Size-related differences in diel activity of two species of juvenile eel (Anguilla) in a laboratory stream

Abstract – The diel activity of three size groups (small=<100 mm; medium=100–199 mm; large=200–299 mm total length) of juvenile shortfinned ("shortfin") eels ( Anguilla australis ) and longfinned ("longfin") eels ( A. dieffenbachii ) was tested in a laboratory flow tank over a 48-h period during summer. All size groups of both species were nocturnally active, with the eels hiding in the substratum during the day and coming out on top of the cobbles from dusk to dawn, to feed. During the foraging period, the numbers and activity of all sizes of longfins visible were greater than those seen of shortfins, with the differences being more pronounced for small and medium eels. The activity of all eels consisted mostly of foraging by crawling, searching and probing for prey among the cobbles. Rate of activity increased with size of eel for both species. Small eels of either species did more swimming than eels of the larger sizes, whereas large eels were observed more frequently with only their head out of the substrate than were the smaller individuals. Feeding of small eels within the interstitial spaces of the streambed may explain their significantly lower activity on top of the substrate at night. The significantly lower rate of activity recorded for shortfins than longfins of all sizes may be due partly to their ability to feed within the interstices of the stream bed, and (or) longer time to recover from handling and habituate to the test environment. ^Note     

Variability in growth rates of freshwater eels (Anguilla spp.) in New Zealand

Abstract— Both species of New Zealand freshwater eels (the shortfinned eel Anguilla australis and the longfinned eel A. dieffenbachii ) are widespread and abundant, supporting important commercial and recreational fisheries. This article reviews growth studies from 35 widely distributed sites throughout New Zealand and discusses factors that influence growth rates. Length at given age is characterized by high intra- and inter-population variability; growth rates for eels >30 cm are typically slow (2–3 cm per year) and linear, with females generally growing faster than males. Water temperature affects the length of the growing season, although growth rates are not correlated with latitude. Other factors suggested as affecting growth rates are eel density, quantity and quality of food, and interactions between both eel species. Al though growth of some New Zealand eel populations is the slowest recorded for any species of Anguilla , growth in culture can be rapid, similar to that of other temperate eel species.     

Synthesis and possible function of 11-ketotestosterone during oogenesis in eel (Anguilla spp.)

The control of 11-ketotestosterone (11-KT) biosynthesis and its physiological roles were examined in female Japanese eel (Anguilla japonica) and New Zealand longfinned eel (Anguilla dieffenbachii). 11-KT was detected in serum of female eels of both species. Among various tissues from Japanese eel, the ovary had the greatest capacity to synthesize 11-KT in vitro. In addition, the oocyte diameters of eels treated with 11-KT had increased significantly. Furthermore, these oocytes were found to have an increased number of oil droplets. These findings suggest that 11-KT in female eels may be mostly of ovarian origin and that this androgen appears to play an important role in controlling pre-vitellogenic oocyte growth.     

Changing population structure of eels (Anguilla dieffenbachii and A. australis) from southern New Zealand

Abstract –  Commercial catches of longfin and shortfin eels ( Anguilla dieffenbachii and A. australis ) were sampled from southern New Zealand in the mid-1990s to determine the status of the eel population. We compared this with historical information on eel populations to highlight changes in the status, and then investigated the likely causes of these changes. A total of 20,722 longfins and 2366 shortfins were sampled from 216 landings. Longfins dominated catches comprising 53–100% (mean = 90%) of eel numbers. Shortfins were predominantly, but not exclusively, coastal in their distribution, whereas longfins were widely distributed through all reaches sampled. Mean size of shortfin was invariably larger than longfin and for both species generally increased with distance inland, particularly for longfin in the longest rivers. There was no clear geographical separation of longfin sexes although females were more common further inland in the longer rivers. Longfin sex ratio was about 4:1 (male:female), whereas in shortfin it was nearly 100% female. There have been marked changes in the status of the eel population in southern New Zealand over the last 60 years. We suggest that commercial fishing has resulted in a reduction in the size of both species, and for longfins a skewing of the sex ratio toward males – fishing may have favoured differentiation into males as well as the removal of the longer-lived longfin females. Skewed sex ratio and small size of longfins compared with those of shortfin suggest that this species has been more affected by fishing pressure than shortfins, possibly allowing shortfins to expand their range further inland.     

