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.     

Dynamics and predicted decline of Anguillicola crassus infection in European eels, Anguilla anguilla, in Neusiedler See, Austria

The eel population in Neusiedler See has been maintained by regular massive stocking since 1958. After the establishment of the National Park Neusiedler See-Seewinkel in 1993, eel stocking was prohibited and the population, together with the specific parasites of eels, was predicted to decline to extinction within 10 years. This investigation was undertaken to document the decline and extinction of the Anguillicola crassus population in eels. From 1994 to 2001, 720 eels were collected from two sites in the lake. Prevalence and abundance of A. crassus were lower in spring than in summer and autumn and larger eels harboured more parasites than smaller ones. Neither year of study nor sampling site were correlated with parasite infection levels. No significant trend in the population parameters of A. crassus was detected over the 8 years of the survey. This suggested that there had been no significant decline in the eel population. This suggestion was confirmed by investigations of the fishery, which also found evidence of regular illegal stocking. The stability of the A. crassus population over the past decade seems to reflect the lack of change in eel population density. No mass mortalities of eels occurred over the period despite the many similarities between Neusiedler See and Lake Balaton in Hungary. Differences in eel size, eel diet and the lack of large-scale insecticide use are discussed as possible explanations for the absence of eel mass mortalities in Neusiedler See.     

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.     

Effects of DDT on the Fish Eagle Haliaeetus vocifer population of Lake Kariba in Zimbabwe

Twenty clutches were collected from nests of Fish Eagles Haliaeetus vocifer at Lake Kariba, Zimbabwe, and a small dam nearby in 1989-90. Unaltered DDT, and metabolites DDD and DDE, were found in every egg. Mean levels of σDDT(= DDT + DDD + DDE) generally varied from 14 to 49 mg/kg dry weight per clutch, but 113–223 mg/kg dry weight were found in clutches from the eastern end of the lake and the mouth of the Sengwa River. SDDT and DDE levels were significantly correlated with the Ratcliffe Index of eggshell thickness. Comparison with museum specimens showed that the Ratcliffe Index has declined by 11% since 1936-41 due to a significant fall in shell weight. Eggshell thinning exceeded 20% at the eastern end of the lake.
Aerial surveys in 1987 and 1990 found that hatching success along the southern lakeshore exceeded 72%, but chicks were seen in fewer than half the nests at the eastern end. However, the density of breeding pairs was greatest here.
Residue levels have increased by about 8% since 1980, rising more steeply in areas recently sprayed for tsetse fly control and falling in others. The threat from DDT may now be receding as regional use has declined and will end, for tsetse fly control, by 1995. None was used for this purpose in 1991. Mercury levels in adult birds were very high and may pose a significant risk.
The breeding population may be limited by availability of safe nest sites. Chicks are sometimes eaten by people. Settlement along the lakeshore is increasing and safe sites are becoming scarcer as dead trees in the lake collapse and large trees onshore are destroyed by elephants.     

The contribution of a pyrethroid insecticide to the massive eel (Anguilla anguilla) devastation, in Lake Balaton, in 1995

