growth life there.     

Migration and habitat use of the tropical eels Anguilla marmorata and A. bicolor pacifica in Vietnam

The patterns of use of marine and freshwater habitats by the tropical anguillid eels Anguilla marmorata and A. bicolor pacifica were examined by analysing the otolith strontium (Sr) and calcium (Ca) concentrations of yellow (immature) and silver (mature) stage eels collected in Vietnamese waters. In A. marmorata, the change in the Sr:Ca ratios outside the high Sr:Ca core was generally divided into three patterns: (1) typical catadromous life history pattern; (2) constant residence in brackish water; and (3) habitat shifting between sea and brackish waters with no freshwater life. In A. bicolor pacifica, no eels had a general life history as freshwater residents. The eels were also divided into three patterns: (1) constant residence in sea water; (2) constantly living in brackish water; and (3) habitat shifting from brackish to sea water with no freshwater residence. The mean Sr:Ca ratio value after recruitment to coastal waters ranged from 1.73 to 5.67 × 10^?3 (mean 3.2 × 10^?3) in A. marmorata and from 2.53 to 6.32 × 10^?3 (mean 4.3 × 10^?3) in A. bicolor pacifica. The wide range of otolith Sr:Ca ratios in both species indicated that the habitat use of these tropical eels was facultative among fresh, brackish, and marine waters during their growth phases after recruitment to coastal areas. Tropical eel species may have the same behavioural plasticity as temperate anguillid species regarding whether to enter freshwater or to remain in estuarine and marine environments.     

Stock delineation of pink snapper and tailor from Western Australia by analysis of stable isotope and strontium/calcium ratios in otolith carbonate

The ¹⁸O/¹⁶O and ¹³C/¹²C ratios in the otolith carbonate of pink snapper Pagrus auratus and tailor (bluefish) Pomatomus saltatrix, from several locations along the Western Australian coast, indicated that pink snapper stocks are location specific but that tailor stocks are less so. The hypersaline Shark Bay, on the coast of Western Australia, generated strongly characteristic isotopic signatures which serve as natural tags. Otolith carbonate from pink snapper from normal oceanic waters just north of Shark Bay showed no evidence that the fish had been in hypersaline water. Similarly, pink snapper from the hypersaline bay showed no evidence of having spent time at normal oceanic salinity. By contrast, some tailor from oceanic waters showed evidence of having spent considerable time in the bay, and some fish from the bay had oceanic signatures. This suggested that tailor were more migratory than snapper. The similarity in the distribution of the isotopic signatures (from oceanic to hypersaline) in otolith carbonate from tailor from oceanic waters north of Shark Bay (Koks Island), and from those within Shark Bay, indicated a single stock in this region (in contrast to pink snapper). Moreover, tailor from coastal south-western Australia and from the Shark Bay area could be considered seperately for some management purposes. For pink snapper stocks from oceanic waters, oxygen isotope signatures were clearly related to water temperature although the temperature relationship was obscured for fish within Shark Bay because of the strength of the signal generated by the hypersaline water. For tailor the temperature relationship was not obvious, probably because migrations of tailor smeared the temperature effect, and the hypersaline Shark Bay waters dominated, and, possibly, at the southern extemity of the range, the freshwater in some estuaries influenced the isotopic signatures of the otolith carbonate. Strontium/calcium ratios in pink snapper otoliths also indicated a separation of stocks, but for tailor overlap of signatures again suggested migratory behaviour.     

Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios

Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2·24 × 10⁻³) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3·23 × 10⁻³) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean ±  s.d . age of 5·2 ± 2·1 years. Eels from the inland Lake Baluošai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.     

Application of otolith microchemistry to estimate the migratory history of Japanese eel Anguilla japonica on the Sanriku Coast of Japan

The age and migratory history of the Japanese eel, Anguilla japonica Temminck & Schlegel, collected in Miyako Bay along the Sanriku coast of Japan, was examined using the otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations conducted with wavelength dispersive X‐ray spectrometry by an electron microprobe. The line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (ca. 15–17 × 10^?3) which corresponded with the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, including sea eels that never entered freshwater (average Sr : Ca ratios, ≥6.0 × 10^?3), and others that entered freshwater for brief periods but returned to the estuary or bay. This evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku coast do not necessarily migrate into freshwater rivers during recruitment as do glass eels at the beginning of their growth phase; even those that do enter freshwater may later return to the marine environment. Thus, anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, but rather a facultative catadromy with seawater or estuarine residents as an ecophenotype.     

