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.     

On the age and growth of the freshwater eel (Anguilla anguilla)

The growth of over a 1000 eels from three different watersheds was investigated. Because of the difficulties of sexing eels we were only able to compare the size of females with that of eels with lobulate organs (some of the latter are potential females). Generally female eels grow faster than the others. There was little difference in the growth of eels with lobulate organs even though there was a great difference in the eel population density in the three localities. But, in any one locality there was a large range of growth. It is suggested that the competition for food is most acute amongst the smaller size groups. As they grow, eels tend to become cannibalistic and find small eels and elvers an easy source of food.
The growth of some continental eels has been related to temperature but this is not confirmed by experimental work. Nevertheless eels eat little or nothing at low temperature as is shown by the large percentage of eels with empty stomachs in our samples.     

Ecological aspects of the Japanese eel, Anguilla japonica, collected from coastal areas of Japan

The ecological characteristics of 597 yellow and silver-stage Japanese eels, Anguilla japonica, were examined and compared among collection sites located at three different latitudes of Japan (Amakusa Islands, Mikawa Bay, and Sanriku Coast) to provide basic data on this unusual catadromous fish species. Eels were sexed and their total length, body weight, age, and growth rate based on otolith analysis was compared among sexes, stages, and collection sites. The overall sex ratio favored females (94%), but the sex ratio differed among the three locations. The frequency of females was highest in the coastal waters at Sanriku in the north (100%), next highest at Mikawa Bay in central Japan (95%), and lowest in the Amakusa Islands in the south (70%). Silver eel males ranged from 41.2-66.3 cm in length and 4-10 years in age, and silver eel females from 44.3-97.2 cm in length and 5-17 years in age. Female eels generally grew faster (8.7+/-2.2 cm/year) than males (6.4+/-2.6 cm/year), and the growth rate slowed in the older eels. The growth rate of A. japonica at all three sites was much faster than that of other temperate anguillid species (< 4 cm/year), and their age at maturation was younger than that of other temperate species (approximately 7 to > 50 years), suggesting this species has important ecological differences from other similar species.     

Induced oogenesis of the European eel (Anguilla anguilla L.) in freshwater condition

European eel is a catadromous fish species, which means that after living in freshwater premature individuals adapt to sea water, and migrate to the Sargasso Sea for spawning. Although male eel can be sexually matured even in freshwater, to date, it was believed that female eel can be matured only in seawater. Here we show that the process of sexual maturation may be induced in freshwater by treating female eels with carp pituitary (GSI = 9.87 ± 1.55%). It is thus proposed that seawater condition is not an obligatory environment for stimulating gametogenesis and for artificial maturation of the European eel in neither gender.     

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.     

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.     

Low fat contents in female silver eels: indications of insufficient energetic stores for migration and gonadal development

The main energetic stores at the silver eel stage were studied by analysing muscle fat concentrations and hepatosomatic indices in female silver eels from various habitats in Sweden. Muscle fat concentrations varied both within and between localities and lean eels with muscle fat concentrations <20% occurred at all study sites. Furthermore, no correlation could be found between muscle fat content and internal or external maturation indices, neither was the relative liver size related to the maturation process, as the correlation between the hepatosomatic and gonadosomatic indices was very weak. Consequently, it was concluded that silvering and the spawning migration may begin also at low muscle fat concentrations. However, most of the energy reserve is stored as muscle fat in eel, and it is highly unlikely that female silver eels with such low fat contents, as were observed occasionally in this study, will ever recruit to the next generation. Therefore, it is suggested that the maturation process in eel is more flexible than previously recognized, and that this process might be temporarily arrested and feeding resumed during the first part of the migratory phase.     

