Influence of different rations and water temperatures on the growth rates of shortfinned eels and longfinned eels

Juvenile (12–152 g) shortfinned eels Anguilla australis and longfinned eels A. dieffenbachia caught in New Zealand streams were fed squid mantle Nototodarus spp. 4 days per week in laboratory experiments. A linear multiple regression equation showed the amount of food eaten (0–2·7% w day⁻¹) explained 77·7% of the variation in specific growth rates (–0·60 to +1·07% w day⁻¹) among individual eels, while previous growth rates, water temperature (10·0–20·6°C), and eel weight (12–152 g) explained a further 5·6, 1·4 and 0·8%, respectively. Growth in length ranged from –0·3 to +0·9 mm day⁻¹. Eels which were starved and then given high rations grew substantially faster than expected. Once growth rates were adjusted for differences in ration and other factors, there were no significant differences in growth rates between species or individual fish. Growth of shortfinned eels fed maximum rations of commercial eel food depended on fish size and water temperatures and ceased below 9·0°C. Growth rates in the wild were substantially less than the maximum possible, after seasonal changes in water temperatures were taken into account, indicating that food supplies and not low water temperatures were controlling growth rates in the wild.     

Diel and seasonal movements of radio-tagged freshwater eels, Anguilla spp., in two New Zealand streams

We studied diel and seasonal movements of 21 radio-tagged shortfinned, Anguilla australis Gray, and longfinned, A. dieffenbachii, eels in two small New Zealand streams. Movements of eels commenced at dusk, with a higher proportion of shortfinned eels moving per night than longfinned eels, and also moving greater distances. Both species often showed extensive movements immediately after tagging, but thereafter movements were limited. In the smaller stream, home ranges averaged 30 and 10m for shortfinned and longfinned eels, respectively, but not all eels were active on every night. There were no seasonal differences in mean distances moved. In both streams, eel movement was almost exclusively bankside, and seldom cross-channel; eels also showed considerable fidelity to a particular bank. Shortfinned eels were most commonly found in runs, and longfinned eels in riffles.     

Variability in growth of longfinned eels among coastal catchments of south‐eastern Australia

Longfinned eels Anguilla reinhardtii were captured by both fishery‐dependent and independent sampling methods from three rivers in New South Wales, south‐eastern Australia. Growth rates were examined in two zones (fresh water and tidal) in the Hacking, Hawkesbury and Clarence Rivers. Mean annual growth increments of sampled longfinned eels ranged from 30 to 60 mm year⁻¹ using age‐length analyses and up to 167 mm year⁻¹ based on tag‐recapture model estimates (GROTAG), with both methods showing high intra‐ and inter‐population variability. Growth was significantly faster in younger (5–15 years) fish than older (>15 years) fish, with females growing an average 10 mm year⁻¹ faster than males of similar age and capture location. Longfinned eels found in tidal areas grew significantly faster than those in non‐tidal freshwater areas as a result of longer growing seasons in the highly productive estuarine habitats. Other possible factors influencing variability in growth rates for this species include habitat preference, density and fishing pressure.     

Densities and swimbladder development of juvenile American eels, Anguilla rostrata (Lesueur) as related to energetics of migration

Glass eels and elvers of the American eel were negatively buoyant. Those adapted to sea water were more dense (1.072 ± 0.001 g cm⁻³) than those adapted to fresh water (1.061 ± 0.001 g cm⁻³). Adaptation to fresh water increased relative body water content, but did not account for the observed decrease in total body density. Histological examination revealed the presence of a potentially functional swimbladder in the glass eels, although this hydrostatic organ did not becomegas-filled until after freshwater residency had occurred. Calculation of lift as used in the selective tidal transport mechanism suggests that hydrodynamic compensation for horizontal swimming during the estuarine phase of migration is energetically adaptive.     

Relative abundance, sex ratio and population structure of the Japanese eel Anguilla japonica in the Tanshui River system of northern Taiwan

From the Gong-Shy-Tyan (GST) Stream and the Tanshui River, Taiwan, eels ranged from 5.53 cm t.l . (elvers) to 72.5 cm t.l . Anguilla japonica accounted for 93-99%, A. marmorata 1-7% and A. bicolor pacifica less than 1% of all eels caught. Mean eel lengths increased from 10 cm t.l . at the downstream sites to 50 cm t.l . at the upstream sites. Females made up 92.8% of the sex-determined eels. Population density, averaged approximately 0.14 eel m⁻² (2.42 g m⁻²) and 0.25 eel m⁻² (0.92 g m⁻²) in downstream sites of the GST and Tanshui River, respectively, and decreased substantially with upstream distance. Eels were rarely found in the heavily polluted and dam constructed areas in the midstream site of the Tanshui River.     