In the summer of 1995, 30 tonnes of eel (Anguilla anguilla) died in Lake Balaton, Hungary. An investigation was carried out to find the causes of this ecocatastrophe. During this investigation, certain biochemical parameters, i.e. the blood sugar level, the acetylcholinesterase (AChE, EC 3.1.1.7), lactate dehydrogenase (LDH, EC 1.1.2.3), glutamic-oxaloacetic transaminase (GOT, EC 2.6.1.1), and glutamic-pyruvic transaminase (GPT, EC 2.6.1.2) activities in the blood serum of the collected surviving and dying eels were examined. Deltamethrin, the active ingredient of the insecticide K-OTHRIN 1 ULV, used against mosquitoes was detected in different animal species, i.e. eel, bream (Abramis brama), pike perch (Stizostedion lucioperca), and the common gull (Larus canus) and in sediment samples from the lake. Additionally, laboratory experiments were carried out to study the effects of deltamethrin on eels. During the investigation in the field it appeared that the AChE activity was significantly lower in the blood serum of the dying eels as compared to that in living animals (P<0.05, Student's t-test). The blood glucose content exhibited a difference, too: it was 2.5 times higher in the dying eels than in the surviving ones. A huge increase in the LDH level was measured in the dying eels. The GOT activities of the serum were twice as high in the dying eels as in the living fish, while the GPT was not significantly changed. Deltamethrin was detected in different tissue samples of the dying eels: 2.70-18.1 microg/kg in the liver, 9.0-31.1 microg/kg in the gill and 3.0 microg/kg wet tissue in the muscle. Deltamethrin residues were found in tissue samples from other animals, in the following concentrations: 0.44 microg/kg in bream, 2.14 microg/kg in pike perch and 1.06 microg/kg wet tissue in dead gulls. The sediment samples collected from the sites of the devastation contained deltamethrin in a concentration of 5.50-30.00 microg/kg wet sediment at the time of the eel deaths, and in a concentration 7.00-8.75 microg/kg wet sediment a month later. Laboratory experiments with the insecticide K-OTHRIN 1 ULV revealed that 1.00 microg/l of its active ingredient, deltamethrin, caused the death of 50% of the eels after an exposure time of 96 h. During this experiments similar trends could be observed in changes of enzyme activities of the treated eels to those that were detected in filed study during the eel devastation in Lake Balaton. At the end of a one-week treatment with the insecticide at the concentration of 0.5 microg/l of its active ingredient the gills of the treated eels contained deltamethrin at 12.6-44.8 microg/kg wet tissue concentration, while at the 24th hour after the treatment (11.2-42.7 microg/kg wet tissue) deltamethrin concentration in the liver of treated eels could be detected. All the above-mentioned changes and the detected deltamethrin residue in the eels appear to demonstrate the contribution of deltamethrin to the severe eel devastation. This information on the ecological risk of pyrethroid insecticides might be useful in their further application.     

Horizontal variations in associations of zooplankton in Lake Kariba

The horizontal distribution of the zooplankton in Lake Kariba was sampled by means of vertical hauls using 55 and 250 μm meshed nets. Crustacea were sparse in the main lake, but increased dramatically in the very shallow water at the end of Crocodile Creek. The composition of the zooplankton is influenced by the presence in the main lake of the Tanganyika sardine, Limnothrissa miodon , and by the large size of the dominant diatom in the phytoplankton. In the creeks the distribution can be explained by the zooplankton occurring in open spaces into which planktivorous fish do not enter because of the possibility of predation by Tiger fish ( Hydrucyori ) and birds.
The systematic composition of the zooplankton in L. Kariba is reviewed and compared with two natural lakes and another man-made lake. It is concluded that the zooplankton and the fishes in L. Kariba have reached approximately the specific diversity that one would expect from a similar sized natural lake in the same geographical area.     

Environmental factors affecting migration of the European eel in the Rivers Severn and Avon, England

Studies were conducted during 1991–1993 on environmental factors affecting the upstream migration of eels in the Rivers Severn and Avon, England. Migrants (> 156 000 pigmented elvers and > 189 000 juveniles) were trapped as they attempted to ascend weir or sluice barriers. Multiple regression models were developed to compare catches per trap per night (C) with data for various key environmental parameters at seven sites, from the tidal limit to a maximum of 42.5 km upstream. The key stimulus for migration of both elvers and juveniles at the tidal limit was water temperature, with some weaker monthly influences related to seasonal temperature increases. Smaller annual influences probably related to earlier glass eel recruitment into the lower estuary. A weak early tidal effect was demonstrated only once, in 1993 in the Severn. Temperature also exerted significant effects on C of juvenile eels at the tidal limit and in the non-tidal rivers, although effects weakened with distance upstream. Year, month, river flows and whether traps were mounted on weirs or sluices made only small contributions at a few sites. Distance between traps also contributed to combined data for upper Severn sites. The threshold temperature in all cases was 14–16°C, with low to zero catches below 10–11°C, catch maxima being achieved above 18–20°C. The implications of strong temperature-dependence of migration in relation to stock recruitment and management are discussed. Special reference is made to recent decreases in recruitment of eels to Europe and N. America and possible long-term effects of global warming.     