Colonisation of freshwater habitats by the European eel Anguilla anguilla

1. The spatial distribution of European eels in 18 U.K. rivers was related to distance from tidal limit using a negative exponential model. This function accounted for between 19 and 90% of the variation in eel density where quantitative data was available. For semiquantitative data the negative exponential function was a significant predictor of eel densities in only six out of 10 cases, although all rivers showed a consistent decline in abundance with distance upstream from the tidal limit. 2. The spatial distribution of different age groups of European eel in River Severn showed an initial rapid dispersion into freshwater followed by a much slower dispersion rate. Movement of the population upstream by a wave‐form migration process does not occur in this system. Instead colonisation of freshwaters can be seen as a two‐phase dispersion. Phase‐1 is a rapid dispersion upstream driven by density at the point source. Phase‐2 commences once the eels become yellow eels and is equivalent to random diffusion of particles. 3. These processes have important implications for the penetration of freshwaters with reduced numbers of eel larvae arriving on the coast of Europe and North America. Eel abundance will decrease more in freshwaters in an upstream direction whilst it may remain stable or decrease to a lesser extent in estuaries. They are also able to explain the demography of eels migrating upstream over weirs and the observations of varying sex ratios within catchments. We conclude that a dispersion model dependent on age, temperature, difficulty of migration, habitat quality and density of eels should be an important part of freshwater eel management.     

The influence of growth rate on the size of migrating female eels in Lake Ellesmere, New Zealand

Lake Ellesmere, a large coastal lake in the South Island of New Zealand, supports an important commercial eel fishery, based mainly on migrating (silver) male Anguilla australis . Lengths of silver female eels from samples collected in 1942, 1974–1982 and 1998–1999 showed an initial decline between 1942 and 1974 but an increase from 1979 onwards. Back-calculated growth rates of 50 female silver eels caught in 1998 showed that most (90%) exhibited a period of accelerated linear growth commencing at lengths between 380 and 660 mm (mean 598 mm); this accelerated growth coincided with a change in diet to piscivory. The onset of maturity was more closely associated with length than age, condition, or growth rate. The increase in average length of female silver eels of 250 mm over the past 20 years is consistent with the hypothesis that female eels adopt a size-maximizing growth strategy to ensure maximum fecundity; this is the first time this hypothesis has been demonstrated from temporal changes within a single population.     

Correlated contemporary evolution of life history traits in New Zealand Chinook salmon, Oncorhynchus tshawytscha

Size at age and age at maturity are important life history traits, affecting individual fitness and population demography. In salmon and other organisms, size and growth rate are commonly considered cues for maturation and thus age at maturity may or may not evolve independently of these features. Recent concerns surrounding the potential phenotypic and demographic responses of populations facing anthropogenic disturbances, such as climate change and harvest, place a premium on understanding the evolutionary genetic basis for evolution in size at age and age at maturity. In this study, we present the findings from a set of common-garden rearing experiments that empirically assess the heritable basis of phenotypic divergence in size at age and age at maturity in Chinook salmon (Oncorhynchus tshawytscha) populations introduced to New Zealand. We found consistent evidence of heritable differences among populations in both size at age and age at maturity, often corresponding to patterns observed in the wild. Populations diverged in size and growth profiles, even when accounting for eventual age at maturation. By contrast, most, but not all, cases of divergence in age at maturity were driven by the differences in size or growth rate rather than differences in the threshold relationship linking growth rate and probability of maturation. These findings help us understand how life histories may evolve through trait interactions in populations exposed to natural and anthropogenic disturbances, and how we might best detect such evolution.     

The leucocyte enzyme cytochemistry of the Indian eel, Anguilla bicolor bicolor McClelland

Activity of peroxidase, alkaline phosphatase, acid phosphatase, α-naphthyl acetate esterase and naphthol-AS-D chloroacetate esterase in the leucocytes of the Indian eel, Anguilla bicolor bicolor McClelland, was studied. But for some minor differences, the enzyme pattern of the leucocytes, especially neutrophil peroxidase, of the Indian eel closely resembled that of the New Zealand eels, suggesting that the Indian eel is phylogenetically more closely allied to the New Zealand eels than to the European eel.     