Spatial organisation of European eel (Anguilla anguilla L.) in a small catchment

Abstract –  Modelling-governing patterns of European eel ( Anguilla anguilla L.) distribution of four eel size classes (<150, 150–300, 300–450 and >450 mm) in the Frémur basin (northwest France) was done using artificial neural network (ANN) techniques and ecological profiles. Our results demonstrate the high predictive power of the ANN models. Some macro- and microscale factors, such as distance from the sea, depth and flow velocity, have the most significant influence on the models. Influence of distance from the sea appears to be very different from the spatial organisation usually described in river systems. In fact, the general tendencies of total eel densities according to the distance from the sea showed that densities increase weakly upstream. Another outcome was the variations in habitat preference according to the eel size, even if this species is spread over practically every type of microhabitat. Small eels were mainly found in shallow habitats with strong abundance of aquatic vegetation, whereas large eels tend to be found in intermediate to high depth with small to intermediate abundance of aquatic vegetation. Finally, we hypothesise that European eels change behaviour and microhabitat characteristic preference around a size of 300 mm.     

Longevity of longfinned eels Anguilla dieffenbachii in a New Zealand high country lake

A sample of 146 longfinned eels (Anguilla dieffenbachii), ranging in length from 231–1203 mm, was collected from Lake Rotoiti, Nelson Lakes National Park, New Zealand. Otoliths (n= 114) of eels were difficult to read, mainly due to the considerable age (maximum age 106 years). Growth rate was slow and linear, averaging 9 mm. year^?1. These are the slowest growth rate and maximum age from any study of Anguilla spp. to date. Lakes within National Parks provide the largest commercially unfished waters within New Zealand and hence provide reserve breeding stocks of longfinned eels; however, the average generation time of 93 years for female eels from Lake Rotoiti implies that stock management should be conservative.     

Estimation of production and long‐term changes in the population of European eel Anguilla anguilla (L.) in an Irish lake

Monitoring of yellow‐phase and silver‐phase Anguilla anguilla during their continental life history is necessary for evaluation of stock recovery measures. Eel population data for an Irish lake (Lough Sheelin) were compiled for the period 1993–2014. Catch data from 2009 to 2014 provided minimum estimates of recent silver eel production ranging annually from 0.79 to 1.84 kg/ha. Long‐term changes in yellow eel abundance and silver eel size structure were assessed as part of a fishery monitoring programme. Yellow eel catch per unit effort (CPUE) in the lake (from longline surveys) was considerably higher in the 1990s (52.2–62.1 eels/100 hooks) than 2002–2008 (1.9–15.8 eels/100 hooks). Conversely, during 1993–2014, the mean size of silver eels migrating from the lake increased significantly (p < .001), from 659 mm to 838 mm. The results suggest that in the absence of direct yellow eel abundance data, interannual variation in silver eel size structure may be a useful monitoring tool for local eel stocks as part of Eel Management Plans (EMP's).     

European eel distribution and body condition in a river floodplain: effect of longitudinal and lateral connectivity

Abstract – We studied eel population characteristics (size classes, densities and body condition) in the lower Loire River floodplain (France) to evaluate the effects of longitudinal and lateral gradients. A total of 36 sites were electrofished in June 2005. The sites were grouped first into three river segments according to the distance inland corresponding to a longitudinal gradient and secondly into three lateral connectivity categories. Results indicate that small eels, especially those ≤300 mm, were very abundant downstream but density rapidly decreased upstream. In addition, eels ≤150 mm tended to be most abundant in connected waterbodies. Conversely, the distribution of larger eels was quite uniform across both dimensions. Eel condition decreased upstream. In parallel, in downstream river segment, eels had a lower condition in disconnected waterbodies than in connected ones. Eel specialists still have only a longitudinal perception of eel habitat. Our results suggest that lateral gradients should be also taken into consideration.     

Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment

Abstract –  Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla , in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996.     

Analysis of distribution patterns of yellow European eels in the Loire catchment using logistic models based on presence–absence of different size-classes

Abstract –  We used logistic models to analyse the longitudinal distribution patterns of yellow European eels in the Loire catchment (France). Our models for predicting the presence of yellow eels supported contrasting spatial patterns according to the size of individuals and catchments. The smallest eels (mainly <150 mm and secondly 150–300 mm) were confined to downstream areas, but rapidly colonised upstream reaches. By contrast, larger eels were distributed further inland but tended to become more sedentary. In addition, a higher density of barriers to migration in the Maine catchment (a large catchment within the Loire catchment) probably delayed the upstream colonisation compared with the rest of the Loire catchment. These results suggest that logistic models may be a useful and efficient tool: (1) for the assessment of eel status and trends in freshwater systems; and (2) for the assessment of the consequences of local perturbations or conversely improvement of colonisation.     

Evidence of niche segregation between freshwater eels and conger eels in Kojima Bay, Japan

The aim of this study was to examine the possible interspecific interactions between the freshwater eel Anguilla japonica and the marine conger eel Conger myriaster. Spatial distribution, foraging time, and diet were compared between 380 A. japonica and 221 C. myriaster juveniles in a large brackish water bay and a tributary river estuary. Almost no C. myriaster were caught in the estuary, but both species coexisted in the bay at similar densities. For both species, percentages of eels with stomach contents were greater in the morning than in the afternoon, indicating they forage at night. Both species mainly fed on benthic crustaceans in the bay, but the size of the main prey species, mud shrimp Upogebia major, was significantly larger in A. japonica. The difference in prey size appears to be related to the eel body size (A. japonica: 559.5 ± 88.0 mm, C. myriaster: 356.4 ± 56.0 mm), which probably reflects the differences in the life history-related movements of these eels, with A. japonica using the bay at large sizes and C. myriaster using the bay as a nursery area before moving to deeper water. The differences in their migratory and movement patterns may result in niche segregation in the brackish bay where both species coexist.     

Variations in the population dynamics of the European eel Anguilla anguilla (L.) along the course of a Cantabrian river

Abstract– The size, density and production rate of eel were determined at 16 contrasting sites of 4 streams along the course of a Cantabrian river over the years 1990–1993. In addition, the diel cycles of feeding activity were determined monthly in another tributary over the year 1991–1992. Eels in the Esva were small, short-lived and mostly males (>99%). The seasonal patterns of eel density, feeding activity and condition were tightly fitted with each other, size-independent and regulated mainly by water temperature. Mean eel size increased with distance from the river mouth, but at each site, it remained similar between seasons and years. In contrast, density decreased upstream and showed marked seasonal and annual fluctuations. For all the sites, the number of eels increased in spring and summer and peaked by late autumn, coincident with higher water temperature and lower discharge. Mortality and migration rates were correlated with eel numbers for each stream, suggesting a densitydependent regulation. Production rates were correlated with distance from the river mouth, which explained only 10.2% of the variation, but they were also correlated with the initial and mean numbers, suggesting that site-specific factors acting upon density also influence production. Average production for the first year was 158.6 kg^–ha^–1. year^–1 (range 56.5–378.0) but decreased to 104.2 and 89.7 in the next 2 years. The reduction in the numbers of eels rather than the later reduction in mean size was responsible for this decrease. Monthly water temperature, distance from the river mouth and the initial number and size of eels for each site explain most of the variation observed in the population parameters along the course of the Esva River.     

