Elver Stage Research Articles

Growth performance of eel fish (Anguilla bicolor) which are maintained in containers of different colors

Performance of Eel (Anguilla bicolor) maintened in containers of different colors. Growth is the main driver in increasing the profitability of aquaculture. In addition to maintenance management the color of the container is also a factor supporting the growth of eels. This study aims to determine the effect of the color of different rearing containers on the growth and appetite response of eel. The test fish used were elver stage eels with an average weight (129 ±131g). The method used is an experiment with two treatments they are grey and green. The test fish were placed in a container measuring 19x4 m2 and a water level of 1m with a stocking density of 1100 individuals each, 30 days of rearing. The variables observed were growth performance including SR (Survival Rate), ADG (Average Daily Growth), FCR (Food Conversion Ratio), SGR (Specific Growth Rate). The results obtained indicated that there was a significant difference in the average growth of fish reared in grey and green containers. Grey container treatment was proven to get higher yields with average final weight values (192.4 ± 4.5 g), FCR (1.01 ± 0.02), ADG (2.01g/day ± 0.06), SGR (1 .25%BW/day±0.02). Based on the results of the study it can be concluded that there is a significant difference in the growth of eels reared in ash and green containers. The color of the ash container proved to be better in supporting growth performance.Keywords: Elver; Green; Grey; Nocturnal

Feeding frequency on survival, growth, blood glucose of glass eel, Anguilla bicolor bicolor cultured in a recirculating aquaculture system

Indonesian shortfin eel, Anguilla bicolor bicolor is one of the eel species that high market demand both for local and export. Feeding frequency is an important that related to production for culture. The purpose of this experiment is to determine the optimal feeding frequencies on survival and growth of shortfin eel at glass eel to elver stages cultured in a recirculating aquaculture system. Four treatments of different feeding frequencies and three replicates each were performed. The treatments were as followed A) 1 time a day; B) 2 times a day; C) 3 times a day; and D) 4 times a day. The result showed that the feeding frequency affect the survival, length and weight growths, and glucose value. The best of survival rate (82.61±3.15%), weight (1.91±0.28 g) and length (10.64±0.45cm) found at feeding frequency 3 times a day compared to the others (P<.0.05). Blood glucose value increases with increasing feeding frequency. Water quality such as dissolved oxygen (DO), temperature and pH were within the optimal range. Three times a day of feeding frequency is recommended for applying on glass eel culture in a recirculating aquaculture system.

An overview of the Anguillid eel culture in Vietnam

The wild eel fishery and eel farming are playing an increasingly important role in aquaculture. Most of the Anguillid eel fries at their glass eel stage are caught in the two provinces of Vietnam, viz., Phu Yen and Binh Dinh, which accounts for over 80% and 10-15%, respectively. The stages of raising eels in Vietnam are very diverse with the survival rate of Anguilla Marmorata is approx. 60% through glass eel and elver stages. The major eel grow-out area of Vietnam is in Mekong delta. Ca Mau province is the highest in eel farming among the 14 provinces in the country. There are over 1,000 eel farms produced about 2,000–5,000 tons of eel in Vietnam at this moment. Around 95% of the total number of farms produced giant mottled eel (A. marmorata), and most of the production (70%) was used for domestic consumption, while30% was exported to the adjacent countries.

Salinity and odor preferences of Japanese eel during the first year of post-recruitment growth in saline water

Freshwater eels grow both as residents of estuaries, spending the yellow eel stage in nearshore saline waters, and as residents of freshwater in rivers. In order to understand how Japanese eels diverge between estuarine and river residence, we investigated their water preferences during the first year of growth in saline water after their recruitment into growth habitats by using Y-maze behavioral trials. In the water-choice trials between natural seawater and river water, glass eels and pigmented elvers showed a significant preference for river water, while small yellow eels showed a significant preference for seawater. Subsequent experiments examining odor and salinity preferences of the eels showed that a preference for riverine odor was maintained during the growth phase. However, as the eels developed into the yellow eel stage in natural seawater, their salinity preference shifted from no preference, or a preference for freshwater, to a preference for saline water. These results suggest that Japanese eels disperse over a wide range of salinities until the elver stage, as long as water has a rich river-originated odor. When individuals grow in saline water until reaching the yellow eel stage, they are likely to become estuary residents.