Populations of Bothriocephalus claviceps (Goeze) (Cestoda) in the European eel, Anguilla anguilla (L.), in three localities in southwest England

The population biology of Bothriocephalus claviceps (Goeze) has been investigated in the European eel, Anguilla anguilla (L.), in three localities in southwest England. Monthly changes in prevalence and abundance show no statistically significant seasonality, in contrast to growth and reproduction of the cestode. The cestode reaches maximum size and becomes gravid mainly in summer. Gravid cestodes die and are lost soon after reproduction. This overlaps with recruitment of the new generation into eels, and may result in a rapid increase in infection levels and in a more over-dispersed distribution of the parasite in some localities, or in a more gradual change in infection levels and over-dispersion in others. Development and growth of the new generation may cease until the following spring, or continue at a slow rate over winter. Parasite numbers in each host decrease well before they actually reach maturity, since the majority of gravid individuals are present in those eels which harbour only one parasite. This suggests that few cestodes develop to maturity and most are lost from hosts as development proceeds.     

11-ketotestosterone synchronously induces oocyte development and silvering-related changes in the Japanese eel, Anguilla japonica

To evaluate the effects of sex steroids on silvering in the Japanese eel, Anguilla japonica, the development of oocytes, eye size, digestive tract, and swim bladder were studied in relation to observations of the profiles of plasma levels of sex steroids (estradiol 17β, E2; testosterone, T; 11-ketotestosterone; 11-KT) during silvering for each sex and by administrating 11-KT to yellow eels. All steroids examined in the study increased in female eels after silvering had begun, whereas in males, only 11-KT increased significantly, and no statistical differences were found in plasma levels of E2 and T between eels in both developmental stages. 11-KT appeared to induce the early stage of oocyte growth, enlargement of the eyes, degeneration of the digestive tract and the development of the swim bladder. This suggested that 11-KT synchronously accelerates early development of the ovaries and the morphological changes, possibly in adaption to oceanic migration, and that 11-KT is one of the most important factors in early stages of development in the Japanese eel, as it appears to be in other anguillid eels.     

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.     

Changes in vision and olfaction during sexual maturation in the European eel Anguilla anguilla (L.)

Retinal structure was examined in sexually immature and artificially matured female Anguilla anguilla . Inner nuclear layer cell numbers decreased from 600 mm⁻¹ retinal cross-section to 300 mm^−I and ganglion cells from 60 to 30 mm⁻¹ cross-section, during sexual maturation. Most of the decrease occurred prior to the stage of maturation at which migration begins. Electroretinograms were recorded from the intact eyes of immature and maturing eels. There was no change in scotopic sensitivities to light of wavelengths 480 and 520 nm, with increasing sexual maturity. Olfactory organs were examined in female eels of a similar range of maturity states and were found to atrophy in artificially matured eels of advanced development. The density of mucous cells in olfactory lamellae decreased from a maximum of 443 mm⁻² in sexually immature eels to as low as 19 mm⁻² in sexually maturing eels. The changes in vision and olfaction were thought to indicate a change in the relative importance of the two sensory modalities with sexual maturation.     

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.     

Sexual size dimorphism in species with asymptotic growth after maturity

If animals mature at small sizes and then grow to larger asymptotic sizes, many factors can affect male and female size distributions. Standard growth equations can be used to study the processes affecting sexual size dimorphism in species with asymptotic growth after maturity. This paper first outlines the effects of sex differences in growth and maturation patterns on the direction and degree of sexual dimorphism. The next section considers the effects of variation in age structure or growth rates on adult body sizes and sexual size dimorphism. Field data from a crustacean, fish, lizard and mammal show how information on a species' growth and maturation patterns can be used to predict the relationships between male size, female size and sexual size dimorphism expected if a series of samples from the same population simply differed with respect to their ages or growth rates. The last section considers ecological or behavioural factors with different effects on the growth, maturation, survival or movement patterns of the two sexes. This study supports earlier suggestions that information on growth and maturation patterns may be useful, if not essential, for comparative studies of sexual size dimorphism in taxa with asymptotic growth after maturity.     

