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Alternative life‐history tactics of masu salmon Oncorhynchus masou are well documented. Subsequent to the freshwater parr stage (age ≥ 1 + years), an anadromous form migrates to the sea after smolting, while a resident form matures without seaward migration. In addition to this typical migratory dimorphism, anecdotal reports based on field observations have indicated that some underyearling masu salmon use estuarine waters. However, no empirical evidence indicates saltwater utilisation and subsequent survival in the early parr stage. Here, we used otolith microchemistry to examine whether a portion of masu salmon parr in northern Japan enters coastal habitats. The otolith Sr:Ca ratios of most juveniles collected from six rivers had consistently low values, indicating that masu salmon parr inhabiting these rivers stay only in freshwater. In contrast, in individuals from a steep‐gradient river the Sr:Ca ratios increased at about a 200‐µm distance from the otolith core. These results suggest that some masu salmon parr might use brackish water or sea water temporarily. In addition, three masu salmon parr were found in another steep river where a culvert located only ten metres from the river mouth completely blocked upstream migration for spawning. The Sr:Ca ratios in these fish increased at about >200 µm from the otolith core, indicating the parr had immigrated to the non‐natal river from the sea. Such flexible behaviour at an early life stage may contribute to the spatial expansion of masu salmon, and the movement could moreover help to stabilise its population dynamics.

A Japanese eel Anguilla japonica was collected off Kozushima Island, one of the volcanic Izu Islands in Japan. The external morphology corresponded to a silvering stage (Silvering Index: S1) with a melanized body coloration of the head, and the tip of the lower jaw to the anus. However, the value of the gonado-somatic index of the female eel was low, and the oocytes were not matured (peri-nucleolus stage). Otolith analyses indicated that the individual had exclusively inhabited the sea for five years without entering freshwater. This record is the first collection of silver-phased Japanese eel in the Kuroshio region, which could be on the spawning migration route of the species, although it was not determined if the eel was settling around the Izu Islands or migrating to the spawning ground.

Oxygen isotope ratios of the core region of otoliths were examined in Anguilla japonica glass eels collected from two rivers in Japan to verify the possible temperature and depth layer experienced by these eels when they were at the egg stage in their spawning area. To determine the relationship between otolith δ18O values and water temperature, the otoliths of glass eels reared under four different temperatures (15, 20, 25 and 30 °C) were analyzed. The otolith δ18O values showed an inverse relationship to ambient water temperature. Linear regression of the fractionation between otolith oxygen isotopic ratio from the δ18O of seawater and water temperature produced a precisely determined relationship from 15 to 30 °C: δ18Ootolith,PDB − δ18Oseawater,SMOW = −0.153 × T (°C) + 1.418. The δ18O core,PDB values of the otolith core region of the glass eels from the two locations were −2.53 ± 0.12 and −2.59 ± 0.07 respectively, and could be converted to water temperatures of 26.3 ± 0.8 °C and 26.7 ± 0.4 °C, respectively, using the equation and assuming a seawater δ18Oseawater,SMOW = 0.06‰. The water depth corresponding to these temperatures is ∼150 m in the water column in the spawning area of Japanese eels, which corresponds to the upper-most part of the thermocline and chlorophyll maximum in the vertical hydrographic profile. These results were consistent with the field studies that egg development after the beginning of otolith formation and hatching occurs around the upper-most part of thermocline, suggesting that stable isotope micro-analysis is a powerful method to extrapolate unknown spawning ecology of fishes.