Isolation and characterization of 12 dinucleotide microsatellites in the European eel, Anguilla anguilla L., and tests of amplification in other species of eels

Twelve polymorphic dinucelotide microsatellites in the freshwater eel Anguilla anguilla L. were isolated and characterized. Genetic diversity was assessed in eels from Lake Constance, Germany. Allele numbers ranged from five to 26 per locus with observed heterozygosities between 0.125 and 0.875. A portion of locus AangCT77 aligns with a transcribed region of the zebrafish gene crystallin beta B2. Cross-species amplification of most markers was possible for nine other Anguilla eel species. The newly developed primer pairs will facilitate population and conservation genetic studies in order to refine the understanding of the subtle population genetic structure typical of eels, and to identify interspecies admixture due to global trade.     

Diel and seasonal feeding activity of the short-finned eel,Anguilla australis schmidtii,in Lake Ellesmere,Canterbury, New Zealand

Synopsis Eel samples from Lake Ellesmere, a brackish water lake in the South Island, New Zealand, were taken at approximately monthly intervals from January 1974 to April 1976. Nets were set for 24 h and emptied every three hours. Total lengths and stomach fullness values of 498 eels were recorded. Analysis using the non-parametric Kruskal-Wallis test showed that eels ⩽40 cm and 40.1–50 cm in length increased in fullness through the night with greatest fullness values at 0300 and 0600 h. Seasonal analyses revealed greatest eel activity in spring, summer and autumn. There was little eel activity in winter.     

Sex differentiation in the European eel: histological analysis of the effects of sex steroids on the gonad

The effects of sex steroids on sex differentiation in the European eel were studied. The steroids, 17α-methyltestosterone (MT) and 17α-ethynylestradiol (EE), were given in the diet to 6–8 cm elvers and to 15–18 cm and 22–25 cm yellow eels. In our rearing conditions a very large percentage of the untreated eels developed as males. No masculinizing effect of MT could be demonstrated. The EE, administered at a dose of 10 mg kg^-1 of diet to 6–8 cm elvers and 15–18 cm eels, induced ovarian differentiation in about 90 and 65% of eels respectively, while in the control <5% of females was recorded. In 22–25 cm yellow eels a moderated feminizing effect was observed.
Histological analysis of the gonads of treated eels showed that sex steroids affect the gonadal structure. The androgen stimulates hypertrophy of compact connective tissue, early differentiation of Leydig cells, Sertoli cells and early formation of the spermatic duct. Oestrogen inhibits the differentiation of these structural components and stimulates the differentiation of follicular cells and an ovarian structure.
The involvement of gonadal structural components is discussed in relation to the effect of steroid treatment and to the peculiarities of sex differentiation in the European eel.     

THE EVOLUTIONARY GENETIC STATUS OF ICELANDIC EELS

The Iceland population of Anguilla eels contains an elevated frequency of fish with vertebral numbers lower than those typical of European localities. Several distinct hypotheses have been advanced to account for these morphologically atypical fish: for example, they could represent (1) genetically “pure” American expatriates, (2) genetically “pure” European types with ontogenetic abnormalities, or (3) hybrids between American and European forms. Here we critically test these and other possibilities by examining the joint distributions of allozyme markers, mitochondrial DNA markers, and vertebral numbers in Icelandic eels. The particular patterns of association among the genetic and morphological traits demonstrate that the Iceland population includes, in low frequency, the products of hybridization between American and European eels. Approximately 2–4% of the gene pool in the Iceland eel population is derived from American eel ancestry. This hybrid zone is highly unusual in the biological world, because the mating events in catadromous eels presumably take place thousands of kilometers from where the hybrids are observed as maturing juveniles. The molecular data, in conjunction with the geographic distributions, strongly suggest that the differences in migrational behavior and morphology between American and European eels include an important additive genetic component. Evolutionary hypotheses are advanced to account for the original separation of North Atlantic eels into American and European populations, and for the presence of hybrids in Iceland.     

