Validation of daily otolith increments in glass-phase American eels Anguilla rostrata (Lesueur) during estuarine residency

Prior to making inferences from otoliths about the residence time and growth rate of glass-phase anguillid eels Anguilla in estuaries, it is necessary to validate the deposition rate of microincrements in the otoliths. Glass-phase American eels Anguilla rostrata (Lesueur), which had been captured near the mouth of an estuary in Maine, USA, prior to freshwater exposure, deposited increments at a daily rate at ambient temperature and salinity in a field and laboratory study. The regression for glass eels not possessing a transition ring was: I=0.976(D-1)+0.434, where I is the number of otolith increments distal to a fluorescent mark placed on the otolith at the beginning of the experiment, and D is the number of days in the experiment, which ranged from 7 to 49. The slope was not significantly different than 1. Unexpectedly, many glass eels deposited the transition ring during the experiment, although this ring had previously been thought to mark entry into fresh water. The regression for these glass eels was: I=0.961(D-1)-3.880, and the slope was not significantly different than 1. The negative intercept suggests that approximately 4 days were lost from the otolith record during deposition of the ring. This study demonstrated daily deposition of increments prior to freshwater exposure and demonstrated that deposition of the transition ring is not linked to freshwater entry.     

The relationship between growth, food conversion and oxygen consumption in developed and underdeveloped American eels, Anguilla rostrata Lesueur

This study examined the influence of body size and temperature on oxygen consumption and food converstion in juvenile American eels ( Anguilla rostrata ). The weight-specific oxygen consumption rate for underdeveloped eels (18 months old) was significantly higher than the weight-specific oxygen consumption rate of developed eels of the same weight (6 months old). Oxygen consumption rates increased linearly with weight at each experimental temperature (15, 20, 25°C) when data were transformed logarithmically. No significant differences were found among slopes oflog transformed data at varying temperatures. Oxygen consumption was significantly higher at night (2300 h) as compared to morning (0900 h). The results indicate that underdeveloped eels use more energy and use less food less efficiently than developed eels.     

The swimbladder nematode Anguillicola crassus in American eels (Anguilla rostrata) from middle and upper regions of Chesapeake Bay.

The patterns of infection of American eels Anguilla rostrata, with the introduced swimbladder nematode Anguillicola crassus, in tributaries of middle and upper Chesapeake Bay are described. A total of 423 subadult eels was collected from 8 Bay tributaries from spring 1998 to fall 1999. Also, 30 elvers were collected from Ocean City, Maryland, in spring 1998. The numbers of juvenile and adult specimens of A. crassus in the swimbladder wall and lumen were counted. No elvers were infected. In subadult eels, prevalence of adult and juvenile stages combined ranged from 13% to 82%; mean intensity ranged from 2.6 to 9.0 worms per eel. Infection levels were highest for Susquehanna River eels (northernmost river) and lowest in the southernmost sites: St. Jerome's Creek and the Pocomoke River. Although eels from these 2 localities were larger, the low infection rates there are most likely due to reduced transmission in higher salinity water and not to eel size. Eels with both adult and juvenile stages of A. crassus were more common than expected by chance. This might be explained by inhibition of juveniles migrating into the swimbladder lumen when adults are already present there.     

Orientation responses of American eels, Anguilla rostrata, to varying magnetic fields

1. The locations of freshwater yellow eels in an eight-chambered octagonal behavior tank were videotaped during six-day intervals while the animals were being subjected to normal and experimental magnetic fields. 2. The earth's magnetic field (0.5 g) was utilized for two control periods at the start and completion of each run for each animal. 3. During each run, the sequence of applied magnetic fields was +1.0, 0.0, -0.5 and -1.0 g, each being applied for a period of 24 hr. 4. Under the influence of the earth's magnetic field, the eels showed a preference for a northeast direction (27.01%). During the second control period (i.e. after being subjected to variations in the magnetic field), the animals showed a dual preference for north and northwest directions (23.02% and 25.9%, respectively). 5. In a 0.0 g field, the eels preferred the north chamber (24.43%) and the vestibule of the behavior tank (19.46%); a preference for north was also obtained with a field of +1.0 g (25.95%). 6. The preferred direction with the -0.5 and -1.0 g fields was southeast (20.93 and 26.71%, respectively).     

Temperature behavioral responses of the American eel,Anguilla rostrata (Lesueur), from Maryland

Acute temperature preference tests were conducted with American eels, Anguilla rostrata, collected from Maryland's eastern shore. Eels were acclimated to temperatures of 6, 12, 18, 24 and 30°C. Final temperature preferendum was 16.7°C. Data differ from the temperature responses of the majority of fishes tested to date in that acclimation temperature did not influence selected temperatures. Similar results were obtained for various other fishes (Oncorhynchus, Salmo, Salvelinus) by other investigators. Behavioral responses at various acclimation temperatures were observed.     