International audience; Seven South Pacific anguillid eel species live from New Guinea to French Polynesia, but their spawning areas and life histories are mostly unknown despite previous sampling surveys. A July–October 2016 research cruise was conducted to study the spawning areas and times, and larval distributions of South Pacific anguillid eels, which included a short 155°E station-line northeast of New Guinea and five long transects (5–25°S, 160°E–140°W) crossing the South Equatorial (SEC) and other currents. This survey collected nearly 4000 anguilliform leptocephali at 179 stations using an Isaacs-Kidd Midwater Trawl accompanied by 104 CTD casts. Based on morphometric observations and DNA sequencing, 74 anguillid leptocephali were collected, which in the southern areas included 29 larvae of six species: Anguilla bicolor pacifica, A. marmorata, A. australis, A. reinhardtii, A. megastoma, and A. obscura (all anguillid species of the region were caught except A. dieffenbachii). Small A. australis (9.0–16.8 mm) and A. reinhardtii (12.4, 12.5 mm) leptocephali were collected south of the Solomon Islands, other A. australis (10.8–12.0 mm) larvae were caught northwest of Fiji along with an A. obscura (20.0 mm) larva, and an A. marmorata (7.8 mm) larva was collected near Samoa. Considering collection sites, larval ages from otolith analysis, and westward SEC drift, multiple spawning locations occurred from south of the Solomon Islands and the Fiji area (16–20 days old larvae) to near Samoa (19 days old larva) during June and July in areas where high-salinity Subtropical Underwater (STUW, ~150 m depth) and the warm, low-salinity surface Fresh Pool were present. Five long hydrographic sections showed the strong Fresh Pool in the west and the STUW formation area in the east.

Research efforts to achieve the production of artificial seedlings of the Japanese eel Anguilla japonica have progressed in recent decades. However, morphological deformities have been frequently observed in reared leptocephali and glass eels. We examined the effect of water current velocities (5.7–8.3 cm/s) on the body size and morphology of reared leptocephali and metamorphosed glass eels. As the current velocity increased, the size of leptocephali became smaller and the occurrence rate of notochord curvature increased. However, even in low velocities, water current had a long-term negative impact on their morphology. Sixty-five percent of metamorphosed glass eels had one of the eight types of vertebral deformities: compression, luxation, fusion, brachyspina, modification, lordosis, kyphosis, or scoliosis. Although their occurrence rate was unrelated to current velocity, there was a tendency for some deformities to be localised in a certain area of the vertebral column. In particular, compression frequently occurred in caudal vertebrae in faster currents. Most vertebral deformities began before the completion of metamorphosis. Therefore, appropriate management of the water current during the leptocephalus stage is important for establishing mass production of morphologically normal glass eels.

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.

Several surveys and studies have examined the Atlantic anguillid eels’ larval distributions, but little is known about their larval growth rates. Otoliths of 17 European eel Anguilla anguilla (8.8–46.0 mm) and 19 American eel Anguilla rostrata (9.8–59.9 mm) leptocephali collected in the Sargasso Sea (25–31°N, 58–70°W) in March and April 2011 were analyzed and their spawning times and larval growth rates were estimated. Ages calculated from the number of otolith increments of European and American eel larvae showed ranges of 10–127 days and 14–233 days, respectively. Linear relationships between age and total length indicated early larval growth rates of 0.31 mm/day for the European eel and 0.35 mm/day for the American eel. This suggested slower growth rates in low temperatures in the Sargasso Sea compared to other anguillid species in the Indo-Pacific, where water temperatures are higher. The back-calculated hatching dates of small leptocephali (8.8–26.7 mm) were in February and March 2011. More American eels hatched in February and more European eels hatched in March. The hatching times of two larger European eel leptocephali (38.7 and 46.0 mm) and a larger American eel leptocephalus (59.9 mm) were back-calculated to November and December 2010 and July 2010, respectively, suggesting hatching times outside of the primary spawning seasons. These novel observations provide important information on the timing of spawning and larval growth characteristics of Atlantic eels, which would benefit from validation by additional otolith studies of leptocephali.

