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.     

Determination of the age of young American eels, Anguilla rostrata, in fresh water, based on otolith surface area and microstructure

The time elvers of the American eel, Anguilla rostrata , spend in an estuary prior to their migration into fresh water was assessed. A distinct mark was formed on elvers' otoliths during their first 2 to 3 weeks in the river estuary. This mark was used to distinguish between growth in fresh water and in salt water. Migrating eels collected at a falls 4 km from the estuary exhibited bimodal length and weight distributions. Frequency distributions showed that eels collected in the estuary were smaller and had smaller otoliths than eels collected at the falls, indicating that elvers do not reach the falls in the same year as they enter the estuary. The three modal groups most probably represent three age–classes. However, the otoliths of elvers collected in the estuary had only the mark of transition whereas eels in the first and second mode at the falls usually had two rings (1–4) and four rings (3–6) per otolith, respectively, in addition to the mark of transition, as viewed under SEM. The possibility that ring formation is not annual means that the use of otoliths for the age determination of eels in this study has not been validated.     

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.     

鳗鲡幼鱼耳石日轮的研究

本文报道采自辽东半岛沿岸鳗鲡(Anguilla japonica)的白仔鳗和经人工培育的当年幼鳗耳石日轮生长的观察结果。白仔鳗和幼鳗耳石平均直径均与体全长成直线相关。12尾白仔鳗耳石的平均日轮数146.3,据此推测其产卵期为11—12月。观察证实从咸淡水转人到淡水生活的幼鳗耳石的环纹有过渡带存在。     

Influence of water temperature and feeding regime on otolith growth in Anguilla japonica glass eels and elvers: does otolith growth cease at low temperatures?

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° 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° 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° C or after 20 days at 15° C, but no feeding occurred at 5 and 10° C. No otolith growth occurred at 5 and 10° C except in two individuals with minimal increment deposition at 10° C. Otolith growth was proportional to water temperature within 15–25° C and not different between 25 and 30° C. At 15, 25 and 30° 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° C, 0·43–0·48 day⁻¹ at 15° C and 0·94–1·07 day⁻¹ at 20–30° C). These results indicated that otolith growth in A. japonica glass eels and elvers was affected by temperature and ceased at ≤10° 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.     

Discrepancies between otoliths of larvae and juveniles of the American eel: is something fishy happening at metamorphosis?

The use of otoliths to interpret early life history in fishes depends upon the assumptions that otoliths record past events accurately and consistently and that records of events in otoliths are continuous. Both the number of growth microincrements ( I ), and the radii ( R ,μm) of otoliths of American eel Anguilla rostrata , leptocephali increased linearly and highly significantly with leptocephalus body length ( L , mm), as expected on the above assumptions ( I , =2·29 L , − 5·75 and R , =1·05 L , + 12·02, r ²,=0·938 and 0·931, n , =20). In contrast, the number of increments and the radii of the leptocephalus growth zones of otoliths of glass-phase American eels were not related to body length, and they were lower than predicted by the relationships developed for leptocephali. Thus, otoliths of American eels apparently violate one or both assumptions. Possibly, the margin of the otolith is resorbed during metamorphosis from leptocephalus to glass eel, perhaps as part of calcium metabolism as skeletal elements are being formed.     

Determination of the freshwater mark in otoliths of Japanese eel elvers using microstructure and Sr/Ca ratios

The microstructure, in particular checks within the otolith edge, of Anguilla japonica glass-eels and elvers and changes in otolith Sr/Ca ratios were examined to ascertain the environmental history of the eels, especially with regard to the time when glass-eels entered the river, and as a benchmark for count daily increments. The percentage of glass eels and elvers with checks and the mean number of checks within the otoliths of glass-eels caught at four localities, Tosa Bay off Tosa City, the mouth of the Gokase River, the mouth of the Saigo River and the dam of the Tsuri River were 0% (0), 15.0% (0.2), 51.6% (1.0) and 100.0% (4.2), respectively. The Sr/Ca ratios and Sr content peaked in the region where checks were formed and the values decreased rapidly towards the edge of the checks; on the other hand, these decreased gradually in the otolith when checks were not formed. These checks were estimated to be formed by stress when the glass-eels were affected by ambient fresh water within the river. The innermost check was called the freshwater mark in the present study and this mark may be useful as a benchmark in studying the growth history of the eel before and after entering freshwater.     

