Young Eels Research Articles

Optimizing the mass marking of fish with alizarin red S: an example with glass eels.

Fish marking is an essential tool for fisheries management, especially for evaluating the stocking of endangered fish species to support conservation and sustainable use of fish stocks. Batch marking of young European eels Anguilla anguilla (L.) prior to stocking is recommended as the benefits of stocking for the spawning stock can be evaluated by recapturing marked fish over time, therefore mass marking of young eels with substances such as alizarin red S (ARS) is becoming increasingly important. To improve the marking method and reduce marking costs when immersing glass eels in an ARS solution, eight laboratory experiments under varying conditions (e.g., temperature, ARS concentration, immersion time, osmotic induction, fish density) and with ARS from different suppliers were carried out. The results show that optimal marking of glass eels can be carried out in the field or during transport by putting approximately 50 g of glass eels per liter in 150 mg L-1 ARS solution for 3 h at 10-15°C. Lower concentrations did not result in reliable marking. Water temperatures of 5°C and below can have a stunning effect on the eels and increase mortality significantly, regardless of the concentration of ARS. Glass eel densities below 50 g L-1 in the marking bath increase marking costs unnecessarily, while a higher density of 100 g L-1 resulted in significantly higher mortality and lower marking success. A somewhat more difficult but less expensive alternative is to bathe the fish in a saline solution of 1% (10 PSU) of 80 mg L-1 ARS for 3 h at 10°C. Costs can also be significantly reduced by choice of supplier for ARS, but care should be taken as the quality of the powder appears to vary (mean percentage of sufficiently marked eels ranged from 59% to 91% among suppliers in the present study) and can lead to marking failure. The optimal marking conditions can help ensure that stocked glass eels can be reliably identified in future studies to assess stocking benefits while reducing costs.

К вопросу обоснования величины коэффициента промыслового возврата европейского угря (Anguilla anguilla) в заливах Калининградской области

The cessation of stocking of the Vistula Lagoon with eels by Poland in the period from 1995 to 2004, and the ongoing depression of the eel population, led to a decrease in eel catches in both parts of the bay to 4–5 tons / year by the end of the first, most of the second decade of the 21st century. The tendency of eel population decrease was even more pronounced in the Curonian Lagoon, which has never been stocked with eel in historical retrospective. Eel catches in the Russian part of the bay fell from 150 tons in 1970 to 3-4 tons by the 1990s. and less than 1 ton in the present century. Stocking pasture reservoirs with young eels is a way not only to preserve eel stocks, but also to reasonably increase catches. In solving this problem, the question of the value of the commercial return coefficient becomes.

Riverine distribution and abundance of Japanese eel in Shikoku, south‐western Japan: Varying importance of regional and local factors

Abstract Regional and local habitat factors affecting the distribution and abundance of Japanese eel (Anguilla japonica) were systematically analysed in 123 reaches of 21 stream systems in south‐western Japan, with particular attention paid to human habitat modification, such as artificial barriers and concrete revetment. The density of Japanese eel decreased upstream, and they were absent from the upper reaches of each stream system. The upper limits of their distribution were explained by the distance from the sea and the channel gradient, rather than barriers. However, the number of barriers had the highest explanatory capacity for the upstream decline in abundance of small eels (<40 cm in total length). Analyses of the relative importance of regional and local habitat factors showed that the number of barriers (regional factor) was more important than local habitat factors in determining small eel abundance, whereas regional and local habitat factors (e.g. concrete revetment or cover habitat) were similarly important for large eel abundance. These results suggest that: (i) barriers are not a major factor limiting the distribution (presence/absence) of the Japanese eel in the streams studied; (ii) barriers can substantially decrease the abundance of young eels; and (iii) as eels grow older and larger, local habitat factors, such as bank conditions and cover, become more important in determining their abundance. The results for the relative importance of regional and local habitat factors suggest that in habitat restoration for the Japanese eel, priority should be given to mitigating barrier effects. The improvement of longitudinal connectivity to facilitate colonization by young eels is primarily needed for a more efficient use of the existing carrying capacity of streams. The improvement of local habitat conditions to increase the carrying capacity should be conducted in stream reaches free from barrier effects.