Interspecific and sexual differences in riverine distribution of tropical eels Anguilla spp.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%). Anguilla bicolor pacifica and Anguilla interioris were rare (1.5 and 0.4%, respectively). Otolith Sr:Ca ratio electron-probe micro analysis (EPMA) for individual migratory histories revealed that 15 A. celebesensis caught in Poso Lake and its inlet rivers were categorized into 14 river eels (Sr:Ca < 2·5) showing upstream migration seemingly at their elver stage and only one sea eel (Sr:Ca ≥ 6·0) that stayed in the marine habitat for the majority of its life after recruiting to Sulawesi Island before its late upstream migration. In A. marmorata, 19 examined eels from Poso Lake and its inlet rivers were all river eels, while 17 eels from the lower reaches of Poso River were two river eels, six sea eels and nine estuarine eels (2·5 ≤ Sr:Ca < 6·0) that mostly lived in the brackish water. The sex ratio of A. celebesensis was highly skewed towards a dominance of females (99%). In A. marmorata, females were predominant in Poso Lake (95·2%), its inlet rivers (94·7%) and Laa River (100%), while males were more frequent in the lower reaches of Poso River (76·5%) and small inlet rivers of Tomini Bay (94·1%). These results indicate that the riverine distribution pattern of tropical eels differs among species and between sexes.

Interspecific and sexual differences in riverine distribution of tropical eels Anguilla spp.

A total of 261 individuals of the four tropical eel species, Anguilla celebesensis, Anguilla marmorata, Anguilla bicolor pacifica and Anguilla interioris, were collected from 12 locations around Sulawesi Island, Indonesia, to gain knowledge about the riverine distribution of tropical eels. Anguilla marmorata was predominant in the lower reaches of Poso River (94·4% of total eel catch in the sampling area), Poso Lake (93·3%), three small inlet rivers of Tomini Bay (100%) and Laa River (92·3%). Anguilla celebesensis occurred frequently in the inlet rivers of Poso Lake (63·5%). Anguilla bicolor pacifica and Anguilla interioris were rare (1.5% and 0.4%, respectively). Otolith Sr:Ca ratio electron-probe micro analysis (EPMA) for individual migratory histories revealed that 15 A. celebesensis caught in Poso Lake and its inlet rivers were categorized into 14 river eels (Sr:Ca < 2·5) showing upstream migration seemingly at their elver stage and only one sea eel (Sr:Ca ≥ 6·0) that stayed in the marine habitat for the majority of its life after recruiting to Sulawesi Island before its late upstream migration. In A. marmorata, 19 examined eels from Poso Lake and its inlet rivers were all river eels, while 17 eels from the lower reaches of Poso River were two river eels, six sea eels and nine estuarine eels (2·5 ≤ Sr:Ca <6·0) that mostly lived in the brackish water. The sex ratio of A. celebesensis was highly skewed towards a dominance of females (99%). In A. marmorata, females were predominant in Poso Lake (95·2%), its inlet rivers (94·7%) and Laa River (100%), while males were more frequent in the lower reaches of Poso River (76·5%) and small inlet rivers of Tomini Bay (94·1%). These results indicate that the riverine distribution pattern of tropical eels differs among species and between sexes. This article is protected by copyright. All rights reserved.

Development of an Arduino Based Fish Counter Prototype for European Eel (Anguilla anguilla L.)

European eels (Anguilla anguilla L.) has a great dispersion from Sargasso Sea to across the Europe and North Africa. Their population have decreased dramatically over the years and listed in IUCN as critically endangered. Stock estimation is one of the important issues in order to sustainable management of this species. Last decades, researchers have focused various studies which based on monitoring of various life stages of eels. Manual counting methods with ladder traps is commonly used based on collection of glass and elver stages of eels from the natural habitats while their migration to upward of streams. During their special migration pattern, a counting device on the eel ladders can be designed so that the eels may be counted while passing through this device. The offered prototype was designed for eel ladders which was briefly, powered by an open-source electronics platform, Light Dependent Resistor (LDR) and 650 nm Laser light were used to count elver eels. Several trials were conducted to test this prototype and results were promising. This study aimed to explain features of prototype and working principle of elver counter. Designed prototype has several advantages such as usable with eel ladders, easy to customisation and affordable characteristics. However, this device needs some improvements in order to effective usages in the field.

