of the swimming compartment of 1.15 meter in order to test the endurance swimming capacity silver eels with a length of 80-90 cm. We applied the very accurate Laser-Doppler
system to demonstrate the homogeneity of the flow in the swim tunnels. The actual flow was measured at different cross-sections and at different distances from the wall. A linear
relationship was observed between the number of revolutions per minute of the motor and the measured water velocity. The linearity existed up to 0.9 m/s. The flow between 40-mm from
the wall to the center stayed within a few percent of the setpoint. So, fish with a width of > 40-mm can not swim in the boundary layer. The eels used in the several studies needed an even wider space because of the amplitude of their tail beat. Furthermore we observed that the head of swimming eels remained between 50 and 100-mm from the wall. Migration: Long term swim experiments over 5,500-km with virus-negative European eel demonstrates that eels are very efficient swimmers. Eels have a fat content of 10-28% with a mean of 20%, which is obviously the predominant energy store. 40% of the total fat reserve of silver eels is required for swimming 60% remains for development of the gonad. Animals with less than 13% fat would not be able to swim 6000-km. In comparison to other fish species like salmon, eels are very efficient swimmers with energy cost for migration that are 4-6 times lower than salmonids. The Cost of Transportation (COT) for eel was 0.68 kJ.kg-1.km-1 while the COT for trout was 2.73 kJ.kg-1.km-1. The estimated fat
use for an adult eel to cross the Atlantic (6,000-km) would be 29% of its fat stores corresponding to 58 g fat/kg eel while this would be for salmon 300 g/kg. At this moment it is not understood why eels are so efficient swimmers. In future studies hydrodynamics has to explain how does undulatory swimming (characteristic for anguilliform movement) work. Therefore two main questions have to be addressed: a) the topic of the muscle design: which muscle arrangement best suits the task of bending the body, b) how does the fish convert muscle power into swimming power.
Environmental effects on migration:
Worldwide, eel populations have been dwindling over the last two decades of the previous century. The exact cause for this phenomenon is unknown, but possible causes include: PCB’s, viruses, and diminished fat stores. In order to study whether these factors had effect on the swimming performance and endurance of European eel, experiments were performed in 22 large swim-tunnels of 127 liter in the laboratory
The results of our study revealed five major observations: First, PCB-exposed animals lose less weight and have lower glucose and cortisol (only swimming) levels compared to their unexposed controls. Second, PCB-concentrations on a lipid basis are 2.7
times higher in swimming compared to resting animals. Third, PCB-exposure significantly reduces oxygen consumption during swimming of the PCB-exposed animals from 400 km on
(18 days) and this effect increases with time. The Cost of Transport (COT, [mg O2. kg-1. km-1]) is significantly lower in PCB exposed animals from 100 km up to 800 km. In addition the
standard metabolic rate measured 2 days after the last swimming activity is significantly lower in the PCB-exposed animals. Fourth, the spleen is increased in the PCB-exposed swim
animals but not in the PCB-exposed Control animals. Fifth, silver eels easily survive resting in marine water and forced swimming in freshwater, but not in a combination of these two
stress factors. Plasma-pH, ion levels (sodium and potassium), plasma lactate acid,haemoglobin and hematocrit were unaffected by PCB-exposure.We conclude, that PCB exposure interferes with the energy metabolism of silver eel in marine water and appears to interfere with cortisol control over (carbohydrate) metabolism. This effect was greater in swimming than in resting eel.
EVEX (Eel-Virus-European-X), HVA (Herpesvirus anguillae) and EVE (Eel Virus European) were detected in wild and farmed European eels (Anguilla anguilla) from the Netherlands, EVEX and EVE from farmed eels from Italy and EVEX from wild eels from Morocco. EVEX was also isolated from wild New Zealand eel (A. dieffenbachi).Elvers
(A.anguilla) collected from eel farms in the Netherlands were mainly infected with HVA. Widespread infection of the eel-population with for instance EVEX virus may result from unlimited intercontinental eel transport. In addition, we show in large swim tunnels that eels
infected with EVEX developed hemorrhage and anemia during simulated migration and died
after 1,000-1,500 km. In contrast, virus-negative animals swam 5,500-km, the estimated
distance to the spawning ground of the European eel in the Sargasso Sea. The virus-positive
eels showed a decline in hematocrit, which was related to the swim distance. The virus-negative eels showed a slightly increased hematocrit. The observed changes in plasma of Lactate dehydrogenase (LDH), Total Protein and Aspartate aminotransferase (AST) are indicative of a serious viral infection. So virus infections and PCB’s can possibly contribute
to decline of eel populations.