Larval history of glass eels, Anguilla anguilla (Linné, 1758) in migration in coastal and estuary areas (Adour, Gulf of Gascogne) as revealed by otolith test]

The embryonic past of glass eels was studied from the interpretation of microstructures registered on otoliths. The aim of this work is to put in evidence possible seasonal modifications of the growth of otoliths so that differences between otoliths of glass eels caught off marine and estuarine environment. So during the season 1999-2000, from November till March, otolith sampling was realised in the southwestern part of France, in an estuarine and coastal zone. We observed a spatial and temporal evolution of proportions of the three various types of otoliths taken into account. Glass eels sampled at sea sometimes have a mark on their otoliths indicating the transition in the estuary, especially at the end of the fishing season. Measures of growth marks of otoliths showed that there were no seasonal differences during phases of the transoceanic migration and the crossing of the continental shelf. The radius of otoliths of glass eels sampled at sea was significantly smaller than those sampled in estuary. These results translated homogeneous environmental modifications met by the various larvae groups during the oceanic crossing and during the principal migration season as well as a turn over of these groups during the transition between marine and continental environment.     

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.     

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.     

Otolith shape analysis and daily increment validation during ontogeny of larval and juvenile European chub Squalius cephalus

This assesses features of otoliths from laboratory-reared embryos, larvae and juvenile European chub Squalius cephalus from hatching to 180 days post-hatching (dph). We observed the development of the three pairs of otoliths (lapilli, sagittae and asterisci) and more precisely shape changes, as well as timing and deposition rate of increments of the lapilli. The lapilli and the sagittae were present at hatching, whereas the asterisci formed between 20 and 30 dph. The lapillus and sagitta shapes were round until 20 dph. From 60 dph the anterior and the posterior rostra of the sagittae were well developed, but very thin, making this otolith too fragile to manipulate for further studies of shape and validation of otolith increment deposition rate. The lapilli provided reliable age estimates for free embryos, larvae and juveniles up to 120 dph. However, caution should be taken when ageing fish older than 150 dph as an underestimation was noticeable. The regression of the number of otolith increments on age showed a slope and an intercept not significantly different from 1 and 0, respectively, which indicated that otolith growth increments were deposited on a daily basis, with the first microincrement occurring at hatching. Increment counts were consistent between three interpreters, indicating a consistent and reliable age estimate. This study validates that the otolith increment deposition rate can be used to assess hatching dates and daily growth of wild S. cephalus under 150 dph and in environments similar to the conditions used in this study.     

The effects of temperature, river flow, and tidal cycles on the onset of glass eel and elver migration into fresh water in the American eel

Anguilla rostrata elvers were collected in the Annaquatucket R., Rhode Island, and their otoliths extracted. Daily increments beyond the check mark formed upon entry into fresh water were counted and the date of freshwater entry was estimated. The effects of river temperature, difference between seawater and freshwater temperature, river flow and tidal stage on the number of elvers arriving on each date were estimated for six collection dates. At the earliest collection date (23 March), increasing river temperatures and reduced flow increased elver migration. At later dates (16 May-12 June), tidal stage was the most important factor in determining the magnitude of elver migration.     

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

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

Validation of annulus in otolith and estimation of growth rate for Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> in tropical southern Taiwan

The age of Japanese eels (Anguilla japonica) is often estimated from otoliths, but this method has not been fully validated, particularly in tropical areas where the annulus in otolith is considered to be less distinct than in temperate areas. To validate the annuli in Japanese eel otoliths from southern Taiwan, known-age (2 year-old) cultured eels from an eel farm and wild eels from Kao-Ping River were collected. It was found that 26 out of 31 cultured eels (83.9%) showed two clear annuli and the remained 5 eels showed either one or three annuli. The mean (± SD) age of the cultured eels was 1.97 ± 0.4 years. Meanwhile, a clear peak in the mean monthly marginal increment ratio of the otolith in wild yellow and silver eels occurred once a year during winter (November to March). The annual deposition of presumed annuli in otoliths of Japanese eel was validated and the age and growth rate estimation for Japanese eels in the tropical southern Taiwan is deemed feasible. The growth rate of cultured eels was significantly faster than that of wild eels, but it did not differ significantly between sexes for wild silver, yellow or cultured eels. The von Bertalanffy Growth Function parameters (K, and t ₀ ) of the wild eels were estimated as 0.114 ± 0.028 year⁻¹, 1178 ± 171 mm and −0.8 ± 0.2 years, respectively.     

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.     

