Relationship between elver recruitment and changes in the sex ratio of silver eels Anguilla anguilla L. migrating from Lough Neagh, Northern Ireland

Between 1932–1947 and from 1960 onwards, elvers have been trapped near the mouth of the River Bann, Northern Ireland, and released into Lough Neagh. Each period of elver transport has been followed by a marked increase in the proportion of male silver eels migrating from the lough. Catches of silver eels were sampled on several nights each year from 1965–1974, and the lengths of a total of 20358 eels measured showed a progressive increase in the percentage of male eels from 9.3-86.0 % during this period. Various reasons for this change were examined. The different ages at which male and female eels migrate to the sea was not important. There was no evidence to support the hypothesis that male elvers normally remain in estuarine conditions, and their transport to the lough was therefore unnatural. An increasing fishing effort for yellow eels, such as occurred following the introduction of trawling in 1960, would favour males since small eels were returned to the lough. It was not thought, however, that this was a major cause of the change in sex ratio. Instead, elver transport appeared to be directly implicated, possibly by the overstocking of Lough Neagh, and the phenotypic determination of progressively more male eels, but the evidence for this suggestion was inconclusive.     

A biological analysis of eel catches, Anguilla anguilla L., from the lagoons of Lesina and Varano, Italy

A study of eel catches from Lesina (444 specimens) and Varano lagoons (325 specimens), in southern Adriatic, Italy, was made. Male silver eels in Lesina ranged from 33.4–51.5 cm in length, with a mean of 42.6 cm; from 50–240 g in weight, with a mean of 141 g and were 1.5–6.5 years old with a mean of 2.5 years. The average length of male silver eels in Varano lagoon was 40.5 cm (range 31–48.5 cm); the average weight was 122 g (range 80–220 g)and a mean age of 2.6 years (range 1.5–7.5 years).
The females are bigger, heavier and older than the males with, in Lesina, a mean length of 61 cm (range 50.9–74.3 cm), a mean weight of 438 g (range 240–730 g) and a mean age of 3.4 years (range 1.5–6.5). The average length of Varano female silver eels was 58 cm (range 50.8–72.5 cm), and the average weight was 383 g (range 225–840 g). They were 1.5–7.5 years old, with an average of 3.8 years. Female silver eels were only 20% of the population at Lesina and 10% at Varano.
In comparison with the silver eel populations of the North Adriatic lagoons, the North Sea or the Atlantic Ocean, the silver eels of Lesina and Varano show a greater growth rate, are younger and have a sex ratio in favour of the males.
The water temperature, higher than in other countries, could be an important factor affecting the differences in age and growth rates between Lesina and Varano silver eels and those of other waters.     

Implications of individual growth status on the future sex of the European eel

Sex-related differences in growth status was demonstrated in eels Anguilla anguilla reared indoors at 17, 20 or 26° C, from the elver stage. Growth status was defined as length increase, weight increase and length–weight relationship. Eels attaining at least 10 g body weight (180–220 mm body length) were tagged with Passive Integrated Transponders (PIT). Length and weight were measured at 6-week intervals, until individuals stopped growing or had attained 150 g weight (380–450 mm). Sex-specific data from potentially undifferentiated eels were provided by retrospective classification of sex. Comparisions between sexes were made within groups graded by length or weight data from the beginning of each 6-week period. There was no consistent difference in absolute length increase between small males and females, but below 40–60 g initial body weight, males displayed on average a higher weight increase than females. Males also had lower length at weight than females, even in the smallest weight groups. Early growth status may influence the future sex of undifferentiated eels, but other approaches are needed for distinction between cause and effect.     

Immunolocalization of steroidogenic enzymes in gonads of European eel, Anguilla anguilla (L.)