Compilation of Japanese fisheries statistics for the Japanese eel, <Emphasis Type="Italic">Anguilla japonica</Emphasis>, since 1894: a historical dataset for stock assessment

Fishery sustainability and the extinction risk of the Japanese eel, Anguilla japonica, are of global concern. The landings of the Japanese eel in Japan comprise a large part of the landings in East Asia. This study provides a compiled dataset of the annual fisheries statistics of the Japanese eel in Japan for stock assessment. The Japanese government has been recording Japanese eel statistics annually since 1984 in five series of annual reports by conducting systematic questionnaire surveys of fisheries managers and associations; however, most of these data are stored in analog format. The key variables in the dataset include the harvest weight of eels, the harvest weight and number of seeds for aquaculture, the number of eels stocked, and the number of management entities engaged in the eel fishery. The levels of spatial aggregation of the variables include the site (river and lake), prefecture, inland and coastal waters, and total in Japan. We also incorporated location data (latitude and longitude) of the site and prefecture into the dataset. Eel harvest includes primarily yellow eels (late juvenile stage) and silver eels (mature stage). Seed harvest in inland waters includes glass eels (intermediate stage between leptocephalus and elver) and elvers (early juvenile stage). Seed harvest from coastal waters comprises glass eels. This dataset provides information to assess long-term trends in the Japanese eel population.     

Variability of mouth width in European eel,Anguilla anguilla,in relation to varying feeding conditions in three Dutch lakes

Synopsis Mouth widths of eels in three Dutch lakes were compared relative to feeding conditions over two years. Average mouth width varied by season and year and per lake. Of two groups of feeding specialists distinguished in each population chironomid feeders showed a smaller mouth width compared to fish feeders of the same body length. Proportions of these specialists differed between years, lakes, and seasons and changed in response to changing feeding conditions. The shift to fish feeding occurred when chironomid biomass decreased and young fish recruited. The shift was greatest in the year when chironomids collapsed completely and in the lakes with high eel densities. Mature males were narrow-headed and left the lakes at sizes between 30 and 40 cm. Larger eels were always broad-headed females. Implications for foraging efficiency and population structure are discussed.     

How have spawning ground investigations of the Japanese eel Anguilla japonica contributed to the stock enhancement?

After Schmidt’s discovery of the spawning area of the Atlantic eels Anguilla anguilla and A. rostrata, the search for the Japanese eel A. japonica began in the Pacific Ocean. In 1991, the spawning area of the Japanese eel was determined to be the western North Pacific. Because of enthusiastic research, eggs and maturing eels have been collected in the Japanese eel. These findings are the first for one of the 19 freshwater eels. The population sizes of the Japanese and Atlantic eels are linearly decreasing. Thus, these eel population sizes are considered outside of safe biological limits, and the current fisheries are not sustainable. Artificial propagation has not yet succeeded for the freshwater eels. Stock assessment and management of the European eel have received increasing attention; however, such assessments and management of the Japanese eel have not yet been seriously considered. This paper is an overview of the results of intensive spawning ground investigations of the Japanese eel and describes how the outcomes of these studies have contributed not only to biological interests but also to stock enhancement. During the past 20 years of expeditions, noticeable findings have only been collected for wild eggs and mature adult specimens in spite of the expenditure of large research grants and the large amounts of time invested. The outcomes throughout an expedition do not necessarily contribute to the development and improvement of artificial breeding techniques and stock enhancement. Thus, eel research should be more focused on the studies related to eel stock management.     

An Examination of Utilizing External Measures to Identify Sexually Maturing Female American Eels, Anguilla Rostrata, in the St. Lawrence River