The use of fyke nets as a quantitative capture technique for freshwater eels (Anguilla spp.) in rivers

Abstract  Fyke netting is a convenient and effective technique for capturing freshwater eels, and catch-per-unit-effort is usually assumed to be an index of eel abundance. The present study investigated the potential of depletion fishing using baited fyke nets to obtain population estimates of longfin eels Anguilla dieffenbachii Gray, in a river in the South Island of New Zealand. The probability of capture (capture efficiency) of a single night's fishing increased with increasing size of eel, and ranged from 0.2 to 0.9 for eels <400 mm, to 0.7–0.9 for eels ≥500 mm. The capture efficiency of baited vs unbaited nets was assessed in a small stream that was electric fished after netting trials were completed. Baited nets proved to be an effective method of assessing abundance of longfin eels (>400 mm) but not shortfins ( A. australis Richardson); capture efficiency of the population of longfins (the sum of fyke caught and electric fished eels) was 0.4 for a single night's fyke netting, increasing to 0.8 over four nights. Comparable efficiencies for shortfins were <0.1 and 0.3 respectively. Unbaited nets were markedly less efficient for both species.     

Gonad histology and plasma steroid profiles in wild New Zealand freshwater eels (Anguilla dieffenbachii and A. australis) before and at the onset of the natural spawning migration. I. Females*

To determine steroid profiles in immature and maturing female eels from the wild, non-migratory and migratory New Zealand longfinned (Anguilla dieffenbachii) and shortfinned (A. australis) eels were caught and blood and ovarian samples collected. Plasma steroid levels were determined and related to the developmental stage of the ovary. Ovaries of non-migrants contained oogonia and previtellogenic oocytes. Vitellogenic oocytes were never observed in these groups, but instead were very common among migrants (up to 88% of oocytes). Concentrations of both androgens (androstenedione (AD), testosterone (T)) and estradiol-17 (E2) were higher in migrants than in non-migrants. Among migrants, T levels were higher in shortfins (2.27 ± 0.14 ng ml–1) than in longfins (0.82 ± 0.10 ng ml–1), whereas E2 levels were higher in longfins (mean 2.46 ng ml–1) than in shortfins. Levels of sex steroids were generally low in non-migrants. In contrast, plasma levels of 17-hydroxyprogesterone were significantly higher in non-migrants than in migrants. Similarly, cortisol levels were higher in non-migrating than in migrating shortfinned, but not longfinned, females. 17,20-Dihydroxy-4-pregnen-3-one, the putative maturation-inducing steroid in anguillids, was near minimum-detectable levels for all animals examined. Surprisingly, very high levels of 11-ketotestosterone (KT) were found in migrants, averaging nearly 3 ng ml–1 in longfins and over 20 ng ml–1 in shortfins. The identity of KT and several 5-reduced androgens was confirmed using gas chromatography - mass spectrometry. The function of KT in females is not known, but we suggest that this steroid hormone may play a role in preparing maturing animals for their spawning migration.     

Two forms of cytochrome P450 cDNA from 3-methylcholanthrene-treated European eel Anguilla anguilla

ABSTRACT:     The cytochrome P450 (CYP) represents a large group of microsomal monooxygenases that catalyze drugs as well as a host of lethal environmental contaminants such as dioxins, leading to either detoxification and excretion from the animal or generation of carcinogenic intermediates. In the present study two forms of cDNA were cloned (Eu MC1 and Eu MC2) for European eel CYP1A genes by polymerase chain reaction (PCR) techniques. The cDNA of Eu MC1 was 3368 bp long coding 521 amino acid residues, and that of Eu MC2 was 2464 bp long coding 517 amino acid residues. Identities of deduced amino acid sequences between Eu MC1 and Japanese eel CYP1A1 and that between Eu MC2 and the second form of Japanese eel CYP1A were 98% and 97%, respectively, showing decisively that Eu MC1 and Eu MC2 are orthologous to Japanese eel CYP1A1 and the second form of CYP1A, respectively. A striking difference between the two eel species was that the Eu MC1 peptide was two amino acid residues longer than that of the Japanese eel CYP1A1. Existence of two loci of CYP1A in Japanese and European eels may suggest that the two forms of CYP1A exist widely among the eel species, because the divergence between the two eel species has been shown to be close to the basal divergence among eels. The identities in CYP1A may help to estimate genetic distance between European and Japanese eels.     

Freshwater protected areas: an effective measure to reconcile conservation and exploitation of the threatened European eels (Anguilla anguilla)?