The localization of cytochrome P450 cholesterol side-chain cleavage (P450scc), 3β-hydroxysteroid dehydrogenase (3β-HSD) and aromatase (P450arom) was investigated using polyclonal antibodies during gonad development in wild European eels, Anguilla anguilla (L.), from the River Po Delta (Ferrara, Italy). The first steroidogenic cells, observed in undifferentiated gonads of 14–16 cm yellow eels, showed no P450scc, 3β-HSD or P450arom activity, but positive regions appeared in head kidney insulae from this stage until the silver eel stage. In undifferentiated gonads of 16–20 cm yellow eels the steroidogenic cells were positive to all enzymes. Pre-Leydig steroidogenic cells, identified in Syrski organs of yellow eels of 22–26 cm evolving into testes, were positive to 3β-HSD and P450scc, but negative to P450arom. However, steroidogenic cells in Syrski organs evolving towards ovaries and in small but fully differentiated ovaries were positive to all enzymes. Immature testes of yellow and silver eels had Leydig cells positive to P450scc and 3β-HSD; the same reactions were also observed in some Sertoli cells of silver eel testes containing meiotic cells. Sex differentiation in A. anguilla apparently occurs through an initial female stage controlled by P450arom activity. Leydig and Sertoli cells appear involved in different steps of hormonal control of spermatogenesis: Leydig cells begin their steroidogenic activity before meiosis, while Sertoli cells begin their activity during meiosis.     

Histological study of the development and sex differentiation of the gonad in the European eel

The histological structure of the gonads was studied in yellow eels sampled from a coastal lagoon and from stocks reared in an aquaculture plant showing different sex ratios. Gonad development related to body size rather than to age and underwent an intermediate stage characterized by a structure of an early testis but containing oogonia and oocytes. This gonad was called the Syrski organ and the stage juvenile ambisexual. Ovaries were found in eels from 22–30 cm in length, possibly derived from undifferentiated gonads or from Syrski organs. Fully differentiated testes were found in eels >35 cm, derived from Syrski organs. These observations support the results of previous research. From elvers and in eels up to 15–16 cm in length, growth of the gonadal primordium is due to primordial germ cell migration. In eels > 15 cm multiplication of primordial cells begins. Oogonial clones were found in eels > 18 cm in length, whilespermatogonium B clones were observed in eels >30 cm in length. The dynamics of sex differentiation was different among stocks with different ultimate sex ratios: ovaries were found in shorter eels in stocks with a prevalence of females, in longer eels in stocks with a prevalence of males. This result supports the hypothesis of a metagametic (environmental) sex determination. The somatic cells in contact with germ cells and those in the interstitium appeared early during gonad development and preceded germ cell differentiation. This suggests that somatic cells are the targets of the environmental factors influencing sex differentiation.     

Influence of different rations and water temperatures on the growth rates of shortfinned eels and longfinned eels

Juvenile (12–152 g) shortfinned eels Anguilla australis and longfinned eels A. dieffenbachia caught in New Zealand streams were fed squid mantle Nototodarus spp. 4 days per week in laboratory experiments. A linear multiple regression equation showed the amount of food eaten (0–2·7% w day⁻¹) explained 77·7% of the variation in specific growth rates (–0·60 to +1·07% w day⁻¹) among individual eels, while previous growth rates, water temperature (10·0–20·6°C), and eel weight (12–152 g) explained a further 5·6, 1·4 and 0·8%, respectively. Growth in length ranged from –0·3 to +0·9 mm day⁻¹. Eels which were starved and then given high rations grew substantially faster than expected. Once growth rates were adjusted for differences in ration and other factors, there were no significant differences in growth rates between species or individual fish. Growth of shortfinned eels fed maximum rations of commercial eel food depended on fish size and water temperatures and ceased below 9·0°C. Growth rates in the wild were substantially less than the maximum possible, after seasonal changes in water temperatures were taken into account, indicating that food supplies and not low water temperatures were controlling growth rates in the wild.     

Biology of the annular seabream, Diplodus annularis (Sparidae), in coastal waters of the Canary Islands

The biology of the Canary Islands annular seabream Diplodus annularis (Linnaeus 1758) was studied from samples collected between January and December 1998. Fish ranged from 82 to 209 mm total length in size and from 8.7 to 137.1 g in weight. The mean length showed an increase with increasing water depth. Males showed a negative allometric growth and females isometric growth. The species was characterized by protandric hermaphroditism. The overall sex ratio was unbalanced in favour of males (1 : 0.79). The reproductive season extended from January to May, with a peak in spawning activity in March–April. Males reached maturity at a smaller length (103 mm, 1-year-old) than females (128 mm, 2-year-old). Fish aged 0–6 years were found. The von Bertalanffy growth parameters for all individuals were: L_∞=247.9 mm, k=0.268 years^–1, and t₀=–0.879 years.     