Do Metabolic Enzymes Explain Osmoregulation in Juvenile American Eels

During the spring, juvenile eels (Anguilla rostrata) (5-6 cm; 50-200 mg) gather in creeks along the Hudson River Tributary Estuary after migration from the Atlantic Ocean. This requires them to quickly adapt to environments with lower salt concentrations. We wondered how this is achieved. To study eels in a lab setting, we transferred them into Instant Ocean® sea salt (100% artificial sea water [ASW]) or dilutions thereof for 2 weeks; mortality was only observed in 100% ASW (37% expired in 3 hours). Interestingly, after 4 hours, 10 or 50% ASW eels showed little change in body mass, suggesting osmoregulation. While much progress has been made in understanding the mature eel response, few studies have reported enzyme activities in young eels. We hypothesized that dehydrogenases and acetylcholine esterase activities might provide a clue to explain osmoregulation and survival. Supernatants of whole juvenile eel extracts obtained from these experiments (IACUC-approved protocol) were run on 7.5% Tris-glycine PAGE gel (150V, 50 min) and the activities of non–denatured isozyme bands were analyzed with Bio-Rad ChemiDoc™ XRS+ Imager. Denatured supernatant extracts were also run on TGX 4-15% Tris-glycine PAGE gel to assess total protein banding. There was little difference in number of protein bands between eels exposed to 10% vs 50% ASW(2 and 3 weeks); however, band intensities were significantly higher for 50% ASW. Further studies revealed that two MDH (malate dehydrogenase) isozyme bands were present in extracts of eels exposed to a broad range of salinities: a relatively heavier band as mitochondrial (m) MDH and a lighter cytoplasmic (c) MDH. Activities of the mMDH bands were more robust than the cMDH bands at 50% and 10% ASW. Although the mMDH activities in eels exposed to either salinity were similar, the cMDH activities in eels exposed to 50% ASW were greater than in eels exposed to 10%. Additional experiments identified four acetylcholine esterase (AChE) isozyme bands in eel extracts; the activity of only one was significantly prominent in all exposures. Transferring the eels from 50% to 10% ASW reduced activity of the relatively heaviest isozyme band. Notably, we could not discern any fluctuations in lactate dehydrogenase (LDH) levels. In conclusion, while these preliminary findings do not reveal any concrete trends in metabolic regulation tied to survival, it does appear several key enzymes have a translational correlation tied to saline homeostasis. We acknowledge that the interpretation of these experimental results is complicated by a lack of genetic validation (i.e., sequences information and parental lines) - “nature” . In addition, the investigators were unable to track or define the environmental stresses faced during migration by individual juvenile eels before capture, which may manifest itself in unique stress responses - “nurture.”

Perspective plan for sustainable eel management in Lake Poso, Central Sulawesi

Lake Poso has high eel potential and a strategic position in supporting the development of eel fisheries in Sulawesi. Lake Poso is out of 15 national priority lakes in natural resources and environmental management programs. The catch of eel from Lake Poso is reported to have been decreasing for the last twenty years. Several factors causing the decline in eel populations are fishing activities, disruption in migration routes, and other factors such as changes in habitat and environmental conditions. The sustainability of the life of the Poso eel is determined by the success of the eel migration process. The migration of adult eels to the sea and young eels from glass eels to freshwater habitats must continue. The perspective plans that can be done to manage Poso's eel resources include 1) Socializing the eco-biological characteristics of eels as catadromous fish; 2) Establishing conservation areas and regulation of eel fishing; 3) Increasing eel population through restocking 4) Integrating fisheries and aquaculture activities; 5) Developing eel culture technology through size segmentation production; 6) Developing an eel migration monitoring system; 7) Building a cross-sectoral forum and put a joint action program; 8) Proposing the Poso eel Research and Conservation Center.

