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.     

Age determination and growth rate of eels, Anguilla anguilla (L)

Measurements taken from a series of photographs of otoliths of 37 eels caught in a stretch of river of 150m length were used to prepare a growth curve based on back-calculated measurements. Values of L ∞= 1045 and K = 0.046 were determined. Mean incremental growth calculated as 33 mm per year agreed closely with increments determined from growth observed up to 4 years after tagging eels from the same location. Inconsistencies in results of age determinations based on burned otoliths are described but it is concluded that the technique yields results of sufficient reliability to be used in practical management situations.     

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.     

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.     

Methods of age determination of band-finned flounder <Emphasis Type="Italic">Liopsetta pinnifasciata</Emphasis> (Pleuronectidae)

Techniques involving chemical treatment are suggested for discrimination of annual and seasonal rings in the otoliths of band-finned flounder Liopsetta pinnifasciata. In the first annual zone, three seasonal rings are discerned, and four seasonal rings are discerned in subsequent zones. It is supposed that these rings correspond to biological seasons. The annual rings on otoliths and scales are formed synchronously: the first ring in December–March, and the subsequent ones in December–February. The time of formation of the first annual zone is less than a year; accordingly, the age of Liopsetta pinnifasciata determined by registering structures is less than its calendar age. According to calculations, metamorphosis of Liopsetta pinnifasciata ends at the length approximately 20 mm.     

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.     

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。     

Growth patterns and age validation from otolith ring deposition in New Zealand longfin eels Anguilla dieffenbachii recaptured after 10 years at large

In 1998, 9500 juvenile New Zealand longfin eels Anguilla dieffenbachii (mean total length, L_T, 42 cm) captured from the lower Clutha River were transferred upstream to Lake Hawea, a high‐country oligotrophic lake in the same catchment where recruitment of juvenile eels has been prevented by hydroelectric dams since 1958. A total of 2010 of the transferred A. dieffenbachii were tagged with coded wire tags. Ten years later in 2008, the A. dieffenbachii population in Lake Hawea was sampled resulting in 399 recaptures (distinguishable by the presence of tags and by L_T from the remnant resident population of large old A. dieffenbachii) of the 1998 transfers; 79 (19·2%) of the recaptured fish had tags compared with 21·3% at release, indicating good tag retention and low mortality due to tagging. All recaptured tagged A. dieffenbachii were female. Mean annual growth over the 10 years since release was 3·80 cm year^?1 for all recaptures and 3·65 cm year^?1 for tag recaptures, and both were significantly greater than the estimate of 2·38 cm year^?1 at release. After release, mean condition (K) increased significantly (P < 0·001) for all recaptures and tag recaptures. Annual length growth increment was linear. Tag recaptures showed significant increases in somatic growth rate post‐transfer, and otoliths from the 2008 recaptured A. dieffenbachii were examined to see whether any similar enhanced growth after transfer was incorporated into the otolith structure that would serve as a date stamp. Measurement of otolith ring radii indicated that an increase in the radius occurred on most otoliths corresponding to the year after transfer. Because there was 9 years of completed growth following the observed growth inflection on the otoliths, this was strong evidence that opaque rings were formed annually.     

Validation of age readings from otoliths of juvenile roundnose grenadier, Coryphaenoides rupestris, a deep-water macrourid fish

Validation of the ageing of deep-water fish is difficult and there are only a few instances where the rings on the otoliths have been shown to be laid down annually. Roundnose grenadier have been fished commercially in the North Atlantic since the 1960s and the adult fish have frequently been aged by counting the rings in otoliths or scales. All the ageing was done on the assumption that the rings in the otoliths or scales were annual. Between 1975 and 1992, the Scottish Association for Marine Science carried out seasonal trawling surveys in the Rockall Trough using a fine-meshed trawl, and collected otoliths from a wide size range of roundnose grenadier. An examination of the growing edge of otoliths from juvenile fish from these collections suggests that the rings in the otoliths are laid down annually. The broader, opaque zones which represent the growth phase were dominant between September and March. The thinner, hyaline zones were dominant between April and July. The apparent delay in the growth phase compared with most shallow-water species is discussed in relation to the availability of mesopelagic prey.     

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.     

Method of age determination of Ponto-Caspian kilka <Emphasis Type="Italic">Clupeonella cultriventris</Emphasis> (Clupeiformes,Clupeoidei) using scales and otoliths

A method of Rybinsk Reservoir kilka Clupeonella cultriventris age determination using otolites is described. Terms of annual rings anlage on both scales and otolites are determined. A comparison of age estimates obtained through studying scales and otoliths by different operators is performed. It is shown that operators’ errors in age determination using otolites are smaller than such when the scales were used.     

Age determination and growth of yellow eels, Anguilla anguilla (L.), from a brackish water, Norway

This paper describes the age determination of European yellow eels from a brackish water area in south-east Norway. Two different methods were used; clearing in 96% ethanol or burning and cracking. When reading the otoliths twice using the same method, the same age was found in 40% of the readings, a discrepancy of 1 year was found in another 40% of the readings (both methods). Burning and cracking yielded age estimates which were on average 1.25 years higher than those obtained by clearing.     

