Abstract
This study compared changes in the biochemical composition of muscles and ovaries of maturing female European eels during hormonal stimulation with carp pituitary homogenate under controlled conditions and in the development of oocytes. It has been found that differentiation of oocyte size in eel is visible from the beginning of the gamete maturation process. This differentiation increases as gonads grow. Furthermore, oocytes are at different development stages and are still highly differentiated in terms of size during ovulation. Moreover, the biochemical composition of the body and gonads was found to change. During the process of maturation, the relative fat content in the eel body decreases both in the muscles (from 21.99 ± 4.3 to 18.48 ± 3.3 %) and in gonads (from 25.76 ± 2.71 to 15.21 ± 4.7 %), with the changes in ovaries being more rapid. Protein content in muscles also decreased (from 15.98 ± 3.13 to 12.35 ± 1.6 %) during the process of female maturation. Different trends were observed for polyunsaturated fatty acids (EPA and DHA). The total amount of these acids decreased in muscles (P 0.05).
Highlights
The population of the European eel (Anguilla anguilla) has decreased dramatically over the past three decades
The European eel is regarded as a trophic opportunist (Prigge et al 2012), it does not consume any food during its migration journey and it takes the energy it needs to travel the thousands of kilometers and undergo the maturation process from the fat accumulated in its body (Clevestam et al 2011)
Calculation of the total amounts of proteins, fats, water and ash in gonads of the European eel during the process of maturation (Fig. 3) has shown that their content decreases in the bodies of females and increases in gonads. This means that fats and proteins are transferred actively from muscles and incorporated in gonads
Summary
The population of the European eel (Anguilla anguilla) has decreased dramatically over the past three decades. The species is outside the limits of biological safety and, it has been placed on the Red List of endangered species in Europe (Belpaire et al 2009; Prigge et al 2012) Such radical changes in the eel population have been attributed, inter alia, to excessive exploitation of the species, deterioration of the environment, climatic changes and infestation of the swimming bladder with the Anguillicoloides crassus nematode (e.g., Friedland et al 2007; Belpaire et al 2009; Clevestam et al 2011). The accumulated energy may even be insufficient for reaching the spawning ground (Clevestam et al 2011), which makes the situation even more difficult for the species
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