Japanese eels, Anguilla japonica, can surmount a 46-m-high natural waterfall of the Amikake River system of Kyushu Island, Japan

Restricted access to upstream riverine habitats is considered to be an important factor affecting abundance declines of wild Japanese eels. However, the characteristics of cross-river structures obstructing upstream movements of Japanese eels have not been well documented, except for weir height. Aiming to establish a case study for estimating the characteristics of barriers to movements, we conducted field surveys for eels downstream and upstream of a 46 m-high natural waterfall. The results of our surveys showed that eels were present in river reaches above the 46 m-high waterfall and other more-upstream obstacles, which indicated that highly motivated eels moved upstream by climbing the rock wall of the waterfall and other obstacles. The eel densities upstream of the waterfall were much higher than the density estimated from modeling based on waterfall height in a previous study, although eels were most abundant downstream of the waterfall. Total length and age of the eels captured upstream of the waterfall ranged widely (217–661 mm in total length; 2+–5+ years), with small, young individuals comprising a higher proportion of the eels captured downstream of the waterfall. Therefore, small, young eels appeared to frequently climb the waterfall and grow in the upstream area after climbing over multiple obstacles. The moist surfaces of the waterfall included cracks that were covered with moss except in areas with strong water flow. These characteristics appear to allow the eels to climb the waterfall and suggest that multiple factors in addition to weir/dam height, such as roughness of the structure surface, may be important for characterizing the barriers to Japanese eels moving upstream. This indicates that even relatively low-height structures may completely or partly block the upstream migration of eels because of their surface conditions. Future studies estimating the characteristics of barriers to upstream movement that consider multiple factors are necessary.

What happens to glass eels after restocking in upland rivers? A long‐term study on their dispersal and behavioural traits

Abstract The European eel Anguilla anguilla is a critically endangered fish species as a result of human activities and climate change in river and oceanic ecosystems. Restocking using glass eels in continental freshwater areas is a potential conservation measure for enhancing local eel stocks and for conserving the species in aquatic habitats, where it may otherwise disappear. However, little is known about the fate of these restocked individuals and the early ecological behaviour of the young eels translocated in rivers. A portable radio‐frequency identification (RFID) telemetry system and 12‐mm tags were used to track restocked eels for a duration of 4 years. The aim was to understand the early movement, behavioural traits, dispersal, and habitat use of elvers after restocking performed in 2013 with glass eels in a shallow riverine environment. From the 241 tagged eels (total length, Q50 = 152 mm), 85% were detected in 1968 positions during a period of 4 years, beginning in 2014. Clear seasonality in eel activity was observed, with higher mobility in summer when the water temperature was high (above 12°C). Dispersal was slowed by numerous artificial obstacles and the high carrying capacity of habitats. There was a negative relationship between the body size of eels at tagging and their mobility. Five behavioural categories of mobility patterns were identified: ascending, descending, oscillating with an upstream trend, oscillating with a downstream trend, and stationary. The first four categories depleted with time, in favour of stationary individuals that displayed a highly sedentary lifestyle. This study provides new knowledge of the long‐term dispersal behaviour of restocked eels and the influence of seasons, barriers, and habitats on their colonization strategy changing with time. The results contribute to a better understanding of the issue of uncommon restocking practices in upland rivers.

