Over the past 25 years the population of European eel has been declining to such degree that major concerns have been raised for its long -term well being. Adult stocks have started to dwindle in the 1940’s in major areas of the European continent, while recruitment (glass eel arrivals) has collapsed since the early 1980’s. There is no sign of recovery and the phenomenon seems to occur over the natural range of the European eel (Anguilla anguilla L.). A parallel development is observed in the closely related American eel (A.rostrata) and the Japanese eel (A. japonica).
The European eel (Anguilla anguilla L.) is a catadromic fish species with its spawning grounds thousands of kilometers away in the ocean. An important aspect of the reproduction of European silver eels is the huge distance they have to swim to reach their spawning
grounds. After leaving the West European coast they still have to swim 5000-6000 km to the Sargasso Sea, the assumed spawning site. To cover this distance eels must swim continuously
for 6 months at 0.5 Body Length per second, which requires an impressive swimming endurance capacity. Also high-energy reserves, coupled with low cost of transport are required. So, in addition it can be hypothesized that long term swimming capacity is a major prerequisite for reproduction. In this thesis we investigated the capacity of European eels to migrate over this distance.
The freshwater phase of growth, sex-differentiation and ‘silvering’ a pre-adaptation to its ocean phase prior to migration will determine the quality of the spawners. This period in the
fresh-water can cover a period of 5-50 years. Thus the habitat quality and habitat factors like shortage of food (leading to diminished fat stores), viruses and toxicants (e.g. polychlorinated
biphenyls: PCB’s) is important for the swimming fitness of the adults and quality of the gametes. In this thesis we will describe some of the topics of the life cycle of the European eel in order to understand more of the possible causes for decline of eel populations and factors that are involved in reproduction.
Freshwaterphase, orientation on the earth’s magnetic field:
In the literature several field studies in tanks, telemetric studies, studies with strong artificial magnetic fields, overriding
the natural directional preference of eels, are indicative that orientation is accomplished through features of the earth magnetic field. Also the observation of magnetic substances in the skull and bones of eels strongly supports this view. We studied the circadian and monthly activity, the distribution patterns and orientation to the earth’s magnetic field of Yellow (non-migratory) female eels in a freshwater ponds means of microchips injected into their
muscles.Detectors for microchips mounted in tubes were placed in the pond to detect if eels oriented themselves with respect to earth’s magnetic field. Based on the frequency of tube visits (search for shelter), the data indicated that the presence of eel in the tubes decreases gradually during the study period. We saw more activity during the night in the first months. There was a seasonal component in the orientation mechanism with a significantly lower preference component in the summer compared to the fall. A preference for tubes oriented in a south-southwest direction (the direction of the Sargasso Sea) in fall suggests an orientation to the earth’s magnetic field.
The transformation of yellow (non-migratory) into silver eel
(migratory) is called ‘silvering’, and takes place prior to migration. The mechanisms involved in the onset of ‘silvering’ of eels are largely unknown. Also a clear description of the different
stages, which characterize this metamorphosis is lacking. Until recently silvering was, mainly based on morphological parameters, split into two separate stages: ‘yellow’ and ‘silver’. This classification did not take into account a possible preparatory phase. We described hormonal profiles of European eel during the silvering. We also used physiological parameters like body constitution and blood substrates. This transformation occurs in association with hormonal surges of testosterone (T), estradiol (E2), cortisol but not
with those of thyroid hormones (TH) and growth hormone (GH) which have a maximum activity in spring and a minimum activity in summer and autumn. In contrast, cortisol levels in fall are elevated which play a role in mobilization of metabolic energy from body stores, to migratory activity and gonadal growth. Based on Principal Component Analysis with
physiological, morphological and endocrinological parameters it is concluded that the transition is gradual and that eels go through several stages.
Freshwaterphase, role of thyroid hormone:
For amphibians like frogs, the metamorphosis from larvae to adult is regulated by thyroid hormones. For other ecotherms like fish, also a
role for thyroid hormone in metamorphosis is assumed like for salmon during parr-smolt transformation. However, from our year-cycle study we observed that thyroid hormones in eel are very high in spring but not in autumn during the ‘silvering’ (=metamorphosis) process.
Therefore we can conclude that the thyroid hormones are possibly not involved in ‘silvering’. Another possibility is that their action is calorigenic and is involved in the control of the
metabolic rate like in birds and mammals.. We measured overall heat production in free moving eels with different thyroid status with an accuracy of 0.1 mW by direct calorimetry.
Hyperthyroidism was initiated by injection of T3 and T4 hormones while the effect of hypothyroidism was studied by exposing the animals to phenylthioureum. The results show for the first time at the organismal level, using direct calorimetry, that neither overall heat production nor overall oxygen consumption in eels is affected by hyperthyroidism. Therefore, we conclude that the thermogenic metabolism-stimulating effect of thyroid hormones is not
associated with a cold-blooded fish species like the European eel.
The new type Blazka swim tunnel:
We developed a Blazka swim tunnel of 127 liter with a total length of 2.0 meter and a length