Swimming activity and behaviour of European Anguilla anguilla glass eels in response to photoperiod and flow reversal and the role of energy status

To better understand migratory divergences among Anguilla anguilla glass eels, the behaviour of individuals caught at the time of their estuary entrance was studied through their response to a light:dark cycle and then to both water current reversal and light:dark cycle. In a first experiment, fish moving with the flow in response to dusk (M⁺ fish) and fish that had not exhibited any movement (M⁻ fish) were distinguished. Anguilla anguilla from these two groups were then individually marked and their response to water current reversal compared. M⁺ individuals mainly exhibited negative rheotaxis with a tidal periodicity, whereas positive rheotaxis was mainly exhibited by M⁻ individuals. Thus, M⁺ A. anguilla glass eels showing negative rheotaxis appear to have the strongest propensity to migrate, the converse applies to M⁻ ones showing positive rheotaxis. A small percentage of individuals (5%) were hyperactive, alternately swimming with and against the current with almost no resting phase. These fish lost c . 2 mg wet mass day⁻¹, whereas individuals which were almost inactive lost c . 1 mg day⁻¹. Wet and dry mass changes in relation to activity levels were compared with previous experiments and it was concluded that A. anguilla glass eel energy status might be involved in differences in migratory tendencies but other factors that might be important are discussed. It is proposed that any decrease in A. anguilla glass eel energy stores associated with global warming might lead to an increase in the proportion of sedentary individuals and thus be involved in the decrease in the recruitment to freshwater habitats.     

A method for direct measurement of the maximum volume of fish stomachs or digestive tracts

A new method for measuring maximum stomach or digestive tract volume of fish incorporates air injection at constant pressure with water displacement to measure directly the internal volume of a stomach or analogous structure. The method was tested with coho salmon, Oncorhynchus kisutch (Walbaum), which has a true stomach, and northern squawfish, Ptychocheilus oregonensis (Richardson), which has a modified foregut as a functional analogue. Both species were collected during July-October 1987 from the Columbia River, U.S.A. Relationships between fish weight (= volume) and maximum volume of the digestive organ were best fitted for coho salmon by an allometric model and for northern squawfish by an exponential model. Least squares regression analysis of individual measurements showed less variability in the volume of coho salmon stomachs ( R² = 0.85) than in the total digestive tracts ( R² = 0.55) and foreguts ( R² = 0.61) of northern squawfish, relative to fish size. Compared to previous methods, the new technique has the advantage of accurately measuring the internal volume of a wide range of digestive organ shapes and sizes.     

An intersexual migratory (silver) longfinned New Zealand eel and its gonadal response to treatment with salmon pituitary homogenate

An intersexual adult New Zealand longfinned eel Anguilla dieffenbachii , displaying testicular tissue within its ovaries, is described. Gonad samples were collected at 0, 21, 42 (by biopsy) and 54 days (autopsy) after the start of an experiment that aimed to induce ovarian maturation by repeated treatment with 5 mg kg⁻¹ salmon pituitary homogenate (SPH). Very small patches of testicular tissue (almost always single seminiferous tubules), not visible macroscopically, were observed on all sampling dates. Treatment with SPH resulted in advanced stages of spermatogenesis in these seminiferous tubules, synchronous with the development of oocytes, which reached the migratory nucleus stage after 53 days. This finding is discussed in relation to current knowledge on sexual development in anguillid eels.     

Osmoregulation during the development of glass eels and elvers

Drinking rates in glass eels and elvers of the European eel increased with environmental acclimation salinity from 0·07±0·02 (FW) to 0·70±0·09 μl g-¹ h-¹ (SW) at month 1 and from 1·12±0·42 (FW) to 12·85±1·05 ± l g-¹ h-¹ (SW) at month 5. Drinking rates increased with time in both FW and SW groups. FW acclimated eels when challenged acutely with SW increased drinking rate rapidly immediately upon transfer (0–15 min) and the magnitude of this response increased with developmental time from month 1 to month 5.     

Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios

Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2·24 × 10⁻³) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3·23 × 10⁻³) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean ±  s.d . age of 5·2 ± 2·1 years. Eels from the inland Lake Baluošai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.     