Young European eels may use magnetic fields to guide them home

Young European eels may use magnetic fields to guide them home

Current status and concerns on tropical anguillid eel stocks

@font-face { font-family: "ＭＳ 明朝"; }@font-face { font-family: "ＭＳ 明朝"; }@font-face { font-family: "@ＭＳ 明朝"; }@font-face { font-family: "Calibri"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0mm 0mm 0.0001pt; text-align: justify; font-size: 12pt; font-family: Calibri; }.MsoChpDefault { font-size: 11pt; font-family: Calibri; }.MsoPapDefault { margin-bottom: 6pt; }div.WordSection1 { } Freshwater eels of genus Anguilla have fascinated biologists for centuries due to the spectacular long-distance migrations between the eels’ freshwater habitats and their spawning areas far out in the ocean and the mysteries of their ecology. The spawning areas of Atlantic eels and Japanese eel were located far offshore in the Atlantic Ocean and the Pacific Ocean, respectively, and their reproduction took place thousands of kilometres away from their growth habitats. Phylogenetic studies have revealed that freshwater eels originated in the Indonesian region. However, remarkably little is known about the life histories of tropical freshwater eels despite the fact that tropical eels are key to understanding the nature of primitive forms of catadromous migration. Recently, the juvenile abundance has declined dramatically: by 99% for the European eel and by 80% for the Japanese eel. Recruitment of the American eel near the species’ northern limit has virtually ceased. Other eel species, including Australian and New Zealand eels, also show indications of decline. The main problem is that all young eels used in cultivation are wild juveniles (glass eels and elvers), which are captured in estuaries. Almost all (90 %) of the total world eel supply comes from aquaculture. Therefore, the supply of eel resources for human consumption is completely dependent on wild catch. Commercial eel industries are now considering tropical eels as possible replacement for European and Japanese eels to compensate for declining stocks. However, useful scientific research and information on the biology and stock assessments of tropical eels are lacking, a situation quite different from that for other temperate freshwater eels, which have been well studied for several decades with trends and recruitment patterns being on record. Nevertheless, the present tropical eel catch has been reported as being less than half that of 20 years ago. The present trends in eel stocks and utilization for human consumption suggest that all eel populations will decline to numbers that fall outside safe biological limits and will be seriously threatened with extinction without protection and conservation from strict enforcement of local and international laws.

To boldly climb: behavioural and cognitive differences in migrating European glass eels.

European eel (Anguilla anguilla) is a catadromous fish species that received substantial attention as its population has markedly declined in the last three decades. The possible causes of this decline include habitat fragmentation factors such as dams and weirs. In some cases, these obstacles are equipped with fish friendly passage devices that may select young eels according to their climbing behaviour. We tested how individual climbing tendency was related to the event of fishway passage experienced in the field and classified fish climbing profiles as climbing ‘leaders’, ‘followers’, ‘finishers’ and ‘no climbers’. Moreover, we analysed the brain transcription level of genes related to neurogenesis and synaptic plasticity and compared it to climbing profiles. We found that fish from the upstream segments of an impounded river had a higher climbing propensity. Their behaviour was also more repeatable throughout the whole test than the obstacle-naive fish from the downstream segment. Moreover, we found that boldly climbing ‘leaders’ had lower levels of transcription of synapse-related genes than the climbing ‘followers’. These differences could be related to coping styles of fish, where proactive ‘leaders’ express a routine and risky behaviour, whereas reactive fish need an environmental assessment before exploratory behaviour. Our study showed that differences in climbing propensity exist in glass eels separated by water obstacles. Moreover, eels could adopt climbing different strategies according to the way they deal with environmental stress and to the cognitive abilities they possess.

Climbing experience in glass eels: A cognitive task or a matter of physical capacities?

The European eel is a panmictic species, whose decline has been recorded since the last 30 years. Among human-induced environmental factors of decline, the impact of water dams during species migration is questioned. Indeed, water impoundments can be a severe obstacle for young eels trying to reach the upstream freshwater zones, even if they are equipped with fish-friendly passes. The passage by such devices could be an important event shaping the outcome of the future life and life history traits of eels. We studied what phenotypic traits were associated with the event of experience of passage by water obstacles. We analyzed specific enzyme activities and/or gene transcription levels in the muscle and brain to test whether the obstacle passage is rather a physical or cognitive task. We found that after a long period of maintenance under homogenous conditions, transcription levels of several genes linked to synaptic plasticity, neurogenesis and thyroid activity differed among the field-experience groups. In contrast, muscle gene transcription levels or enzymatic activities did not show any differences among fish groups. We suggest that cognitive processes such as learning and memory acquisition rather than swimming-related metabolic capacities are involved in passage of water obstacles by young eels.

