Changes in Body Weight and Eye Size in Female European Eel Kept in Fresh and Salt Water

Abstract

The eel is a catadromous fish which spends most of its life in freshwater and adults swim to the Sargasso Sea region to spawn. While preparing for the reproductive process, eels undergo a metamorphosis to become what is called silver eel; a process involving changes in the colour and weight of the body and an increase in the eye size. These are indicators of fish maturity and they facilitate the selection of fish for reproduction under controlled conditions. During this study, changes in the body weight (BW) and eye size in female European eel were observed while being given weekly hormonal injections of 20 mg kg -1 carp pituitary homogenate and kept in 15°C freshwater and in 15°C saltwater with a salinity of 32-33‰. Fish kept in saltwater but not subjected to hormonal stimulation were used as a control group. Furthermore, after the experiment was finished, females in the control group were kept for next 5 months, with the same environmental parameters maintained. Differences between the treated groups were observed as early as week 4 of the experiment. An increase in fish BW was observed in fish kept both in salt and freshwater which were subjected to hormonal stimulation. On the other hand, changes in eye size were observed in the fish kept in saltwater, both in those hormonally stimulated and otherwise. The eye diameter in the fish kept in fresh after hormonal stimulation did not change significantly.

Changes in European eel ovary development and body and ovary chemistry during stimulated maturation under controlled conditions: preliminary data

“…The oocyte diameters observed in this study after 5 weeks of stimulating the female fish were larger (0.176 ± 0.018 mm) than those observed by other authors after 6 weeks of the experiment (0.109 ± 0.041 mm) in a study examining the effect of swimming on the size of oocytes (Palstra et al 2007). It may be concluded that hormonal treatment considerably accelerates the process of gamete maturation, which was also confirmed by a study conducted by Nowosad et al (2014). The oocytes obtained at the end of the experiment had diameters of up to 0.853 mm, while eggs of a similar size (0.763-0.930 mm) obtained by Pedersen (2003) were regarded as pre-ovulation eggs.…”

“…The hormonal stimulation was started after the desired water salinity level was achieved using the method described by Nowosad et al (2014). After being taken out of the tanks, the fish were put in a container with an anesthetic.…”

“…Many intensive physiological and morphological changes take place in the eels during the maturation process (Müller et al 2004a;van Ginneken et al 2005a;Clevestam et al 2011;Nowosad et al 2014). Although 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).…”

Use of an ultrasound device to determine sex and to perform gonad biopsy in the European eel Anguilla Anguilla

“…The data from reports published to date, along with the authors' own research (PALSTRA et al 2005;HORVÁTH et al, 2011;NOWOSAD et al 2014NOWOSAD et al , 2015, indicate that the effectiveness of artificial reproduction of eel depends closely on the maturity of gonads. The maturity of ovaries in eel can be determined by biopsy or by such indicators as an increase in body weight index -BWI (NOWOSAD et al 2014.…”

“…The maturity of ovaries in eel can be determined by biopsy or by such indicators as an increase in body weight index -BWI (NOWOSAD et al 2014.…”