To clarify the role of thyroid function during metamorphosis from leptocephalus to glass eel in the Japanese eel, we examined the histology of the thyroid gland and measured whole-body concentrations of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), and thyroid stimulating hormone β-subunit TSH (TSHβ) mRNA expression levels in five stages of artificially hatched eels (leptocephalus, early-metamorphosis, late-metamorphosis, glass eel, and elver). During metamorphosis, the inner colloid of thyroid follicles showed positive immunoreactivity for T4, and both T4 and T3 levels were significantly increased, whereas a small peak of TSHβ mRNA level was observed at the early-metamorphosis stage. Similarly, TSHβ mRNA levels were highest in the glass eel stage, and then decreased markedly in the elver stage. In contrast to TSHβ mRNA expression, thyroid hormones (both T4 and T3) increased further from the glass eel to elver stages. These results indicated that thyroid function in the Japanese eel was active both during and after metamorphosis. Therefore, the thyrotropic axis may play important roles not only in metamorphosis but also in subsequent inshore or upstream migrations. J. Exp. Zool. 321A: 357–364, 2014. © 2014 Wiley Periodicals, Inc.

Freshwater eels of the Anguillidae are diadromous because they migrate between ocean and freshwater environments, but other anguilliform fishes are generally considered to be strictly marine species. A few marine eels of the Muraenidae and Ophichthidae have occasionally been found in freshwater or estuaries, indicating that anguillids are not the only anguilliform eels that can use freshwater in some parts of the world. The moray eel Gymnothorax polyuranodon is one species that is known to be present in freshwater in the Indo-Pacific, but its life history is unknown. One way to evaluate what types of habitats are used by fishes is to determine the ratio of strontium (Sr) to calcium (Ca) in their otoliths, because this can show if they have used freshwater or saltwater environments. To evaluate the patterns of freshwater use by this unusual species of marine eel, the otolith Sr/Ca ratios of four G. polyuranodon (275–344 mm) caught in a freshwater stream of Fiji were analyzed. The consistently low Sr/Ca values (0–4) indicated upstream movement after settlement and freshwater or estuarine residence of all four individuals. These eels did not appear to have entered freshwater just for a short time period, which is consistent with other reports that this species is present in estuarine and freshwater habitats. This suggests that G. polyuranodon may be a catadromous species of marine eel. The similarities and differences between the life histories of anguillid eels and the few marine eels that have evolved the ability to invade freshwater habitats is discussed in relation to the evolutionary origin of diadromy in anguilliform fishes that originated in the marine environment.

We evaluated the migratory patterns of white-spotted charr in eastern Hokkaido, Japan using: (1) field observation of fish, (2) salinity-recording archival tags, and (3) analysis of otolith Sr and Ca concentrations. Field observations suggest that the majority of anadromous white-spotted charr outmigrate to the sea in spring (April to June) and ascend the river between late summer and autumn (August to November). Salinity records from archival tags revealed a fluctuating pattern consisting of both low and high values within a month, indicating frequent short-term movements between freshwater (including non-natal rivers) and brackish/salt water habitats. Otolith Sr:Ca ratios exhibited a sharp rise at about 40–50% of the otolith radius, which was followed by several spikes consisting of both low and high values, indicating that fish migrated between freshwater and marine environments multiple times during their life history. However, the peaks in Sr:Ca ratios were generally associated with annulus zones on the otolith, indicating that the anadromous migration was generally an annual event.

Anguillid eels have been observed, studied, fished, traded and consumed by humans for centuries. As a consequence, societies have developed various cultural phenomena regarding cuisine, folklore, art, literature, legend. In recent years, however, some populations of eels have been in sharp decline across the globe, some species are now considered to be endangered, and there is an ever increasing need to share knowledge amongst the scientific, management and conservation community to protect these enigmatic fish, to reverse declines and elsewhere to prevent their occurrence. To facilitate this knowledge exchange, a congress session focussed on anguillid eels was held at the sixth World Fisheries Congress in May 2012 at the Edinburgh International Conference Centre. This special issue of Ecology of Freshwater Fish contains a number of the papers presented at the congress session, each focussed on a particular scientific problem. While the scientific study of eels is required to understand and manage eel resources, and to which end the majority of this volume is related, it is also necessary to take the cultural and social aspects of eels into account when considering how to ensure that this fascinating family of fishes has a sustainable future.