Decline in non-native freshwater eels in Japan: ecology and future perspectives

The occurrence, distribution, and biological characteristics of non-native freshwater eels were analyzed using 5524 eels collected from 16 sites in Japan between 1997 and 2005. Three hundred seventy-four fishes (6.8%) were identified as non-native European eels, Anguilla anguilla, while the remainder (93.2%) were native Japanese eels, A. japonica. The European eel was found at 7 sites (44%), including 3 rivers, 2 freshwater lakes, one brackish lake, and one sea bay, suggesting a wide rage of habitat use. This variability of habitat use was also evidenced by the otolith microchemistry, which showed that they had lived in not only freshwater but also in seawater habitats. The sites with European eel were localized within the vicinity of southern Japan where a number of these eels were cultivated in the early 1970’s, suggesting that some had escaped from the culture ponds or were released intentionally into nearby natural waters. The large body size (mean total length: 803 mm), pigmented skin, enlarged eyes, and relatively matured gonads (mean gonad somatic index: 1.9) found in non-native European eels indicated that most had metamorphosed into the migratory silver phase, suggesting their ability to initiate spawning migration. However, the proportion of European eels in Mikawa Bay in 1997 was more than 12%, which decreased markedly to less than 2% after 2001, corresponding to the recent decline in import of European glass eels for aquaculture. This suggests that the population of European eels will decrease in Japanese waters in the future.     

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.     

Differences in the Trace Element Deposition in Otoliths Between Marine- and Freshwater-resident Japanese Eels, Anguilla japonica, as Determined by Laser Ablation ICPMS

Synopsis In order to determine whether the trace element composition in otolith of the Japanese eel Anguilla japonica could be used to determine its habitat use, we used laser ablation inductivity coupled plasma mass spectrometry (LA-ICPMS) to assay sectioned otoliths of both marine-resident (sea eels) and freshwater-resident (river eels) eels. A close linear relationship in the Sr:Ca ratios between EPMA (X-ray analysis with an electron microprobe) and LA-ICPMS analyses was found, suggesting that the latter technique could be used to separate the marine and freshwater life phases. Elemental signatures in the otolith outside the elver mark showed significant differences in Cr:Ca, Mn:Ca, and Ba:Ca ratios as well as Sr:Ca ratios between sea and river eels. These results indicate that the elemental compositions may reflect environmental variability between marine and fresh water masses. Thus, those elemental ratios determined by LA-ICPMS analysis seem to have the potential to help distinguish the habitat of the eel.     

Linking isotopic and migratory patterns in a pelagic seabird

The value of stable isotope analysis in tracking animal migrations in marine environments is poorly understood, mainly due to insufficient knowledge of isotopic integration into animal tissues within distinct water masses. We investigated isotopic and moult patterns in Cory’s shearwaters to assess the integration of different stable isotopes into feathers in relation to the birds’ transoceanic movements. Specimens of Mediterranean Cory’s shearwater Calonectris diomedea diomedea caught accidentally by Catalan longliners were collected and the signatures of stable isotopes of C (δ¹³C), N (δ¹⁵N) and S (δ³⁴S) were analysed in 11 wing and two tail feathers from 20 birds, and in some breast feathers. Based on isotopic signatures and moult patterns, the feathers segregated into two groups (breeding and wintering), corresponding to those grown in the Mediterranean or Atlantic regions, respectively. In addition, feathers grown during winter, i.e. moulted in Atlantic waters, were grouped into two isotopically distinct profiles, presumably corresponding to the two main wintering areas previously identified for Mediterranean Cory’s shearwater in tracking studies. N signatures mainly indicated the Mediterranean-to-Atlantic migration, whereas C and S signatures differed according to the Atlantic wintering area. Our results indicate that isotopic signatures from distant oceanic regions can integrate the feathers of a given bird and can indicate the region in which each feather was grown. This study thus underscores how stable isotope analysis can link marine animals to specific breeding and wintering areas, and thereby shed new light on studies involving assignment, migratory connectivity and carry-over effects in the marine environment. Xavier Ruiz deceased 27 April 2008.     