Effects of Dietary Surfactin Supplementation on Growth Performance, Intestinal Digestive Enzymes Activities, and Hepatic Antioxidant Potential of American Eel (Anguilla rostrata) Elvers

The 70-day trial was conducted to investigate the effects of surfactin on growth performance, intestinal digestive enzymes activities, and hepatic antioxidant potential on American eel (Anguilla rostrata) at elver stage. Six hundred American eel elvers were randomly divided into five treatments with three replicates per group, 40 fish per replicate. Dietary surfactin levels of the five treatment groups were 0, 25, 50, 100, and 200 mg/kg, respectively. Final body weight, weight gain rate, and feed efficiency were significantly affected by surfactin supplementation (P&lt;0.05). There were no significant differences in feeding rate and survival rate among all groups (P&gt;0.05). Protease and lipase activity in the intestine were significantly increased by surfactin supplementation (P&lt;0.05), and amylase activity was similar among all groups (P&gt;0.05). Malondialdehyde level, total antioxidation capacity level, and activities of superoxide dismutase, and glutathione peroxidase in intestine of fish were significantly (P&lt;0.05) affected. No significant differences in CAT activities were found among all groups (P&gt;0.05). The results demonstrated that dietary 25 mg/kg surfactin supplementation improves growth performance, some digestive enzyme activities in the intestine, and hepatic antioxidant potential of American eel at elver stage.

Effects of Different Levels of Dietary Surfactin Supplementation on Intestinal Morphology, and Intestinal Microflora of Growth Retarded Marbled Eel Juveniles (Anguilla marmaorata)

This 70 day trial was conducted to investigate the effects of surfactin on intestinal morphology and intestinal microflora of growth retarded marbled eels (Anguilla marmorata). A total of six hundred and forty marbled eels at elver stage were randomly allocated to 4 treatment groups on the basis of body weight. There were 4 replicate tanks in each treatment group with 40 fish per tank. Four diets were formulated with four levels of dietary surfactin: 0, 25, 50, and 100 mg/kg, respectively. Fish were fed twice daily to apparent satiation for 70 days. Retarded marbled eels fed diets with surfactin supplementation had enhanced mucosa folds in their intestines (P&lt;0.05). It seems that the quantity of mucosa folds increased with the increasing surfactin levels in the diet. Dietary surfactin supplementation decreased the E. coli population significantly (P&lt;0.05), and increased the Lactobacillus population significantly (P&lt;0.05), but had no effect on total bacterial population in intestines of marbled eels (P&gt;0.05). Results of the present study indicate that surfactin improves intestinal function in retarded elvers of marbled eels.

Effects of Different Levels of Dietary Surfactin Supplementation on Intestinal Morphology, and Intestinal Microflora of Growth Retarded Marbled Eel Juveniles (Anguilla marmaorata)

This 70 day trial was conducted to investigate the effects of surfactin on intestinal morphology and intestinal microflora of growth retarded marbled eels (Anguilla marmorata). A total of six hundred and forty marbled eels at elver stage were randomly allocated to 4 treatment groups on the basis of body weight. There were 4 replicate tanks in each treatment group with 40 fish per tank. Four diets were formulated with four levels of dietary surfactin: 0, 25, 50, and 100 mg/kg, respectively. Fish were fed twice daily to apparent satiation for 70 days. Retarded marbled eels fed diets with surfactin supplementation had enhanced mucosa folds in their intestines (P&lt;0.05). It seems that the quantity of mucosa folds increased with the increasing surfactin levels in the diet. Dietary surfactin supplementation decreased the E. coli population significantly (P&lt;0.05), and increased the Lactobacillus population significantly (P&lt;0.05), but had no effect on total bacterial population in intestines of marbled eels (P&gt;0.05). Results of the present study indicate that surfactin improves intestinal function in retarded elvers of marbled eels.