A new anguillid eel species, Anguilla luzonensis, was recently discovered on Luzon Island of the northern Philippines, but little is known about its life history. DNA identification was used to determine that five leptocephali of this species (29.2–51.2 mm) were collected offshore in the western North Pacific (13°–17.5° N, 125°–141° E) in April, June, and July between 2002 and 2009. One leptocephalus was caught west of Luzon and four were close to the spawning areas of Anguilla japonica and Anguilla marmorata. Otolith microstructure showed that the leptocephali were up to 103–138 days old. Glass eels from northern Luzon in previous studies that were thought to be Anguilla celebesensis now appear likely to have been A. luzonensis, and they were estimated by otolith analysis to have long larval durations similar to those of A. marmorata. Estimated hatching dates of these glass eels and the A. luzonensis leptocephali were both in the February to May season. Offshore presence of leptocephali of A. l...

The migratory history of tapertail anchovy Coilia nasus in the Yangtze River Estuary, China was investigated using otolith Sr:Ca ratios and two-dimensional images of the Sr level from an X-ray electron probe microanalyzer (EPMA). The results showed that 17 of the 22 young-of-the-year (YOY) specimens had low Sr:Ca ratios (1.2–2.4 × 10−3;1.5 ± 0.3 × 10−3) at the central otolith area, indicating their riverine origin and initial freshwater residence. In addition, 11 of the 14 adult specimens had low Sr:Ca ratios (1.3–2.2 × 10−3; 1.7 ± 0.4 × 10−3) at the central otolith area but showed alternating changes between high (>4.0 × 10−3) and low (

Remarkably little is known about the life histories of the many tropical anguillid eels distributed across the Indo-Pacific region, and since the Danish expedition to study eels in the region in 1928 and 1929, research on these eels has only begun again in recent years. Sampling for anguillid leptocephali in the Indonesian Seas has been carried out recently to learn about the spawning ecology and larval distributions of tropical eels there. The leptocephali of Anguilla marmorata, Anguilla bicolor pacifica, Anguilla borneensis, Anguilla interioris, and Anguilla celebesensis were collected around Sulawesi Island both in May 2001 and October of 2002. The development of genetic identification techniques has enabled these leptocephali to be identified to species, and their distributions and sizes during different seasons indicated that there are differing life history patterns among sympatric species in the region. A. celebesensis was found to have been spawning in Tomini Bay of northeastern Sulawesi Island in March and April 2001, but apparently, no spawning had occurred in the late summer and fall of 2002. Studies on anguillid glass eels have suggested that tropical anguillids may spawn throughout much of year, but our research on leptocephali in Tomini Bay and data on the downstream migration of tropical anguillids in the major tributary to Tomini Bay indicate that A. celebesensis may have a distinct seasonal pattern of spawning possibly related to the regional monsoon cycles. This is the first evidence of seasonality of spawning in tropical anguillid eels whose life histories are only just beginning to be revealed.

The timing of catches of anguillid glass eels and their otolith microstructure and microchemistry were studied in southwest Iceland, where the European eel, Anguilla anguilla and American eel, A. rostrata have been thought to live sympatrically, to learn about their early life history and the possible mechanism of the separation between these two species ranges. Catches at the site studied suggest that glass eels may have started upstream migration as the river temperature warmed in late June and early July. The glass eels were mitochondrially identified into two species, A. anguilla and A. rostrata, although the latter were likely hybrids between the two species based on a different study. Otolith analyses showed no sharp increases in otolith increment width or sharp decrease of otolith Sr:Ca ratio in either species, which are the characteristic changes corresponding to the onset of metamorphosis in many anguillid species including A. rostrata collected in North America and A. anguilla in Europe. The mean age at recruitment determined for the glass eels in Iceland were similar between the two species (336.6 ± 41.7 and 319.3 ± 36.0 days for A. anguilla and A. rostrata, respectively), as were their total lengths (range 58.0–78.5 mm and 58.5–73.0 mm). In addition, mean age at metamorphosis (278.0 ± 36.8 and 254.0 ± 47.7 days) and total age (372.3 ± 50.8 and 352.9 ± 42.6 days) were also similar between the two species. However, these ages of A. rostrata in Iceland were older than those in North America, and those of A. anguilla collected in Iceland were roughly intermediate between the rest of Europe and North Africa. These findings support the hypothesis that the timing of metamorphosis is a key factor for determining the place of recruitment of glass eels and maintaining the geographic separation between the two species.