Changes in otolith microchemistry of the Japanese eel, Anguitta japonica, during its migration from the ocean to the rivers of Taiwan

The newly recruited Japanese eel, Anguilla japonica , elvers and 1-year-old eels collected in estuaries and in rivers, respectively, were studied. The microstructure and chemical composition of the sagittal otolith of these eels were examined by SEM and wavelength-dispersive spectrometer (WDS), A transition zone or'elver mark'was observed in the otolith of the young eels. A comparison of the otoliths of elvers with those from the 1-year-old eels suggests that this transition zone was deposited during upstream migration, a change from a marine to freshwater environment. Strontium (Sr) content in the primordium of the otolith of both elvers and young eels was low, probably due to the maternal or freshwater origin of the oocyte. The concentration of Sr in the otolith increased gradually during marine life and reached a peak approximately 1 month before upstream migration. As the elvers entered the estuary, the Src concentration dramatically decreased and remained at a low level thereafter. These findings indicate that the history of the migratory environment of the eel can be reconstructed from a combined study of otolith microstructure and microchemistry analysis.     

Discrimination of French grunts (Haemulon flavolineatum, Desmarest, 1823) from mangrove and coral reef habitats using otolith microchemistry

Using French Grunts (Haemulon flavolineatum) held captive within mangrove and reef sites, we determined (a) whether otolith microchemical differences existed between mangroves and reefs separated at a biologically relevant spatial scale (0.25-7.1 km), (b) whether patterns in elemental concentrations were consistent across years, and (c) whether it was possible to identify whether a given fish occupied a mangrove as a juvenile.Three sites were established at Great Exuma, Bahamas (two reefs and one mangrove, May 2001) and at Turneffe Atoll, Belize (one reef and two mangroves, August 2001 and 2002). Using concentrations of Sr and Ba, discriminant function analysis (DFA) indicated unique spatial microchemical signatures of fish from each of the three Bahamas sites allowing an average correct classification of 77%. Using concentrations of Sr, Ba, Sn and Pb (2001), and in addition Li, Mg, Cu and Rb (2002), DFA of the three Belize sites indicated an average correct classification of 68% and 85% in 2001 and 2002, respectively. To assess temporal variability in otolith microchemistry, we compared microchemical signatures of Belize fish from 2001 to those from 2002. On average, 42% of fish from 2002 were correctly classified to their captive sites using chemical information from 2001, thus suggesting considerable temporal variability in otolith microchemistry. Finally, to identify whether a given fish occupied a mangrove during its juvenile stage, we ablated the juvenile portion of the otolith taken from reefs in Belize 2002. Results of this analysis indicated that 36% of 39 individuals taken from the reef had a signature more representative of one of the mangrove sites. Although otolith microchemistry varied temporally and our analysis was restricted to the grouping of individuals to only one of three sites, mangroves appeared to contribute to reef populations.     

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.     

A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post‐larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5–25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.     

Otolith trace element and stable isotopic compositions differentiate fishes from the Middle Mississippi River, its tributaries, and floodplain lakes

Naturally occurring stable isotope and trace elemental markers in otoliths have emerged as powerful tools for determining natal origins and environmental history of fishes in a variety of marine and freshwater environments. However, few studies have examined the applicability of this technique in large river-floodplain ecosystems. This study evaluated otolith microchemistry and stable isotopic composition as tools for determining environmental history of fishes in the Middle Mississippi River, its tributaries, and floodplain lakes in Illinois and Missouri, USA. Fishes were collected from 14 sites and water samples obtained from 16 sites during summer and fall 2006 and spring 2007. Otolith and water samples were analyzed for stable oxygen isotopic composition (δ¹⁸O) and concentrations of a suite of trace elements; otoliths were also analyzed for carbon isotopic composition (δ¹³C). Tributaries, floodplain lakes, and the Mississippi and Lower Missouri Rivers possessed distinct isotopic and elemental signatures that were reflected in fish otoliths. Fish from tributaries on the Missouri and Illinois sides of the middle Mississippi River could also be distinguished from one another by their elemental and isotopic fingerprints. Linear discriminant function analysis of otolith chemical signatures indicated that fish could be classified back to their environment of capture (Mississippi River, floodplain lake, tributary on the Illinois or Missouri side of the Mississippi River, or lower Missouri River) with 71–100% accuracy. This study demonstrates the potential applicability of otolith microchemistry and stable isotope analyses to determine natal origins and describe environmental history of fishes in the Middle Mississippi River, its tributaries, and floodplain lakes. The ability to reconstruct environmental history of individual fish using naturally occurring isotopic markers in otoliths may also facilitate efforts to quantify nutrient and energy subsidies to the Mississippi River provided by fishes that emigrate from floodplain lakes or tributaries.     