Factors that influence cover selection by Japanese eels at elver stage

Adult eels are known to prefer narrow spaces for cover but factors influencing cover selection at the elver stage are unclear. I examined factors likely to be important in cover selection by elvers of the Japanese eel (Anguilla japonica). Pipes with a range of diameters, lengths, internal illumination (brightness), and direction of flow were provided for elvers in small-, medium- and large-sized classes. All sizes chose pipes without preference for diameter. Medium and large elvers preferred longer pipes. All size classes selected darker environments for pipes with similar dimensions. Medium to large elvers chose pipes without preference for orientation to the current up to flow speeds at which they were unable to stay in the pipe. The proportion of small elvers that remained in the pipes was lower than that of medium and large elvers.

Effects of Dietary Surfactin Supplementation on Growth, Digestive Enzyme Activity, and Antioxidant Potential in the Intestine of Growth Retarded Marbled Eel (Anguilla marmaorata) at Elver Stage

A 70 day trial was conducted to investigate the effects of surfactin on growth, digestive enzyme activity, and antioxidant potential in the intestine of growth retarded marbled eels (Anguilla marmorata) at the elver stage. Six hundred and forty marbled eels at elver stage were randomly divided into four treatment groups with four replicates per group, and 40 fish per replicate. The dietary surfactin level of the four experimental diets was 0, 25, 50, and 100 mg/kg, respectively. Final body weight, weight gain rate, survival rate, protease activity in the intestine, malondialdehyde level, total antioxidation capacity level, and activities of superoxide dismutase, and glutathione peroxidase in the intestine of fish, increased significantly with surfactin supplementation, and feed conversion rates improved significantly (P&lt;0.05). No significant differences of growth performance, lipase activity (except in the 25 mg/kg surfactin supplementation group), amylase activity and CAT activities were found among surfactin supplemented groups (P&gt;0.05). Results demonstrated that dietary surfactin supplementation could improve growth performance, some digestive enzyme activities, and antioxidant potential, in the intestine of growth retarded marbled eel at elver stage. Surfactin could be used as a dietary growth promoter for growth retarded eels.

Effects of Dietary Surfactin Supplementation on Growth, Digestive Enzyme Activity, and Antioxidant Potential in the Intestine of Growth Retarded Marbled Eel (Anguilla marmaorata) at Elver Stage

A 70 day trial was conducted to investigate the effects of surfactin on growth, digestive enzyme activity, and antioxidant potential in the intestine of growth retarded marbled eels (Anguilla marmorata) at the elver stage. Six hundred and forty marbled eels at elver stage were randomly divided into four treatment groups with four replicates per group, and 40 fish per replicate. The dietary surfactin level of the four experimental diets was 0, 25, 50, and 100 mg/kg, respectively. Final body weight, weight gain rate, survival rate, protease activity in the intestine, malondialdehyde level, total antioxidation capacity level, and activities of superoxide dismutase, and glutathione peroxidase in the intestine of fish, increased significantly with surfactin supplementation, and feed conversion rates improved significantly (P&lt;0.05). No significant differences of growth performance, lipase activity (except in the 25 mg/kg surfactin supplementation group), amylase activity and CAT activities were found among surfactin supplemented groups (P&gt;0.05). Results demonstrated that dietary surfactin supplementation could improve growth performance, some digestive enzyme activities, and antioxidant potential, in the intestine of growth retarded marbled eel at elver stage. Surfactin could be used as a dietary growth promoter for growth retarded eels.

Nearshore habitat associations of stocked American eel, Anguilla rostrata, in Lake Ontario and the upper St. Lawrence River

Abstract Approximately 3.9 million glass and elver stage American eels ( Anguilla rostrata ), life stages never before represented in Lake Ontario, were stocked into two locations, the upper St. Lawrence River and the Bay of Quinte, to help recover a segment of the panmictic population under severe decline. We characterized near-shore patterns of abundance and size and indicated habitat associations in the two stocking locations. Generalized additive models identified that in spring, eel presence was positively associated with the percentage of soft (organic or silt) substrate, and negatively associated with gravel substrate. In the fall, eel presence was positively associated with rubble substrate. Canonical correspondence analysis demonstrated eel habitat preferences that change as eels grow, and that these preferences do not vary with season. Small eels ( 1 m). These habitat shifts are likely due to a combination of physical space requirements, habitat availability and prey preference changing with increasing body size. The availability of suitable habitats differed between main stocking locations, yet neither location had an ideal mix of coarse substrates (for smaller eels) and fine substrates (for larger eels). The observed habitat shifts resulted in the stocked American eels utilizing an array of habitats, fitting the general view of the species as a habitat generalist, but it is apparent that in the Great Lakes eels use specific substrate features that change with increasing length.