Assessing the Efficiency of an Elver Ladder Using a Multi‐State Mark–Recapture Model

AbstractThe decline of the European eel (Anguilla anguilla) stock has led the European Commission to enforce a regulation (Council Regulation N° 1100/2007), in which each member state was required to establish an eel management plan. Various measures in the French plan aim at restoring river connectivity by mitigating the impact of obstacles on the colonization of continental water by eels. Consequently, many obstacles are going to be equipped with elver ladder in the near future. In this context, a method to assess the passability of an obstacle seems essential. In this study, we developed a tag–recapture method, appropriate to glass eels and elvers, and an associated multi‐state mark–recapture model (i) to assess the passability of a ladder and (ii) to quantify the effect of various environmental factors on this passability. An application to a specific obstacle is applied as an illustrative example that demonstrates the relevance of the assessment method, and how the results can be used to propose technical solution to improve the efficiency of the ladder. Nine tag–recapture campaigns were carried on this obstacle, and about 4400 young eels were tagged. The model demonstrates that the efficiency of the ladder was rather low, especially during low river flow periods, mainly because of accessibility problems. The model also demonstrates the major influence of the river flow on the probability for an eel to pass the ladder; consequently, managing river flow during the migration period can be a relevant measure to improve river connectivity and facilitate colonization of the watershed. Copyright © 2014 John Wiley & Sons, Ltd.

극동산 뱀장어, Anguilla japonica의 성비와 성장에 미치는 수온 및 estradiol-17β의 효과

This study investigated the effects that water temperature and the administration of estradiol-17s (E2) had on the sex ratio and growth of the Japanese eel, Anguilla japonica. Glass eels (total length≒6.5 cm) were differentiated into an E2 group and an E2-free group and then they were reared for about four months at three water temperature levels of 20C, 24C, and 28C. The results showed that the young eels survived normally at the rearing water temperature of ≥24 o C, and grew to a mean size of 20 cm (total length). In the E2-free group, temperature was not found to increase the sex ratio (feminizing rates); however, the sex ratio of the E2-administrated group was found to be a little higher at a high temperature (28C). The growth of the E2 group was lower than the growth of the E2-free group at 24 o C and the E2 concentration levels in the plasma at 24 o C were found to be significant after the end of the E2 administration period (178 days). Therefore, we thought that long-term administration of E2 must be considered to be the reason for growth decline in spite of the prominent sex ratio effect. Our results indicate that temperature was not related to an increase in the feminizing rate (sex ratio) in the Japanese eel, Anguilla japonica, and other environmental factors (rearing density, salinity, etc.) that have the possibility of inducing ovarian differentiation must be investigated.

극동산 뱀장어, Anguilla japonica 자성화에 미치는 estradiol-17β 유도 효과

The effects of oral administration of estradiol- (E2) on glass eels (Body weight: , Total length: ) and young eels (Body weight: , Total length: ) on gonadal sex and growth were examined, respectively. Glass eels were fed a diet containing E2 at a dose of 10 mg/kg or 25 mg/kg, respectively, for five months. The female ratio significantly increased in all E2-treated groups (10 mg/kg diet group: 70%; 25 mg/kg diet group: 90%) when compared to the control group (10%). Young eels were fed a diet containing E2 at a dose of 25 mg/kg for four months. The female ratio also significantly increased in the E2-treated groups (60%) compared to the control group (20%). The highest female ratio was observed in the stage of glass eels rather than young eels. In all experiments, however, the growth of eels treated with E2 was similar to that of controls to the end of the experiment. Thus, oral administration of E2 could be a good approach to controlling sex differentiation.