The use of sectioned otoliths to age barramundi (Lates calcarifer) (Bloch, 1790) [Centropomidae]

The relationship between the number of rings present in sagittal otoliths and the age of barramundi, Lates calcarifer (Bloch, 1790) [Centropomidae], was investigated by examining cross sectioned otoliths of 37 tagged fish of known age between 1 and 5 years from the Johnstone River, north Queensland. Concentric rings were clearly visible in all otolith sections and were validated as annual marks. The technique was then used to estimate the age and calculate von Bertalanffy growth parameters for 70 barramundi from the Fitzroy River, central Queensland. Growth appeared to be rapid but variable in the first year; the von Bertalanffy growth parameters for length versus age were L =690 mm, K=0.53, t ₀= 0.003 years. October 1 was designated as the birth date. Whole otolith length, width and thickness were also approximated well by the von Bertalanffy equation. We suggest that examination of otoliths is a useful technique for ageing barramundi but note that further validation of the ageing method is still needed for fish older than six years.     

Age and growth of eels Anguilla japonica in a Chinese river

Age and growth of Anguilla japonica sampled from the Qiulu River in southern China were studied. Mean body lengths and weights of the eels at each age were back-calculated from their otoliths. The growth parameters for the von Bertalanffy function gave L _∞= 98–2cm, K =0·07 and t ₀=− 1·94. Scales were observed to form firstly at the middle part of the lateral line, at a minimum body length of 160 mm and were inappropriate for ageing.     

Freshwater habitat use by a moray eel species,Gymnothorax polyuranodon,in Fiji shown by otolith microchemistry

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.     

Age and growth of migrating tropical eels,Anguilla celebesensis and Anguilla marmorata

The age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata from central Sulawesi, Indonesia, were examined. Migrating eels (63 A. celebesensis and 38 A. marmorata ) were obtained from weirs near the Poso Lake outlet and non‐migrating eels (35 A. celebesensis and 119 A. marmorata ) were captured by baited hooks, eel pots, scoop net and electro‐fishing in the Poso River system, Laa River system, Baluga River, Tongku River and Padapu River from February 2009 to October 2010. In both species, the proportion of eels with opaque otolith edges showed a single peak in July, suggesting that one annulus (a pair of translucent and opaque zones) was formed each year in their otoliths. Mean ± s.d . and range of total length (L _T) and age was 785·2 ± 114·9 (585–1083) mm and 7·5 ± 1·6 (5–11) years in migrating female A. celebesensis and 1132·2 ± 173·7 (800–1630) mm and 11·6 ± 3·3 (7–23) years in A. marmorata . The age of migrating female eels was negatively correlated with annual growth rate, 100·7 ± 17·2 (68·1–145·0) mm year^?1 in A. celebesensis and 97·9 ± 19·3 (66·6–131·6) mm year^?1 in A. marmorata , but there was no significant correlation between the L _T and annual growth rate in either species. The annual growth rates of these female tropical eels were typically higher than those of temperate anguillid species, suggesting a latitudinal cline in growth rate in the genus Anguilla reflecting the environmental conditions of their growth habitat.     

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.     

Location,size and age at onset of metamorphosis in the Japanese eel Anguilla japonica

This study clarifies the location, size and age at the onset of metamorphosis in Japanese eels Anguilla japonica through oceanic surveys, rearing experiments and analyses of the morphology and otoliths of leptocephali and glass eels. Twenty‐eight metamorphosing leptocephali were collected in the mesoscale eddy region to the east of Taiwan during research expeditions in 2004. Rearing experiments showed that the total length (L_T) of leptocephali decreased by an average of 12·5% during metamorphosis and 13·9% during the 2–12 h after death. Thus, the mean back‐calculated L_T at the onset of metamorphosis for 630 glass eels from Taiwan and Japan was estimated at 67·8 ± 2·7 mm (mean ± S.D.). The estimated mean ante‐mortem size of the fully grown pre‐metamorphic leptocephali collected in 2004 was 64·6 ± 3·4 mm, which was consistent with the L_T estimate for glass eels. Otolith analysis showed that the mean age at the onset of metamorphosis was 137 ± 15 days and indicated that Japanese eels may have a recruitment route through the mesoscale eddies to the east of Taiwan in addition to the direct transfer route from the North Equatorial Current to the Kuroshio Current.     

Demographic characteristics of an intertidal bay goby (Lepidogobius lepidus)

Synopsis In a fourteen month study (May 1976 – June 1977) I examined the following characteristics of an intertidal bay goby (Lepidogobius lepidus) in Morro Bay, California, U.S.A.: annual and seasonal patterns of abundance, age composition and growth rates, survivorship and mortality patterns, and the reproductive cycle for female gobies. Fishes were collected with the aid of quinaldine and otoliths and ovaries removed. Age and growth rates were estimated from otolith annuli using a back calculation formula and a Brody-Bertalanffy growth curve. Mortality rates were derived using the methods of Heincke (1913), Robson & Chapman (1960), mean age, and a catch curve (Ricker 1975). A gonad index was used to describe the annual reproductive cycle. Results indicated that abundance fluctuated seasonally and that these fluctuations appeared to be caused by reproductive emigrations. Bay gobies reached an age of 7+ and a standard length of 87 mm. Growth was relatively constant (6 mm yr⁻¹) until age 5, at which point it began to decline. The mean rates of survivorship, mortality, and instantaneous mortality were 0.75, 0.25, and 0.29 respectively. Mortality rates for individual age classes ranged from 0.13 to 0.51 and increased with age. This stock appears to reproduce mainly during the winter.     

中华鲱鲇年龄鉴定及生长特征

对2009-2010年采自澜沧江下游的188尾中华鲱鲇(Clupisoma sinensis)进行了年龄与生长的研究.采用耳石、脊椎骨、鳃盖骨、胸鳍棘4种材料对全部样本进行年龄鉴定,耳石年龄鉴定最为准确,4-5月是年轮形成高峰期.渔获种群由1-6龄共6个龄组组成,以1-4龄个体为主.体长与耳石半径呈线性函数关系.体长与...