Vital population statistics based on length frequency analysis of the exploited Japanese eel (Anguilla japonica) stock in the Kao‐Ping River,southern Taiwan

Vital statistics such as growth, mortality, and maturity parameters are crucial in understanding the population dynamics of a species. A total of 7 074 Japanese eels (Anguilla japonica) in the lower reach of the Kao‐Ping River, southern Taiwan, were collected with eel tubes in 1998 ～ 2004 and shrimp nets in 2004 ～ 2007. Data from 2004 were excluded due to mixed gear information and escapement of cultured eels; in subsequent years escaped cultured eels were identified and excluded from analyses. The estimated asymptotic length in the von Bertalanffy growth function (84.5–110 cm) was smaller, while the Brody growth parameter (0.30–0.44 year^?1) was higher using electronic length frequency analysis (ELEFAN) than when using Shepherd’s length composition analysis (SLCA). The total instantaneous mortality rate (Z) was around 1 for periods 1998–2003 and 2 year^?1 for 2005–2007 using length‐converted catch curves. The 95% confidence intervals of Z did not overlap for two of the periods, suggesting that the mortality rates were significantly higher during 2005–2007, possibly due to the introduction of shrimp nets. The maturity function differed significantly between sexes, indicating that females become silver eels at a larger size. The Japanese eels in the lower reach of the Kao‐Ping River were likely heavily exploited, thus management and conservation actions are strongly recommended.     

Variability in growth of longfinned eels among coastal catchments of south‐eastern Australia

Longfinned eels Anguilla reinhardtii were captured by both fishery‐dependent and independent sampling methods from three rivers in New South Wales, south‐eastern Australia. Growth rates were examined in two zones (fresh water and tidal) in the Hacking, Hawkesbury and Clarence Rivers. Mean annual growth increments of sampled longfinned eels ranged from 30 to 60 mm year⁻¹ using age‐length analyses and up to 167 mm year⁻¹ based on tag‐recapture model estimates (GROTAG), with both methods showing high intra‐ and inter‐population variability. Growth was significantly faster in younger (5–15 years) fish than older (>15 years) fish, with females growing an average 10 mm year⁻¹ faster than males of similar age and capture location. Longfinned eels found in tidal areas grew significantly faster than those in non‐tidal freshwater areas as a result of longer growing seasons in the highly productive estuarine habitats. Other possible factors influencing variability in growth rates for this species include habitat preference, density and fishing pressure.     

Growth rate and age at migration of Anguilla anguilla

The length, age and growth rate were investigated for downstream migrating male and female eels in the unexploited Burrishoole system, western Ireland. Significant differences were found in the age and length at migration with the larger, older female eels also showing faster annual growth as early as the first year in fresh water. Female eels normally migrated at lengths from 40.5 cm, exceptionally to 92.9 cm, and male eels at lengths between 28.9 and 46'0 cm. Back-calculation showed an irregular pattern of fast and slow annual growth. Mean annual growth increments were almost always greater for females than for males.     

Critical swimming speed of yellow‐ and silver‐phase European eel (Anguilla anguilla,L.)

The prime objective of this study was to evaluate differences between the swimming performance of two distinct life stages of European eels. The critical swimming speed (U_crit) of 29 yellow‐ and 33 silver‐phase eels was evaluated in a swim tunnel. Silver‐phase eels showed a better swimming performance (U_crit = 0.66 ms^?1) than yellow individuals (U_crit = 0.43 ms^?1). Male and female silver eels reached an identical U_crit despite their different sizes, which may be a strategy to increase the synchronization of arrival at the spawning grounds.     

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.     

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.     