We observed that in hormone treated European eel spawning behavior could
be induced. This behavior of eels was collective and simultaneous corresponding to spawning
in a group. This is the first time spawning behavior has ever been observed and recorded in
eels and opens new perspectives for future research. Another important research result
concerning the reproduction of this species was the observation that in 3 years old (juvenile) European eels which swam for 173 days in Blazka swim tunnels, covering a distance of
5,500-km, at the end of the swim trial, the maturation parameters 11-ketotestosterone,
pituitary levels of lutheinizing hormone (LH) and plasma levels of estradiol were higher
(although not significantly) in the swim compared to the rest group. In contrast, the oocyte
diameter was found to be significantly higher in the swim compared to the rest group. Based
on these observations we conclude that a period of prolonged swimming might be a
physiological stimulus necessary for the onset of maturation in the European eel. Experiments in
future studies with adult virus free animals have definitely to prove that endurance swimming
might be the natural trigger for gonadal maturation in European eel.
Panmixia, molecular work:
The hypothesis that all European eel migrate to the Sargasso
Sea for reproduction and comprise a single randomly mating population, the so called panmixia theory, was until recently broadly accepted. However, based on field observations, morphological parameters and molecular studies there are some indications that the Danish Biologist Schmidt’s (1923) claim of complete homogeneity of the European eel population and a unique spawning location in the Sargasso Sea may be an overstatement. Recent
molecular work (overview of different authors given in a review) on European eel indicated a genetic mosaic consisting of several isolated groups, leading to a rejection of the panmixia
theory. Nevertheless, the latest extensive genetic survey of our Belgian colleagues from Leuven University indicated that the geographical component of genetic structure lacked temporal
stability, emphasising the need for temporal replication in the study of highly vagile marine species. So, the Panmixia theory can still not be excluded nor confirmed.
During the last two decades of the previous century eel populations
declined with 90-99%, possibly due to the synergy between human activities and oceanic fluctuations, making it an endangered species. For 3 million years the catadromous European eel succeeded in surviving and maintaining their characteristic life style with spawning areas in the ocean (possibly the Sargasso Sea) and its juvenile life phase of growth and sex differentiation in the freshwater at the European continent. The following recommendations can be made to prevent a total extinction of this species:
a) Reduce fisheries pressure by establishing nature reserves in the Inland waters to conserve
this territorial bounded fish species,
b) Development of early warning systems at hydropower stations in order to protect a
substantial part of the downstream migrating silver eels in fall during the migration
c) PCB contamination should be monitored in all major hydro-systems and areas with low
levels should be protected,
d) To prevent the spread of parasites and virus infections international global instructions
and sanitary standards for transportation of aquatic animals should be introduced,
e) Research of artificial reproduction of the eel should be extended by focussing on
administration techniques of hormones, more research is needed to understand the
spawning behaviour of eels and the role of pheromones, and research should be focussed
on understanding the natural trigger for reproduction.
f) Molecular techniques like building in cells with genes (gentherapy), which produce
maturation hormones, are a challenge.
Considering the recent catastrophic decline of eel populations throughout Europe, not much
time is given to answer these questions and prevent the irreversible loss of this mysterious
We can conclude that European eels are very efficient swimmers and that healthy well fed eels are able to reach the Sargasso Sea leaving enough reserves for the reproduction. Endurance swimming of eels and the ability to migrate are negatively influenced by infections with viruses like EVEX. Also PCB’s which are released from depleted lipid stores after migration may interfere with energy use and metabolism. This may result in reduced oxygen consumption, lower glucose and cortisol levels, a diminished
conversion of amino acids in the gluconeogenesis and a reduced Standard Metabolic Rate. So, although we have found several habitat factors that may interfere with fitness and
endurance swimming such as viruses and toxicants (PCBs) which could be a contributing factor to declining eel populations we found two positive results which in future studies may be of importance for reproduction of eels under artificial conditions. a) First, we found strong evidence that swimming triggers silvering and early maturation, b) Second, we observed for the first time in hormone treated eels that group spawning was collective and simultaneous.