The sagittal otolith morphology and microchemistry of reared juvenile ayu, Plecoglossus altivelis, were examined to describe the occurrence and microchemical characteristics of the abnormal otoliths in this species. Juvenile ayu (N = 31) were collected in June 2004 at three different locations, Wakayama, Kumamoto, and Biwa Lake in Japan, where they were being reared in freshwater aquaculture ponds after having been collected in the wild as larvae. Otolith abnormality was found in the sagittae of 26% (N = 8) of the individuals examined, of which five fish had abnormal otoliths only on one side, while the otolith on the other side was normal. Abnormal otoliths were more transparent and crystalline in appearance with irregular shapes compared to normal ones that were more opaque and less irregular. Abnormal otoliths were divided into two types, semi-abnormal (Type 1) with a normal part in the center, and fully-abnormal (Type 2) that were completely crystalline in appearance. The line transects and whole otolith concentration maps showed that the contents of Sr, Na and K were lower in the abnormal otolith regions compared to the normal ones, while those of Ca and S were almost constant in both. The appearance and microchemical properties of the abnormal ayu otoliths were similar to the abnormal otoliths in other species in which vaterite replaces the aragonite. Abnormal formation of otoliths occurred in ayu from Biwa Lake (30%) and Kumamoto (45%), while the Wakayama samples had no abnormality. The microchemical analyses of the normal and abnormal otoliths indicated that some abnormal otoliths had formed before the fish were captured and transferred to the hatchery, so the possible causes of otolith abnormality in ayu are discussed.

A sampling survey for leptocephali was conducted in the eastern Indian Ocean to the west of Sumatra from 5 to 20 June 2003 in an overlapping area with the historical survey in September to November 1929 during the Danish Round the World Expedition. The age and larval growth rate of 34 Anguilla bicolor bicolor collected in 2003 were estimated using their otolith microstructure to obtain new information about the early life history of this sub-species. The total lengths and ages of these leptocephali were 44·1–55·5 mm and 114–158 days, respectively. Their hatching dates ranged from 3 January to 20 February 2003. Combining these age data and the estimated age of leptocephali collected by others in the same area, this sub-species was estimated to have a wide range of spawning periods. Individual growth rates of the leptocephali in 2003 ranged from 0·32 to 0·39 mm day−1 with a mean ±s.d. of 0·35 ± 0·02 mm day−1. These values were lower than the growth rates of leptocephali of other tropical eels such as Anguilla celebesensis or Anguilla borneensis, suggesting that A. b. bicolor from the eastern Indian Ocean have a longer leptocephalus period of oceanic migration.

The influences of water temperature and feeding regime on otolith growth in Anguilla japonica glass eels and elvers were investigated using individuals reared at 5, 10, 15, 20, 25 and 30 degrees C and in fed or unfed conditions at salinity 32 after their otoliths were marked with alizarin complexone (ALC). To eliminate the difficulty of observing the edges of otoliths with optical (OM) or scanning electron (SEM) microscopes, three to 10 individuals were sampled from each tank at 10, 20 and 30 days during the experiment and reared for an additional 10 days at 25 degrees C after their otoliths were marked a second time. Otolith growth and the number of increments were measured using both OM and SEM. Most A. japonica commenced feeding after 10 days at 20-30 degrees C or after 20 days at 15 degrees C, but no feeding occurred at 5 and 10 degrees C. No otolith growth occurred at 5 and 10 degrees C except in two individuals with minimal increment deposition at 10 degrees C. Otolith growth was proportional to water temperature within 15-25 degrees C and not different between 25 and 30 degrees C. At 15, 25 and 30 degrees C, the mean otolith growth rate in fed conditions was higher than in unfed conditions. The number of increments per day was significantly different among water temperatures (0.00-0.01 day(-1) at 5 and 10 degrees C, 0.43-0.48 day(-1) at 15 degrees C and 0.94-1.07 day(-1) at 20-30 degrees C). These results indicated that otolith growth in A. japonica glass eels and elvers was affected by temperature and ceased at < or =10 degrees C under experimental conditions. Hence, future studies analysing the otoliths of wild-caught A. japonica glass eels and elvers need to carefully consider the water temperatures potentially experienced by the juveniles in the wild.