Listeria monocytogenes and ready-to-eat seafood in Spain: Study of prevalence and temperatures at retail

The aim of this study was to obtain data from refrigerated ready-to-eat seafood products at retail in Spain (young eels, crabstick and smoked salmon), regarding prevalence and levels of Listeria monocytogenes, storage temperatures and the impact of transport conditions (type of bag) on the temperature of the product. The one-year surveillance period was carried out according to the EC Regulation No. 2073/2005, taking 5 units/batch and analyzing 250 samples following ISO 11290-1/A1 and ISO 11290-2/A methodologies. Low prevalence of L. monocytogenes was observed in surimi products, while 4.8% of smoked salmon samples were positive for Listeria with low levels (<10 cfu/g) and uneven pathogen distribution. A single company was responsible for 80% of the positive lots. All purchased products showed values higher than 4 °C at retail and an average increase of 2.5 °C or up to 6.2 °C was recorded when isothermal or plastic shopping bags were used for transport, respectively. To avoid noncompliance of the Food Safety Objective for L. monocytogenes in seafood RTE products more efforts from all stakeholders are needed, with special attention so as to improve control and maintenance of refrigerators at retail and to enhance consumer education regarding food safety practices.

Determinants of dietary specialization: a comparison of two sympatric species of sea snakes

Why do some predator species specialize on only a single type of prey whereas others take a broad range? One critical determinant may be the ontogenetic range of body sizes of the predator compared to that of its prey. If any single prey taxon spans only part of the range of prey sizes ingestible by the predator, then the predator will be more likely to take multiple prey taxa. We exploit a model system that provides a robust opportunity to test this hypothesis. We studied two sympatric species of predatory sea snakes, similar in size and general ecology that feed on anguilliform fishes from different habitats in the Great Lagoon of New Caledonia. Eel species from soft‐bottom habitats must construct their own burrows, and thus tend to be more slender‐bodied and less variable in body size than eel species that inhabit variable‐sized crevices among hard coral. As a result, a laticaudine sea snake species (Laticauda saintgironsi) that feeds on hard‐coral‐dwelling eels relies primarily on a single prey species: juveniles take young eels whereas adults consume adult eels of the same species. In contrast, a laticaudine species (L. laticaudata) that forages on soft‐bottom eels switches its prey ontogenetically: juveniles take small eel species whereas adults consume large eel species. Thus, habitat‐imposed constraints on the range of body sizes within each prey taxon generate a striking difference in the degree of dietary specialization of two closely related, sympatric predator species.

Effectiveness of Atimycin‐A as a Toxicant for Control of Invasive Asian Swamp Eels

AbstractThe Asian swamp eel Monopterus albus is native to southeast Asia but has established populations in several locations within the United States. These eels grow up to 1 m long and may prey on or compete with native fishes and amphibians. Additionally, these eels are obligate air‐breathers, can survive long periods out of water, and will travel over land in search of new habitats when necessary. As early as 1990, Asian swamp eels were introduced to several ponds at a local nature center near Atlanta, Georgia, within the Chattahoochee River drainage basin; individuals have since migrated to an adjacent marsh of the Chattahoochee River. Because this population is relatively contained, several methods for control, containment, and possible eradication are being investigated. Juvenile and adult swamp eels primarily obtain oxygen through surface air, although very young eels absorb oxygen directly through their skin. Because they are obligate air‐breathers, standard piscicides (e.g., rotenone and antimycin‐A) that are transmitted across the gill membrane may not be effective. Nonetheless, we tested the effectiveness of antimycin‐A on 2–3‐week‐old Asian swamp eels in case transmission occurred across the epidermis. We tested antimycin‐A concentrations of 2, 5, 10, and 100 nL/L (ppb). No concentration was effective at killing Asian swamp eels, although golden shiners Notemigonus crysoleucas were susceptible at 10 nL/L. Eels that have not undergone metamorphosis (&lt;10 d old) may be susceptible to piscicides such as antimycin‐A, but the lack of effectiveness at the early life stage we tested indicates that eradication through such toxicants probably is not a feasible control method.