Abstract – For decades, the European eel Anguilla anguilla (L.) population has been declining strongly despite several management attempts, so additional experiments need to be conducted on management measures. The use of freshwater protected areas has been advocated but their efficiency has never been assessed. In this study, we investigated whether the population structure and the silver eel (mature migrating stage) production differ in fished and protected areas within a marsh wetland (Brière, 7000 ha, Northwest France), using an intensive biological study (electrofishing and trapping) and a survey of the traditional fishery (licenses, questionnaires and creel surveys). First, we found that fishermen mainly targeted >320‐mm yellow eels (sedentary stage) using pots and square dipping nets and that harvest by fishermen was highly variable at different locations in the study area. Secondly, we found differences in the size‐class structures and mortality rates between protected and fished areas. Mortality rates of eels >320 mm was positively correlated with harvest by fishermen. Furthermore, the proportion of potentially migrating eels in the total population was found to be higher in the protected areas than in fished areas (6.38% vs. 1.42%, respectively). Thirdly, we found that protected areas potentially produce 8.4% of the total silver eel production whereas they only account for 2.4% of the aquatic habitat area. We estimated that a size adjustment of protected areas to 31.1% with maintaining the current fishery would produce 50% of the potential silver eel of a fully protected marsh. Protection of freshwater areas appears to be a promising management measure and a constructive consensual way to integrate the patrimonial and societal value of the traditional fishery and the international management plans for European eels. Furthermore, freshwater protective measures can be an effective local solution if they are integrated into the framework of freshwater biodiversity management and accompanied by other management measures that focus on all eel life stages.     

A caution for conservation stocking as an approach for recovering Atlantic eels

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Overwintering dormancy behaviour of the European eel (Anguilla anguilla L.) in a large lake

Overwintering dormancy behaviour was studied in female silver eels in Lake Mälaren in Sweden between 2008 and 2010. Depth choices and movements in relation to temperature were analysed from pressure and temperature records for 13 eels with implanted data storage tags, covering 17 overwintering periods and three intervening summer periods. Dormancy commenced in October–November as temperatures fell below 4–12 °C. Eels tended to remain motionless throughout the winter, with some short periods of activity signalled by small changes in depth distributions. During dormancy, the eel shows a clear avoidance of shallow areas <5 m in favour of the 10–25‐m‐depth interval. Activity tended to resume 4–6 months later in April–May as temperatures rose above 3–7 °C and ice cover broke, and eels spent more time at shallower depths of <5–10 m. The majority of the eels were assessed as being in the silver eel stage at the time of tagging. During the autumn months, the diving behaviour, with frequent and large vertical excursions and periods at the surface, was similar to that seen in migrating eels in the Baltic and Atlantic Ocean. In spring and summer, the behaviour differed, being dominated by more gradual depth variations, implying that the eels reverted from silver eel migration behaviour to yellow eel foraging behaviour. Body weight declined during dormancy, but other studies of starvation over comparable time periods showed significantly higher average specific weight losses, implying that the Mälaren silver eels must have fed between the end of dormancy and recapture.     

Notes on the occurrence of two tropical species of Anguilla in reservoirs in south-eastern Sri Lanka and preliminary data on the populations

Abstract. The occurrence and abundance of eels were investigated in five lowland reservoirs in south-eastern Sri Lanka. Paired fyke nets and longlines were used for sampling eels. Two tropical species of eel. Anguilla bicolor McClelland and Anguilla nebulosa McClelland, were caught. In two of the five reservoirs investigated only A. bicolor was found and in another two reservoirs both species were caught. Eels seemed to be absent from one reservoir. The CPUE in reservoirs with eel was between 0·13 and 2·60 eels of both species per fyke net and night, and the total weight was between 81 and 814g. A small capture-recapture experiment in Badagiriya, where eels seemed to be most plenliful, gave a population estimate of only six eels or 1·2 kg per ha (eels longer than 40cm). Such a low abundance was regarded as unrealistic. Stocks of eel could presumably, at least in some reservoirs, withstand commercial exploitation but more information is needed on recruitment, growth and migration before any advice can be given on the management of stocks.