Seasonal changes in ionoregulatory variables of the glass eel Anguilla anguilla following estuarine entry: comparison with resident elvers

In the present study, glass eels Anguilla anguilla in the Minho River estuary (41·5° N, 8·5° W) decreased in size (standard length, L _S and mass, M ) from the beginning (autumn) to the end of the sampling season (summer). On the other hand elvers increased in L _S and M from spring to summer and were significantly larger than glass eels in paired comparisons. Branchial Na⁺/K⁺-ATPase and vacuolar (V-type) proton ATPase ( in vitro activities), two important ion transporting pumps, did not show significant seasonal changes in either glass eels or elvers although in glass eels Na⁺/K⁺-ATPase (activity) expression was significantly higher than in elvers. In a single month comparison Na⁺/K⁺-ATPase branchial mRNA expression was also higher in glass eels as was the protein level expression of both Na⁺/K⁺-ATPase and NKCC (Na⁺:K⁺:2Cl⁻ co-transporter). Immunofluorescence microscopy indicated apical CFTR Cl⁻ channel labelling in Na⁺/K⁺-ATPase positive chloride cell in glass eels which was absent in elvers. Whole body sodium concentration and percentage water did not show significant seasonal differences in either glass eels or elvers although there were significant differences between these two groups during some months.     

Long-term numerical changes and regulation in a river stock of European eel Anguilla anguilla

1. The objective was to explore the regulatory mechanisms underlying the numerical dynamics of a river stock of European eel Anguilla anguilla based on a purely observational study. We used a 21-year data set on the Rio Esva (Asturias, northwestern Spain) stock of eels, encompassing eel density at an estuary site and at nine sites distributed among three tributaries selected to differ in the distance from the tidal limit and fishery data on glass eels from the nearby Rio Nalón estuary.
2. The abundance of glass eels and estuarine and in-stream densities declined simultaneously over a 15-year period. However, following a historical minimum in the abundance of glass eels around the year 2000, the density continued to decline in the estuary whereas density in the streams began an increase that has continued to the present.
3. In-stream cohort mortality rate ( Z ) varied from 0.0016 day⁻¹ to only 0.0003 day⁻¹ (on an annual basis from 0.57 to 0.099 year⁻¹). The negative effects of intra-cohort and inter-cohort densities on mortality rate emphasized the responses of the riverine stock to compensatory density-dependence. As a consequence, the number of eels surviving to commence the silvering stage was similar in magnitude in the strongest and weakest cohorts.
4. These results suggest that, even if the low abundance of glass eels persists, in-stream density may increase towards a stable value regulated by in-stream density-dependence.     

Endurance swimming of European eel

A long‐term swim trial was performed with five female silver eels Anguilla anguilla of 0·8–1·0 kg ( c . 80 cm total length, L _T) swimming at 0·5 body lengths (BL) s⁻¹, corresponding to the mean swimming speed during spawning migration. The design of the Blazka‐type swim tunnel was significantly improved, and for the first time the flow pattern of a swim tunnel for fish was evaluated with the Laser‐Doppler method. The velocity profile over three different cross‐sections was determined. It was observed that 80% of the water velocity drop‐off occurred over a boundary layer of 20 mm. Therefore, swim velocity errors were negligible as the eels always swam outside this layer. The fish were able to swim continuously day and night during a period of 3 months in the swim tunnel through which fresh water at 19° C was passed. The oxygen consumption rates remained stable at 36·9 ± 2·9 mg O₂ kg⁻¹ h⁻¹ over the 3 months swimming period for all tested eels. The mean cost of transportation was 28·2 mg O₂ kg⁻¹ km⁻¹. From the total energy consumption the calculated decline in fat content was 30%. When extrapolating to 6000 km this would have been 60%, leaving only 40% of the total energy reserves for reproduction after arriving at the spawning site. Therefore low cost of transport combined with high fat content are crucial for the capacity of the eel to cross the Atlantic Ocean and reproduce.     

Evidence of silver eels contamination by microcystin‐LR at the onset of their seaward migration: what consequences for breeding potential?

Thirty migrating silver eels Anguilla anguilla were collected in a river system where algal blooms occurred yearly. Fifty per cent of eel livers were contaminated by microcystin-LR (mean ± s . d . toxin level: 28·1 ± 22·4 ng g⁻¹). Contaminated silver ( v. healthy) eels had lower fish condition. Consequences of this impact for the breeding potential of these migrating eels are discussed.     

Environmental factors as forces structuring the fish community of the Elbe Estuary

Smelt, flounder, eel, three-spined stickleback, twaite shad, ruffe and herring were the major species among 62 forming the fish community of the Elbe estuary. Species richness, species diversity, evenness and total fish biomass decreased in the upstream direction. Total abundance was greatest during summer. Salinity was the most significant physico-chemical factor affecting species richness and total fish biomass, whereas water temperature was the best predictor of total abundance. Marine fish species decreased with decreasing salinity, while the freshwater species roach and ide were absent at salinities > 15‰. Bream, blue bream, white bream, perch and pikeperch occurred more frequently in shallow marginal regions with lower current velocities, while herring and other marine species occurred closer to the deep central regions with high current velocities. Annual and seasonal variations of the community structure mainly reflected in population dynamics of smelt, flounder, twaite shad, three-spined stickleback and eel, especially those of 0-age smelt. Populations of smelt and ruffe and their importance in the fish community increased between 1989 and 1992, while those of twaite shad and eel decreased. At oxygen concentrations <3 mg I¹, smelt and flounder were rare, but eels were at their maximum at 3.5 mg I⁻¹. Changes of species composition and fish abundance were closely related to daytime and tidal cycle.     