Infection of newly recruited American eels (Anguilla rostrata) by the invasive swimbladder parasite Anguillicoloides crassus in a US Atlantic tidal creek

Abstract Little is known about the infection status of glass eel and elver stages of the American eel Anguilla rostrata by the invasive swimbladder parasite Anguillicoloides crassus. This study examined infection by adult and larval A. crassus in glass eels (n = 274) and elvers (n = 199) collected during March–December 2013 from an eel ladder at a dammed creek near Charleston, SC, USA. Among all the eels examined [total lengths (TLs), 34–156 mm], the prevalence (±SE), mean abundance, and mean intensity of A. crassus worms was 29.4 ± 2.1%, 0.88 ± 0.12, and 2.98 ± 0.34, respectively. Infection by A. crassus was not detected in the earliest glass eel development stages (pigment stages 1–3), but it was detected in more advanced stages (pigment stages 4–7) and fully pigmented elvers. From March to July, parasite prevalence increased significantly with eel TL, and all eels 125 mm or longer (n = 13) were infected. From August–December, when fewer eels were caught, parasite prevalence was generally lower and less dependent on the eel TL. Our study demonstrates the potential risk of spreading A. crassus to new areas by transporting live glass eels and elvers. This is of particular relevance because our study site was located in the Cooper River drainage, one of the few locations in the USA that permits a glass eel harvest.

Changes in the role of the thyroid axis during metamorphosis of the Japanese eel, Anguilla japonica

To clarify the role of thyroid function during metamorphosis from leptocephalus to glass eel in the Japanese eel, we examined the histology of the thyroid gland and measured whole-body concentrations of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), and thyroid stimulating hormone β-subunit TSH (TSHβ) mRNA expression levels in five stages of artificially hatched eels (leptocephalus, early-metamorphosis, late-metamorphosis, glass eel, and elver). During metamorphosis, the inner colloid of thyroid follicles showed positive immunoreactivity for T4, and both T4 and T3 levels were significantly increased, whereas a small peak of TSHβ mRNA level was observed at the early-metamorphosis stage. Similarly, TSHβ mRNA levels were highest in the glass eel stage, and then decreased markedly in the elver stage. In contrast to TSHβ mRNA expression, thyroid hormones (both T4 and T3) increased further from the glass eel to elver stages. These results indicated that thyroid function in the Japanese eel was active both during and after metamorphosis. Therefore, the thyrotropic axis may play important roles not only in metamorphosis but also in subsequent inshore or upstream migrations.

Regional variation in otolith Sr:Ca ratios of African longfinned eel Anguilla mossambica and mottled eel Anguilla marmorata: a challenge to the classic tool for reconstructing migratory histories of fishes

Otolith Sr:Ca ratios of the African longfinned eel Anguilla mossambica and giant mottled eel Anguilla marmorata from nine freshwater sites in four rivers of South Africa were analysed to reconstruct their migratory life histories between freshwater and saltwater habitats. For A. mossambica, the Sr:Ca ratios in the otolith edge differed significantly among rivers and had large effect sizes, but did not differ among sites within a river. Otolith Sr:Ca ratios did not differ among rivers for A. marmorata. When rivers were pooled, the edge Sr:Ca ratios of A. mossambica were not significantly different from those of A. marmorata. According to the river-specific critical Sr:Ca ratio distinguishing freshwater from saltwater residence, most A. mossambica and A. marmorata had saltwater habitat experience after settlement in fresh water. This was primarily during their elver stage or early in the yellow eel stage. During the middle and late yellow eel stage, freshwater residency was preferred and only sporadic visits were made to saltwater habitats. The data also suggest that regional variations in otolith Sr:Ca ratios affect the critical Sr:Ca value and are a challenge for the reconstruction of migratory life histories that should be explicitly considered to avoid bias and uncertainty.