Eels meet again

One of the most mysterious fish of the western Atlantic is the European eel. Its range is vast: adults spend the bulk of their life in freshwater systems from Iceland to Morocco. And, indeed, the species has supported one of the biggest fisheries in Europe, employing up to 25,000 people at its height. But, as with many other commercial species, stocks are now in dangerous decline.The number of young eels returning from the sea to river systems has declined to one per cent of the numbers recorded in the 1970s. Action is urgently needed if the species is to survive but its geographic range and complex life history hampers conservation efforts.Adults are thought to breed in the Sargasso Sea, near Bermuda, although breeding has never been directly observed. But the smallest seen larvae have been found in this area and these are thought then to drift on the Gulf Stream and North Atlantic Current back to Europe and north Africa. Here they change and enter freshwater environments, where they live for many years before changing again and heading back out to sea on their final journey to breed.Recent evidence has also suggested further complications, in that the population appears to be genetically fragmented, so that returning adults may perhaps only breed with others from the same river systems from which they left, adding to the problems facing the species.But new work by Johan Dannewitz at the Institute of Freshwater Research in Drottningholm, Sweden and colleagues in Sweden and Belgium (reported in the Proceedings of the Royal Society series B, published online), describes a genetic analysis of eels sampled at more than 40 sites across Europe and, more importantly, those examined from some sites over eight years between 1994 and 2002. The results represent ‘the most extensive genetic study, with respect to the number of locations and individuals analysed, of the European eel to date,’ the authors say.On the basis of the statistical analysis of their results, genetic differences over time at particular sites outweigh geographical differences, suggesting that the eels might comprise a single breeding population.The authors believe their results suggest that a single population-wide conservation strategy is therefore more desirable, rather than national efforts, if the species is to be effectively conserved.Silver declining: European eel populations are falling dramatically but new studies suggest they form a single breeding population which may help conservation strategies. (Picture: Photolibrary.com)View Large Image | View Hi-Res Image | Download PowerPoint Slide

Tracking continental habitat shifts of eels using otolith Sr/Ca ratios: validation and application to the coastal, estuarine and riverine eels of the Gironde - Garonne - Dordogne watershed

To enable a relevant interpretation of otolith strontium : calcium (Sr/Ca) variations in terms of habitat shifts of eels, the Sr/Ca-salinity relationship in eel otoliths was validated. Downstream and upstream migrations of young eels were reproduced in the laboratory by transferring groups of fish every 2 months between aquaria filled with water coming from the Dordogne river (salinity = 0), the upper Gironde estuary (salinity = 5), the lower Gironde estuary (salinity = 25) and the coast (salinity = 30), which represented the salinity gradient observed in the Gironde–Garonne–Dordogne watershed. Ontogenetic changes in otolith Sr/Ca were assessed in two groups of control fish that were kept in one of either two constant salinities (fresh water or seawater). X-ray electron microprobe (wavelength dispersive spectrometry, WDS) analyses of Sr/Ca ratios in the otoliths showed that the change of aquarium was recorded as a Sr/Ca increase (downstream migration) or a Sr/Ca decrease (upstream migration). No ontogenetic effect was detected in otoliths of control fish outside glass eel marks in either group of fish. The electron microprobe (WDS) analysis of the Sr/Ca life (transected in several otoliths of eels caught in the Gironde–Garonne–Dordogne watershed) showed that some of them were migrant eels that had experienced one major habitat shift during their continental life.

Long-term trends in the fish communities of Lakes Peipsi and Võrtsjärv (Estonia)

This study was undertaken to determine the direction and extent of time related changes in the fish communities of the large shallow eutrophic lakes Peipsi and Võrtsjärv. The official statistics of commercial fisheries for the periods 1931–1940 and 1950–1999 were used for trend analysis. In the ichthyocoenosis of both lakes, several drastic and economically important changes have occurred during recent decades. Some trends in the population dynamics of fish (e.g., sharp decline of the stock of vendace Coregonus albula (L.) in Lake Peipsi) are probably irreversible owing to the eutrophication of the lake. A sharp decrease in the stock of smelt Osmerus eperlanus (L.), caused by nighttime water anoxia in the summer of several years, was followed by a quick recovery. Periodic changes in the population of pike Esox lucius L., are related to water level fluctuations. The population dynamics of several fishes, particularly pikeperch Stizostedion lucioperca (L.), are, to a large extent, controlled by fishery policy, although climatic factors are also of importance. The state of the stock of eel Anguilla anguilla (L.), depends entirely on introduction of young eels.