Introduction
Control over gamete quality is a key issue in aquaculture, both to increase productivity of commercially important fish and for the management of threatened species. One of the current challenges is to understand if and how rearing conditions can affect ejaculate quality. In order to investigate the effect of captivity in European eel, sperm number and performances of six wild and seven farmed males were compared starting from 11 weeks after weekly hormonal induction. Sperm fatty acids content and plasma testosterone levels were also analysed to evaluate dietary influences and hormonal male response, respectively.
Materials and methods
Wild European silver eels were caught early in spring, four months before the experimentation, in brackish water lagoon near the sluices of the North Adriatic Sea Val Noghera (Friuli Venezia-Giulia, Italy), whereas the farmed eels came from Allevamento Ittico Leonelli Antonio Succi freshwater fish farm (Goro, FE, Italy). The eels were caught, selected, transported, acclimated and maintained in RAS as previously described by Mordenti et al. (2013). Before catch and transfer to the experimental facilities, farmed eels were fed with a commercial fish meal and fresh mollusc mash while the wild ones fed natural diet whereas all animals were starved throughout the experimental period. After acclimation eels were individually tagged and spermiation was induced by weekly administration of 1.0 IU g ̄1 hCG for 17 weeks (Mordenti et al., 2013). Ejaculate analysis began 24 h after the 11th hormonal administration and lasted until 3 weeks after hormonal administration suspension. Sperm number was evaluated by spermatrocrit measure and haemocytometer counting (ejaculate dilution 1:1000 with inactivating solution, Sørensen et al., 2013). Sperm longevity and viability were evaluated by counting the time of total motility and the number of motile sperm at different time interval (sample dilution: 1:500).
The fatty acid profile of spermatozoa was determined by gas-chromatographic technique on samples prepared according to Matyash et al. (2008) and Jenkins (2009). Plasma testosterone concentration was measured by RIA as described by Parmeggiani et al (2015).
Statistical analysis were performed with SPSS21. Data were analysed by means of a linear mixed models (restricted maximum likelihood estimation of parameters) in which treatment (wild vs farmed), hormonal injection (present in weeks 13-17, absent in weeks 18-20), week, interactions between treatment and week and between treatment and injection were considered as fixed factors; male identity was included as random factor to account for repeated measures.
Results
Spermatocrit and hemocytometer counting were positively correlated in all analysed weeks (all r>0.7; all p<0.01). Hormonal administration had a positive effect on sperm number and this effect was significantly higher in wild specimens (no hormone:12.825±4.777millions/μl; hormone: 13.608±3.536 millions/μl) (treatment x hormone injection: spermatocrit: F1,88.55 =5.937, p=0.017; hemocytometer: F1,88.58 =10.37, p=0.002). The highest sperm concentration was observed in wild males at 14th week. Sperm viability was not influenced by the treatment and by hormonal administration neither in wild nor in farmed eels (treatment: F1,11.02 =1.818, p=0.205; hormone injection: F1,78.97 =3.163, p=0.079; week; F1,76.75 =1.071, p=0.387). Sperm longevity analysis on the slide showed a significant effect of treatment (wild vs farmed, F1,11.36 =11.989, p=0.005), of hormonal administration (F1,90.16 =59.345, p<0.001), of sampling week (F1,90.16 =2.960, p=0.008). Sperm of wild males showed a higher longevity on the slide (wild: 240.478±110.441 sec; farmed: 189.688±68.118 sec). Higher longevity was observed at week 16th both in wild and farmed males and a decrease in all the week after the hormonal administration suspension was also observed (week 18th: 165.000±36.458 sec.; week 19th:161.727±51.59; week 20: 132.416±29.059 sec.). Also longevity in vial showed similar results with a significant effect of the treatment (F1,10.17 =12.634, p=0.005) and of the hormonal administration (F1,87.77 =74.161, p<0.001). As regard to the fatty acid composition, the treatment significantly influenced the content of unsaturated fatty acids (UFA) with higher values in farmed male sperm (treatment: F1,10.02=6.009, p=0.034). OMEGA 3 were significantly higher in farmed male sperm (treatment: F1, 9.35 =44.065, p<0.001) whereas OMEGA 6 were higher in wild male sperm (treatment: F1, 10.96 =77.912, p<0.001). No effect of the treatment was observed in the testosterone level, but only an effect of the sampling week was present (treatment: F1,12.17 =0.001, p=0.075; week: F1,104.23 =12.14, p<0.001).
Discussion
The present study compared sperm number and performances of wild vs farmed silver eels in order to investigate possible effects of captivity. In both wild and farmed animals, the hormonal administration induced similar plasma testosterone levels and increases sperm number and performances confirming the results of previous studies (Perez et. al, 2000). However, to our knowledge our study is the first demonstration of a differential, higher, effect of hormonal stimulation in wild animals compared to farmed ones. The farmed eels showed, indeed, lower sperm number and longevity on slide than the wild ones and these worst performances could be due to the high levels of OMEGA3 in sperms probably due to the high levels of these fatty acids in fish meal used to prepare the artificial feed. These results confirmed those reported by Peìrez et al. and Butts et al. (2000 and 2013 respectively) and highlighted the importance of diet composition on sperm quality of fish in captivity. Sperms obtained 14 and 16 weeks after first hormone administration showed the highest performances suggesting to select this timing to improve artificial reproductive protocol in this species.
References
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