Transgenerational mass marking of viviparous fish larvae in vivo was validated by intra-muscular injection of elemental strontium chloride (SrCl(2)) in gestating females and detection of the Sr in the otoliths of developing larvae. All otoliths of brown rockfish Sebastes auriculatus larvae produced from SrCl(2)-injected females showed enriched Sr:Ca ratios near the otolith edges, and the signatures did not appear to be affected by the anterior, centre and posterior positions of larvae within the ovary. Results from the present study indicate that transgenerational marking is a highly reliable technique for marking large numbers of extremely small viviparous fish larvae.

International audience; A total of 4,172 freshwater eels have been collected by electroWshing in upper estuaries from Madagascar (East coast), Mascarene (Réunion and Mauritius Is.), Comoros (Mayotte Is.) and Seychelles (Mahé and Praslin Is.) Archipelagos, between October 2003 and February 2006. Eel species composition in the sampling stations was contrasted between eastern Madagascar (Anguilla mossambica 96.0%, A. marmorata 3.9% and A. bicolor bicolor 0.2%), the Comoros (A. marmorata 56.1% and A. bicolor bicolor 43.9%), the Mascarene (A. marmorata 91.4%, A. bicolor bicolor 5.4% and A. mossambica 3.2%) and the Seychelles Archipelagos (A. bicolor bicolor 100.0%). This gradient in species composition, even concerning the short time-range of our sampling, argued for separate migration routes between species. A total of 168 eels were aged by reading their otolith microstructure, and otolith growth rates were calculated from pre-leptocephalus stage (post-hatching) to metamorphosis, until freshwater check. For all species, mean otolith growth rate (OGR) was related to specific migration routes: A. bicolor bicolor is distributed in the lowest latitudes and showed the highest OGR during leptocephalus stage, whereas A. mossambica, endemic of the Malagasy area, has the most southern distribution and showed the lowest OGR. OGR during leptocephalus stage was negatively correlated to the leptocephalus stage duration, showing a decrease of global metabolism with time, classical in leptocephali. This relationship was found significant for A. marmorata and A. mossambica, probably because all these larvae crossed successively the same environments, but not for A. bicolor bicolor, probably because their larvae crossed diVerent pelagic environments, opening the hypothesis of larvae from diVerent origins.

International audience; Freshwater eels are important fisheries species in parts of the western South Pacific, but little is known about their oceanic early life history or spawning areas. The age, growth, morphology and geographic distribution of five species of genetically identified anguillid leptocephali collected in 1995, 2000 and 2005 were compared. The sizes and ages of the leptocephali collected, Anguilla australis (n = 18), Anguilla marmorata (n = 15), Anguilla reinhardtii (n = 12), Anguilla megastoma (n = 2) and Anguilla obscura (n = 1), ranged from 19.0 to 50.9 mm and from 25 to 155 days, respectively. Leptocephali were mostly collected in the South Equatorial Current region. The total myomere ranges overlapped among species, but anodorsal myomere numbers clearly divided shortfinned and longfinned eels. The myomere ranges of the leptocephali were similar to the reported ranges of the numbers of vertebrae in adults. Larval growth rates suggested that the temperate species A. australis had slightly slower growth than the tropical species A. reinhardtii. The present study suggests that both temperate and tropical anguillid eels use the South Equatorial Current region for spawning and larval development, although some species might have different early life parameters and migration routes to their recruitment areas