Pathogenic bacteria associated with cultured American eels, Anguilla rostrata Le Sueur

American eels, Anguilla rostrata , obtained from an aquaculture facility in South Carolina were examined for diseases and bacterial pathogens were isolated and identified. During the first year of culture, only a small number of bacterial pathogens and diseases were detected in glass and elver stages. This observation appears to be associated with a natural immunity to disease and/or administering the antibacterial compound, nitrofurazone. There was also a positive correlation between the use of nitrofurazone and the isolation of nitrofurazone resistant strains of Aeromonas hydrophila . A higher incidence of bacterial pathogens and/or disease occurred during the process of culling and in older eels during warmer months. The primary aetiological agent of disease of cultured eels was A. hydrophila . Other potential pathogens isolated included A. salmonicida, Vibrio spp. and Pseudomonas spp.     

The glucose and lactate kinetics of American eels, Anguilla rostrata (LeSueur), under MS 222 anaesthesia

Glucose and lactate kinetics were examined in fed and food-deprived American eels, Anguilla rostrata (LeSueur), under MS 222 anaesthesia (AE). These values are compared to free-swimming, non-anaesthetized animals (FSE) reported previously (Cornish & Moon, 1985). The AE group demonstrated a steady but minor hyperglycemia during the 5-h experiment and significant decreases in both glucose turnover and metabolic clearance rates compared to the FSE groups. Food-deprivation further depressed these kinetic parameters.
Blood lactate continuously increased during the experiment, reaching values 300 times (fed) and 100 times (food-deprived) higher than the similar FSE groups. Consistent with these higher concentrations, rates of lactate appearance in and disappearance from the blood generally increased with anaesthesia.
This study supports the view of Soivio et at. (1977) that MS 222 acts as an asphyxiant. The associated metabolic disruptions are consistent with a reduction in oxidative in favour of anaerobic tissue metabolism. The specific changes in glucose and lactate parameters reflect the nervous and hormonal disruptions occurring with anaesthesia. These studies urge caution in the interpretation of data using an anaesthetized fish preparation.     

Kinematics and hydrodynamics of linear acceleration in eels, Anguilla rostrata

The kinematics and hydrodynamics of routine linear accelerations were studied in American eels, Anguilla rostrata, using high-speed video and particle image velocimetry. Eels were examined both during steady swimming at speeds from 0.6 to 1.9 body lengths (L) per second and during accelerations from -1.4 to 1.3 L s(-2). Multiple regression of the acceleration and steady swimming speed on the body kinematics suggests that eels primarily change their tail-tip velocity during acceleration. By contrast, the best predictor of steady swimming speed is body wave speed, keeping tail-tip velocity an approximately constant fraction of the swimming velocity. Thus, during steady swimming, Strouhal number does not vary with speed, remaining close to 0.32, but during acceleration, it deviates from the steady value. The kinematic changes during acceleration are indicated hydrodynamically by axial fluid momentum in the wake. During steady swimming, the wake consists of lateral jets of fluid and has minimal net axial momentum, which reflects a balance between thrust and drag. During acceleration, those jets rotate to point downstream, adding axial momentum to the fluid. The amount of added momentum correlates with the acceleration, but is greater than the necessary inertial force by 2.8+/-0.6 times, indicating a substantial acceleration reaction.     

First record of Anguillicoloides crassus (Nematoda) in American eels (Anguilla rostrata) in Canadian estuaries, Cape Breton, Nova Scotia

In the summer of 2007, American eels, Anguilla rostrata, from 2 localities on Cape Breton Island, were found to be infected with the swim bladder nematode Anguillicoloides crassus. This is the first documented report of this highly invasive parasite in Canadian waters. More than half of the yellow eels in Mira River (6 of 10), and 1 eel (of 5) from Sydney Harbour were infected. Parasite intensity ranged from 1 to 11 worms per eel. The occurrence of A. crassus at these 2 localities suggests the need for a more extensive survey on the distribution of this exotic parasite in eel populations throughout Cape Breton Island.     