It is well established that Anguillid eels undergo a complex suite of morphological and physiological changes during their transformation from resident, yellow-phase juveniles to actively migrating silver-phase eels. While it has been shown that some morphological measures can be used successfully to identify sexually maturing European eels, Anguilla anguilla, as well as Australian short fin, Anguilla australis, and long fin, Anguilla dieffenbachii eels, this relationship has never been quantitatively assessed for American eels, Anguilla rostrata. American eels of varying sexual development were collected from three locations on the St. Lawrence River: Lake St. Lawrence, Quebec City and Kamouraska. Sexual development of each eel was assessed with gonadosomatic index (GSI), oocyte diameter and degree of oocyte development. Morphological measures of total length, weight, head width, pectoral fin length and vertical and horizontal eye diameters were obtained from each fish. We used this data to test two hypotheses: (i) resident yellow phase eels, suspected migrants and known migrants are morphologically indistinguishable; and (ii) if differences exist, they cannot be used to reliably predict gonadal development or migratory status. Univariate analysis (ANOVA and ANCOVA) indicated that there were highly significant differences in all of the measured parameters and thus we were able to reject the first hypothesis. However, we failed to reject the second hypothesis as the high degree of overlap between groups eliminated the ability of any single measure to differentiate between resident and migratory eels. A multivariate discriminant model was developed that could classify only 72–80% of the eels correctly based on their morphological characters. While morphological measures may have some potential as a rapid, cost-effective method of pre-screening individual eels, morphological measures should not be considered a definitive indicator of sexual maturity or migratory status for female American eels in the Upper St. Lawrence River.     

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.     

Synopsis of scientific results of the sarec sponsored project on the ecology of Lake Kariba

A summary of current research on Lake Kariba is given. Lake Kariba is now a phosphorus limited oligotrophic lake, dependent on annual nutrient input for the maintenance of production. Nitrogen fixation by cyanobacteria has become an important source of nitrogen in the dry season and that fish harvesting is an important phosphorus sink. An up to date plankton composition list has been produced and plankton biomass determined. Diving studies indicate large biomass of bivalves. The aquatic vegetation displays both longitudinal as well as depth gradients, related to light regimes. Only preliminary data are available on mud/water interactions, but the available data indicate considerable fluxes in the river mouth stations and shallow protected littoral areas. The role of predatory birds and crocodiles in the fishery economy of the lake is evaluated and indicates no major conflict between these predators and human fishing interests. The research also draws attention to pesticides in Lake Kariba as indicated by work on fish eating birds and crocodile research. Since the publication of ‘Lake Kariba’ by Balon & Coche (1975) the research project, THE ECOLOGY OF LAKE KARIBA, being undertaken by the University Lake Kariba Research Station constitutes the first multidisciplinary study of the lake, 30 years after the Zambezi river was impounded at Kariba Gorge. It could have offered a unique opportunity of comparing the lake now with what it was soon after impoundment.     

Identification of Anguilla anguilla (L.) and Anguilla rostrata (Le Sueur) and their hybrids based on a diagnostic single nucleotide polymorphism in nuclear 18S rDNA

The variability of the nuclear 18S rDNA was examined for 10 species of freshwater eels: Anguilla anguilla, A. australis, A. dieffenbachii, A. japonica, A. marmorata, A. mossambica, A. nebulosa labiata, A. obscura, A. reinhardtii and A. rostrata. We observed that a single nucleotide polymorphism (SNP) separated A. anguilla and A. japonica from the remaining taxa. While this SNP marker may be used to identify hypothetical hybrids of A. japonica and sympatric eel species, we focused on Atlantic eels to evaluate the potential for this diagnostic chromosomal marker to discriminate between individuals of A. anguilla, A. rostrata and their hybrids.     

A preliminary assessment of the biomass of the pelagic sardine Limnothrissa miodon in Lake Kariba

The sardine Limnothrissa miodon was very patchily distributed in Lake Kariba in 1981–1983 and its mean biomass was 59 23 kg ha⁻¹, declining from 90–91 kg ha⁻¹ in 1981 to 38–66 kg ha⁻¹ in 1983. In addition, the mean size of the fish fell from 55–2 mm in 1981 to 49–9 mm in 1983. These changes are attributed to the effects of drought and reduced river flows as well as to the high level of commercial fishing for this species. There was a correlation between commercial catches and biomass estimates which allowed estimation of biomass in previous years from fishing statistics. Fishing effort appeared to be a major influence on sardine abundance: total biomass in 1985 was about 10% of what it was in 1974 when the fishery began. When expressed in terms of unit area, as is usual for African lakes, the pelagic biomass appeared to be greater in Lake Tanganyika, where Limnothrissa is endemic, than in Lake Kariba, although there is little difference in their primary productivity, but consideration in terms of volume rather than area shows that this was not the