Profiles of mRNA expression for prolactin,growth hormone,and somatolactin in Japanese eels,Anguilla japonica: The effect of salinity,silvering and seasonal change

For understanding the functions of the growth hormone (GH)/prolactin (PRL)/somatolactin (SL) family of hormones, we examined pituitary mRNA expression of these hormones in anguillid eels in relation to salinity difference, silvering, and seasonal change. Female Japanese eels (Anguilla japonica) were collected in the brackish Hamana Lake and its freshwater rivers from July to December. To clarify the effect of salinity, the habitat use history of the eels were determined using otolith microchemistry. Expression levels of mRNA of each hormone were determined using real time PCR. Although GH and PRL have been known to be osmoregulatory hormones, there were no consistent differences in expression levels of these hormones between different salinity habitats. In contrast, SL mRNA expression was higher in eels from freshwater rivers than from the brackish lake. GH mRNA expression clearly decreased during silvering, whereas PRL and SL mRNA expression did not change. We also showed that PRL mRNA and SL mRNA decreased in the brackish lake and PRL mRNA increased in freshwater rivers from autumn to early winter. These findings provide basic knowledge for a further understanding of the role of these hormones.     

Influence of host habitat on the occurrence of gill monogeneans Pseudodactylogyrus spp. on wild Japanese eels Anguilla japonica

With technological improvements in otolith microchemical analysis, the flexible use of habitat from coastal marine to fresh waters has been discovered in Japanese eels Anguilla japonica. We examined the occurrence of 3 congeneric gill monogeneans-Pseudodactylogyrus anguillae, P. bini, and P. kamegaii-on wild Japanese eels, in relation to the host's flexibility. From April 2008 to October 2009, 114 eels were collected from a brackish-water cove and 2 rivers flowing into the cove in Ehime Prefecture, western Japan. Based on otolith microchemical analysis, the eels were discriminated according to the following 4 types of habitat use: freshwater residents (Type I), individuals utilizing low-salinity habitats (Type II), downstream habitat-shifters (Type III), and cove residents (Type IV). P. anguillae occurred mainly on Type I and II eels, while P. bini was primarily found on Type I eels. In contrast, P. kamegaii occurred mainly on Type III and IV eels. Thus, we conclude that species composition and infection levels of Pseudodactylogyrus spp. clearly differed with habitat-use patterns of Japanese eels. Also, since P. anguillae was scarcely found on either Type III or IV eels, this study suggests that previous identifications of monogeneans collected from European brackish-water localities as P. anguillae may require verification.     

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.     

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.     

Stable isotope analyses provide new insights into ecological plasticity in a mixohaline population of European eel

Recent studies have shown that anguillid eel populations in habitats spanning the marine–freshwater ecotone can display extreme plasticity in the range of catadromy expressed by individual fishes. Carbon and nitrogen stable isotope analysis was used to differentiate between European eels (Anguilla anguilla) collected along a short (2 km) salinity gradient ranging from <1‰ to ~30‰ in Lough Ahalia, a tidal Atlantic lake system. Significant differences were recorded in mean δ¹³C, δ¹⁵N and C:N values from eels collected from fresh, brackish and marine-dominated basins. A discriminant analysis using these predictor variables correctly classified ca. 85% of eels to salinity zone, allowing eels to be classified as freshwater (FW), brackish (BW) or marine (MW) residents. The results of the discriminant analysis also suggested that a significant proportion of eels moved between habitats (especially between FW and BW). Comparisons of several key population parameters showed significant variation between eels resident in different salinity zones. Mean condition and estimated age was significantly lower in MW eels, whilst observed length at age (a correlate of growth) was significantly higher in MW eels, intermediate in BW and lowest in FW eels. This study has demonstrated that the ecology of eels found along a short salinity gradient can be extremely plastic and that stable isotope analysis has considerable utility in demonstrating intra-population variation in diadromous fishes.