Histological changes in the gonad, skin, intestine and olfactory epithelium of artificially-matured male American eels, Anguilla rostrata (LeSueur)

Migratory (silver) male American eels were injected weekly for 5 weeks with human chorionic gonadotropin (hCG), salmon pituitary extract, hydrocortisone 21-hemisuccinate (cortisol), or saline to determine the effects of induced maturation on the histology of their gonads, skin, intestines and olfactory epithelia. Treatment with hCG induced full sexual maturity, salmon pituitary evoked only limited spermatogenesis, and neither hydrocortisone nor saline had any effect on the gonad. Eels injected with hCG and salmon pituitary experienced no changes in skin morphology, but the epidermal thickness of saline- and hydrocortisone-treated fish decreased. Intestinal morphology did not change in any of the treatment groups. Both the presumed sensory and non-sensory portions of the olfactory epithelium of hCG- and pituitary-treated males exhibited a decrease in thickness, structural degeneration, and non-significant reductions in mucous cell density. Some of these changes in maturing male eels are similar to those previously observed in maturing females but others are different; it appears likely that maturing Anguilla are sexually dimorphic and that these changes are adaptive and not artifacts of the hormone treatments.     

Furunculosis in cultured American eel Anguilla rostrata (Le Sueur)

Diseased American eels, Anguilla rostrata , were collected from an eel culture facility in South Carolina and examined. The disease was characterized by skin lesions located on the body. A bacterium identified as Aeromonas salmonicida was isolated from the lesions and shown to be the causative agent. The bacterium was also isolated from the kidney during later stages of the disease.     

Osmoregulation in North American eels (Anguilla rostrata LeSueur) on land and in freshwater: effects of the corpuscles of Stannius

Hypercalcemia, hypomagnesia and hypophosphatemia were observed in freshwater (FW) eels (Anguilla rostrata LeSueur) after removal of the corpuscles of Stannius. These changes did not occur if Stanniectomized (CSX) eels were removed from FW and placed on land for 12 days but did occur after the eels were returned to FW. Therefore, changes in plasma electrolyte concentrations after CSX depended upon the branchial and/or integumental influx of ions. Plasma Na⁺, Cl⁻ and osmolal concentrations decreased gradually in both sham-operated (SHM) and CSX eels on land (12 days) and in FW (12 days). Plasma K⁺almost doubled in both SHM and CSX eels after 4 days on land, remained elevated, and fell abruptly to normal within a day after the eels were returned in FW. After 2 days on land, urine flow rates in SHM and CSX eels had decreased by approximately 85%, osmolar clearance by 50% and positive free-water clearance by more than 90%. Body weights did not decrease when eels were on land so it was concluded that the reduced but continuous renal loss of water was counterbalanced by the integumental uptake of condensed water. Accepted: 21 October 1998     

Retinal response in the post metamorphic American eel (Anguilla rostrata) (Pisces,Teleostei)

Retinas of light and dark adapted post metamorphic American eelAnguilla rostrata were examined. The retinal epithelial pigment migrates vitreally in light and sclerally in darkness. Two layers of rods and a layer of single cones are present. Some cones elongate slightly in the dark and contract in the light. The cone synaptic ribbons show no difference between the light and dark adapted stages. It appears that this eel stage is capable of functioning in bright and dim environments.     

The induction of maturation and ovulation in American eels, Anguilla rostrata (LeSueur), and the relevance of chemical and visual cues to male spawning behaviour

Female American eels, Anguilla rostrata (LeSueur), were artificially matured with injections of salmon and carp pituitary and human chorionic gonadotropin. In vivo ovulation was induced with 4-pregnene-17α,20β-diol-3-one and eggs were spontaneously released. Eggs were fertilized in vitro and survived to the gastrula stage. Males were matured with injections of human chorionic gonadotropin. They were attracted by the sight and odour of maturing and mature females. Ovulated females released a sex pheromone which was especially attractive to mature males and triggered the release of sperm.     

Determination of the age of young American eels, Anguilla rostrata, in fresh water, based on otolith surface area and microstructure

The time elvers of the American eel, Anguilla rostrata , spend in an estuary prior to their migration into fresh water was assessed. A distinct mark was formed on elvers' otoliths during their first 2 to 3 weeks in the river estuary. This mark was used to distinguish between growth in fresh water and in salt water. Migrating eels collected at a falls 4 km from the estuary exhibited bimodal length and weight distributions. Frequency distributions showed that eels collected in the estuary were smaller and had smaller otoliths than eels collected at the falls, indicating that elvers do not reach the falls in the same year as they enter the estuary. The three modal groups most probably represent three age–classes. However, the otoliths of elvers collected in the estuary had only the mark of transition whereas eels in the first and second mode at the falls usually had two rings (1–4) and four rings (3–6) per otolith, respectively, in addition to the mark of transition, as viewed under SEM. The possibility that ring formation is not annual means that the use of otoliths for the age determination of eels in this study has not been validated.