Effects of feeding stimulants on the feed consumption, growth and survival at glass eel and elver stages in the European eel (Anguilla anguilla)

Elvers and glass eels of Anguilla anguilla were fed diets containing two types of feeding stimulants (FS) that were based on processed marine (MBFS) and yeast proteins (YBFS). Elvers (1.5 ± 0.3 g) were fed seven diets (MBFS and YBFS diets at 20 g kg−1, 40 g kg−1 and 60 g kg−1 plus control) for 60 days. Glass eels (250 ± 100 mg) were weaned to 60 g kg−1 MBFS, 60 g kg−1 YBFS and control diets for 30 days. Diets containing 60 g kg−1 FS had a beneficial effect in terms of growth, homogenous size distribution and feed intake in elvers. Elvers fed 60 g kg−1 MBFS and YBFS diets grew 11.9% and 5.6% faster than the control group. No differences in growth and size distribution were detected in glass eels fed 60 g kg−1 MBFS and YBFS diets. However, FS affected the digestive system maturation; fish fed the 60 g kg−1 MBFS and YBFS diets showed higher and intermediate values in the specific enzyme activities in comparison with the control group. This study revealed that the incorporation of FS into a pelleted diet was beneficial on the overall performance of European glass eels and elvers. However, the observed results were different depending on the eel’s stage of development, as well as the type and inclusion level of the FS.

Preliminary Evaluation of a Large-Scale American Eel Conservation Stocking Experiment

Abstract Significant declines in the recruitment of American eels Anguilla rostrata to formerly productive habitats in the upper St. Lawrence River and Lake Ontario resulted in the implementation of an experimental conservation stocking program. Nearly 3.8 million American eels (glass eel and elver stages) were stocked during 2006–2009. Our study objectives were to (1) assess the adequacy of sampling procedures for following temporal changes in stocked eel abundance, (2) examine captured eels for evidence of spinal trauma, (3) qualitatively evaluate whether stocked eels would disperse outside of stocking locations, and (4) provide initial data on biological variables describing young stocked yellow eels. Boat electrofishing was successful at capturing all four stocked year-classes, and the densities of stocked eels in the main stocking locations ranged from 25 to 275 eels/ha. Estimated sampling precision ranged from 0.15 to 0.28, and the estimated sample sizes required to detect a 50% change in stocked eel densities ranged from 27 to 112 electrofishing transects depending on location and season. The stocked American eels dispersed throughout Lake Ontario and demonstrated among the fastest recorded growth rates for this species: 60 to 123 mm/year. The first male American eels ever identified in the St. Lawrence River watershed were among the stocked individuals assessed for gender. We conclude that boat electrofishing for yellow American eels has the potential to measure stocking effectiveness along shallow shorelines with limited aquatic vegetation. We also recognize that the ultimate assessment of the conservation stocking experiment will not be made until future studies on the population demographics, migratory behavior, and spawning physiology of stocked American eels are complete. Received December 10, 2010; accepted May 11, 2011

Distribution of pepsinogen- and ghrelin-producing cells in the digestive tract of Japanese eel (Anguilla japonica) during metamorphosis and the adult stage

Pepsinogen is the precursor form of the gastric-specific digestive enzyme, pepsin. Ghrelin is a representative gastric hormone with multiple functions in vertebrates, including the regulation of growth hormone release, stimulation of food intake and gastrointestinal motility function. We investigated chronological changes in the distribution of pepsinogen-expressing cells by in situ hybridization and ghrelin-immunoreactive cells by immunohistochemistry in the Japanese eel (Anguilla japonica) during metamorphosis from the leptocephalus sage to the elver stage. The ghrelin-producing cells first appeared in the gastric cecum and pyloric portion of the stomach in the late phase of metamorphosing leptocephali, whereas the pepsinogen-producing cells were first detected in the early phase of the glass-eel stage. These suggest that endocrine cells differentiated earlier than exocrine cells in the eel stomach. Accompanying eel development, the distribution of ghrelin-producing cells spread to the esophagus and other regions of the stomach, but not to the intestine. These results may be related to the changes in dietary habits during metamorphosis in the Japanese eel.