Some morphometric and biochemical features of ready-to-migrate silver and pre-migratory yellow stages of the shortfin eel of south-eastern Australian waters

The mean total length ( L _T), mass and age of ready to migrate female silver shortfin eels Anguilla australis from the Hopkins River estuary and the mouth of the Merri River in south-eastern Australia, were 83·2 ± 1·2 cm, 1051 ± 51 g, and 17·2 ± 1·79 years, respectively. The eye index ( I _E) of the silver shortfin eels was < 5·2 (mean 7·64 ± 0·29) and differed significantly from that of the yellow shortfin eels collected from two other sites. The I _E increased with L _T (mm) and was related by log I _E= 2·656 log L _T6·925. The per cent moisture, protein and ash content of the liver of silver shortfin eels was significantly lower than in yellow shortfin eels, but lipid content was significantly higher in the former (35·5 ± 2·0%). The mean mass μg mg lipid ⁻) of saturates (230·4 ± 2·6 v . 181·7 ±2·6), monoenes (367·4 ± 6·3 v . 290·8 ± 8·9) and PUFA (177·3 ± 5·3 v . 159·7 ± 4·6) in muscle was significantly higher, and the great majority of individual fatty acids was found also in higher quantities in silver shortfin eels. In the liver, the PUFA found in the highest quantity was 22:6n-3, except in shortfin eels from Hopkins River estuary, and the amount of 18:2n-6 in the liver of silver shortfin eels was significantly higher than that in yellow shortfin eels but the reverse was true of 20:4n-6. In both muscle and liver tissues the saturate 16:0 and the monoene 18:ln-9 collectively accounted for >50% of all the fatty acids in the lipid.     

Effects of diet supplementation with carp ovary on gonad differentiation and growth of the European eel

Treating elvers of European eel Anguilla anguilla with mature carp ovary for 3–6 months during early growth induced female differentiation in 51·6–66·7% of treated animals compared with c . 5% in controls. The treatment also induced differentiation of ovaries in eels <13 cm L _T and a higher number of Syrski organs with ambisexual characters, and was most effective when administered at an early growth stage. The results could be attributed to the natural steroid content of the carp ovary. The total weight of treated animals at the end of the farm experiment was 84·7% higher than controls. The specific growth rate for weight was significantly higher in female yellow eels than in males, for both control and treated groups. The enhanced growth was related to induced feminization. A diet supplementation with mature carp ovary could be a good approach to control of sex differentiation and growth in eels.     

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.     

The silvering process of Anguilla anguilla: a new classification from the yellow resident to the silver migrating stage

The identification of five stages for female and two stages for male eels Anguilla anguilla using multivariate analysis was carried out on a large sample of individuals collected at six different locations in France. Stages corresponded to a growth phase (stages I and II), a pre‐migrant phase (III) and two migrating phases (IV and V). It is likely that an important period of growth triggered silvering through the production of growth hormone (GH) in stage III eels. In migrating eels gonad development, gonadotropin hormone (GTH‐II) production and increase of eye surface were similar at all sites. Differences among locations were found in gut regression and pectoral fin length. As variability for these increased with the size of the watershed and values were highest for the most downstream locations, fin length and gut regression may indicate the time since an eel started its migration.     

Growth of European eel Anguilla anguilla along the southern Baltic coast of Germany and implication for the eel management

To detect growth differences of European eel Anguilla anguilla along the southern German Baltic coast 728 yellow eels, with total lengths ranging from 256 to 944 mm and ages ranging from 3 to 15 years were collected from six coastal areas from 2005 to 2009. The estimation of the growth performance was based on the otolith increments. The mean growth rate of the female yellow eels varied from 56 to 62 mm?year^–1. No significant differences in the mean growth rate were detected between eels from inner and open coastal areas. The overall mean annual increment of eels was estimated at 59 mm?year^?1. Specific growth rates (SGR) of female yellow eels decreased with increasing age from 0.68 %?day^?1 in the first year to 0.05 %?day^?1 in the tenth year. Results indicate that no separation is needed in the development of population models or management initiatives based on the growth performance of eel in inner and open coastal waters of the southern German Baltic coast.     

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.