Ecological factors influencing the composition of the parasite fauna of the European eel, Anguilla anguilla (L.), in Ireland

The parasites of 121 eels from three contrasting sites in the Corrib catchment area, western Ireland, were investigated. Thirteen species, Ergasilus gibbus, Diplostomum gasierostei, Diplostomum spathaceum, Sphaerostoma bramae. Bothrocephalus claviceps, Proteocephalus macrocephalus, Camallanus lacustris, Cucullanus truttae, Paraquimperia tenerrima, Raphidascaris acus, Acanthocephalus clavula, Acanthocephalus lucii and Pomphorhynchus laevis , were recorded. Two species, P. macrocephalus and P. tenerrima , have not previously been reported from Ireland. Microhabitat preferences of the parasites were noted. Variations in the occurrence and intensities of the parasites observed were analyzed in relation to sampling period, host habitat and characteristics of the eel populations studied. A variety of factors were shown to be of importance, including composition of the fish communities and distributional patterns of intermediate hosts and piscivorous birds. Differences were noted in the parasit-ocoenoses of eels in still and running water sites. The occurrence and intensities of infection of several parasite species were shown to be related to age and size of host: the occurrence of B. claviceps. C. truttae, P. tenerrima and R. acus was shown to be related to either age or size of eels, which is accounted for by the fact that eels become increasingly piscivorous with age and increasing size. Little evidence of interspecific interactions was noted.     

Sexual bimaturation and sexual size dimorphism in animals with asymptotic growth after maturity

Many animal taxa exhibit a positive correlation between sexual size dimorphism and sex differences in age at maturity, such that members of the larger sex mature at older ages than members of the smaller sex. Previous workers have suggested that sexual bimaturation is a product of sex differences in growth trajectories, but to date no one has tested this hypothesis. The current study uses growth-based models to study relationships between sexual size dimorphism and sexual bimaturation in species with asymptotic growth after maturity. These models show that sex differences in asymptotic size would produce sexual bimaturation even if both sexes approach their respective asymptotic sizes at the same age, mature at the same proportion of asymptotic size and have otherwise equivalent growth and maturation patterns. Furthermore, our analyses show that there are three ways to reduce sexual bimaturation in sexually size-dimorphic species: (1) higher characteristic growth rates for members of the larger sex, (2) larger size at birth, hatching or metamorphosis for members of the larger sex or (3) smaller ratio of size at maturity to asymptotic size (relative size at maturity) for members of the larger sex. Of these three options, sex differences in relative size at maturity are most common in size-dimorphic species and, in both male-larger and female-larger species, members of the larger sex frequently mature at a smaller proportion of their asymptotic size than do members of the smaller sex. Information about the growth and maturation patterns of a taxon can be used to determine relationships between sexual size dimorphism and sexual bimaturation for the members of that taxon. This process is illustrated for Anolis lizards, a genus in which both sexes exhibit the same strong correlation (r 0.97) between size at maturity and asymptotic size, and in which the relative size at maturity is inversely related to asymptotic size for both sexes. As a result, sexually size-dimorphic species of anoles exhibit the expected pattern of a smaller relative size at maturity for members of the larger sex. However, for species in this genus, sex differences in the relative size at maturity are not strong enough to produce the same age at maturity for both sexes in sexually size-dimorphic species. Members of the larger sex (usually males) are still expected to mature at older ages than members of the smaller sex in Anolis lizards.     

Morphological and histological changes in the swim bladder during maturation of the Japanese eel

Silver eels Anguilla japonica ( I _G 1·5–3·5) had more developed rete mirabile, gas gland and submucosa than yellow eels ( I _G 0·4–1·5) and the development of swim bladder components increased with sexual maturity only in the early maturation process ( I _G 3·5). These observations indicate that the Japanese eel develops its swim bladder in either the river or shallow sea water and leaves for the open sea after the swim bladder has become adapted to a deeper-sea environment.