Differences in size and growth rates of male and female migrating Japanese eels in Pearl River, China

The Sr/Ca ratios in otoliths of silver Japanese eels Anguilla japonica , in Pearl River, China, indicated that both sexes did not stay in brackish water and grew in fresh water from the glass eel stage until spawning migration. This did not support the hypothesis that females tended to distribute upstream and males might be restricted to estuaries. The back-calculated total length of males at glass eel stage was not significantly different from that of females, indicating that the hypothesis that small glass eels became males and larger ones became females may not be true. The mean (±S.D.) age and total length of males at migration were 6·4±1·6 years and 48·3±4·5 cm, which were significantly smaller than for females, 8·3±1·6 years and 61·4±4·1 cm. The age of migration was related inversely to growth rate for both sexes. Growth parameters of the von Bertalanffy growth equation were K =0·21 cm year°1, L _∞=55·7 cm and t _o=-0·55 year for males and K =0·14 cm year⁻¹, L _∞=77·5 cm and t _o=-0·60 year for females. The difference in asymptotic length ( L _∞) between males and females may be because females postpone migration to achieve larger size for maximizing reproductive success.     

The influence of body size and temperature on oxygen consumption of the European eel, Anguilla anguilla

The effect of temperature and body size on oxygen uptake of European eels, Anguilla anguilla , was studied. The mass specific oxygen uptake of large eels was lower at all temperatures than that of small eels. The effect of temperature on metabolism was greater on small eels than on large eels. The relationship between oxygen consumption and body size is described by the equation M = a W^b , where M is metabolism (O₂ h^-1 ), W is body weight (g), b is the slope of the function which describes the relationship between body weight change and metabolism, and a is the temperature-dependent constant of the equation. In this study it was found that increased temperature caused an increase in a and a decrease in b .     

Reproductive migration of brown trout in a small Norwegian river studied by telemetry

The movement of 34 large (39–73 cm standard length) brown trout Salmo trutta was monitored using radio telemetry for up to 74 days in Brumunda, a small Norwegian river (mean annual discharge 3·3 m³ s⁻¹) flowing into the large Lake Mjøsa. The maximum range of movement in the river was 20 km. No clear relationships existed between individual movement and water discharge, temperature and barometric pressure. Brown trout migrated at all levels of water discharge. At low discharge (<2 m³ s⁻¹) movements were nocturnal. A weir 5·3 km from the outlet restricted ascending brown trout at low ( c . 6° C), but not at high ( c . 8° C) water temperatures. Spawning occurred in September to October and tagged individuals spent 2–51 days at the spawning sites. Mean migration speed from tagging to when the fish reached the spawning area, and from when they left the spawning areas and reached the lake was 1·0 and 2·3 km day⁻¹, respectively. All tagged brown trout that survived spawning returned to the lake after spawning.     

Impaired spermatogenesis in the Japanese eel, Anguilla japonica: Possibility of the existence of factors that regulate entry of germ cells into meiosis

In the cultivated male Japanese eel, spermatogonia are the only germ cells present in the testis. Weekly injections of human chorionic gonadotropin (HCG) can induce complete spermatogenesis from proliferation of spermatogonia to spermiogenesis. In some cases, however, HCG injection fails to induce complete spermatogenesis. Testicular morphological observations revealed that HCG-injected eels could be classified into three types based on their testicular conditions. Type 1 eels had a well-developed testis and the milt could be acquired by hand-stripping. In type 2 eels, spermatogenesis was also induced by HCG injection, but testicular size was remarkably smaller than that of type 1 eels, and the milt could not be hand-stripped. At the end of the experiment, type 2 fish had only spermatogonia and a small amount of spermatozoa, but no spermatocytes or spermatids, in their testis. Type 3 eels had thready testis, which did not develop any germ cells during the experimental period. These results suggest that, despite elevations of plasma 11–ketotestosterone levels, HCG injections were not successful in inducing the completion of spermatogenesis in type 2 and type 3 eels. In most spermatogonia of type 2 eels, meiosis was not induced by HCG injections. Furthermore, only few mitotic divisions had occurred as evidenced by the presence of 2³ to 2⁶ late type B spermatogonia in most cysts. This suggests that spermatogonial stem cells undergo four or five, and occasionally six, mitotic divisions before the interruption of spermatogenesis in type 2 eels. It is proposed that those numbers of mitotic divisions are related to a mediator that regulates entry of spermatogonia of the Japanese eel into meiosis.