Occurrence of sea eels of <Emphasis Type="Italic">Anguilla japonica</Emphasis> along the Sanriku Coast of Japan

 The age and migratory history of the Japanese eel, Anguilla japonica, collected along the Sanriku Coast of Japan, were examined using otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations. 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 and others which had entered freshwater for brief periods but returned to the estuary or bay. This first 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. Received: May 15, 2002 / Revised: August 4, 2002 / Accepted: August 15, 2002 Acknowledgments We thank Messrs. S. Yamane and K. Morita, and crews of the Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, for their assistance in collecting the eels. This work was supported in part by Grant-in-Aid No. 13760138 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Correspondence to:Takaomi Arai     

The early life history of the tropical eel Anguilla marmorata (Quoy & Gaimard, 1824) from four Pacific estuaries, as revealed from otolith microstructural analysis

Anguilla marmorata glass eels or elvers were collected separately during anadromous migration from four Pacific estuaries: Hamuta, Poso, Shuang Hsi and Tanshui. The total length at arrival in these estuaries was (mean ± standard error) (51.50 ± 0.90) (51.80 ± 0.90) (46.95 ± 0.84) and (47.33 ± 0.80) mm, respectively. The sagittal otolith microstructure, increment patterns and daily age were examined by scanning electron microscope. Based on the number of increments of presumed daily deposition, the overall mean age at arrival in the estuaries was estimated to be about 3–4 months, with an estimated period of 73–86 days for the leptocephalus stage. Two zones, i.e. the leptocephalus growth zone (L) and the metamorphosis growth zone (M) were recognizable in the otolith cross section. The increment width of L and M varied from the otolith's centre to its margin, reflecting different growth rates. The spawning grounds of these eels are presumably not far from the estuary. Their locations are discussed.     

Validation of the periodicity of increment formation in the otoliths of a cichlid fish from Lake Tanganyika, East Africa

Tetracycline was used as a chemical tag in a mark‐recapture study to examine the pattern of increment formation in the otoliths of Tropheus moorii , a rock‐dwelling cichlid from Lake Tanganyika. A total of 256 fish were captured by divers and injected with tetracycline. Of these, nine were recaptured after either 1 or 2 years at liberty and eight retained tags within their otoliths. Comparison of the number of growth increments formed after the tag and the time at liberty demonstrated that increments were deposited on an annual basis in the otoliths of this species. Furthermore, there was a strong relationship between otolith mass and age suggesting that otoliths grew at a predictable rate throughout the life of the fish. Validation of an annual pattern of increment deposition allowed age and growth information to be derived from otoliths. This showed that T. moorii grew rapidly to attain adult size by 3 years of age. Males grew faster than females and also attained a larger size than females (8·74 v . 7·91 cm L _S respectively). The longevity of some of these small freshwater fish was surprising; the oldest individual had an age of 10 years, while the average age of adults was 4 years.     

Age of European silver eels during a period of declining abundance in Norway

The European eel (Anguilla anguilla) is critically endangered throughout its range. Knowledge about age distribution of future spawners (silver eels) is essential to monitor the status and contribute to the recovery of this species. Determination of age in anguillid eels is challenging, especially in eels from the northern part of the distribution area where growth is slow and age at maturation can be up to 30 years or more. Eels from the river Imsa in Norway have been monitored since 1975, and this reference time series has been used to assess the stock at the European level. Population dynamics in this catchment were analyzed during the late 1980s by estimating ages on whole cleared otoliths. However, techniques for revealing annual increments on otoliths have evolved over the years sometimes yielding significant differences in age estimates. In this study, the historical otolith data were reanalyzed using a grinding and polishing method rather than reading the whole otolith. The new age estimates were considerably higher than the previous ones, sometimes by up to 29 years. Since the 1980s, mean age of silver eels only slightly increased (from 19 to 21 years in the 2010s). This was mainly due to the disappearance of younger silver eels (<15 years) in the 2010s. The new age estimates agreed with the steep decline in recruitment which occurred in the late 1980s in the Imsa catchment. Mean growth (30 mm/year, min–max: 16–64 mm/year) has not changed since the 1980s, although density in the catchment has decreased. Revealing and reading age of slow‐growing eels remain a challenge but adding a measure of otolith reading uncertainty may improve age data collection and contribute to recovery measures for this species.     

Daily growth increments in the otoliths of Atlantic salmon parr, Salmo salar L., and the influence of environmental factors on their periodicity

Daily increments were demonstrated in the sagitta otoliths of fast- and slow- growing Atlantic salmon parr, Salmo salar L., when held under natural photoperiod and temperature. Otolith increments continued to be deposited at a daily rate when fish were held under constant light and/or temperature and on single or multiple feeding regimes. However abnormally short photoperiods of 6L: 6D induced two increments per day. The results suggest that an endogenous rhythm, synchronized lo light/dark transitions within a 24 h period, controls otolith increment deposition.