A balanced broodstock diet is considered imperative for the achievement of satisfactory egg production, and egg and larval quality in aquaculture. Therefore, specialized and expensive broodstock feeds are produced to ensure proper intake of nutrients, such as proteins, lipids, vitamins and carotenoids. Depending on species, some fish cease feeding during reproductive development and/or spawning, while others reduce their feed intake. In gilthead seabream, although feeding continues throughout the reproductive period, it has been shown that fish use their liver and muscle reserves during this period (Almansa 2001). In the present study, we were interested in investigating whether the energy and nutrients stored in fish tissues are adequate to cover the demands of spawning, and if or how soon diet deprivation may have negative effects on the spawning performance of fish, both in terms of fecundity, but also fertilization success and embryo/larval development. This information may provide guidance for optimal broodstock feeding practices in fish aquaculture.
The preliminary experiment in the present study examined the effect of fasting for 43 days in the beginning of the spawning season on egg fecundity and fertilization success, demonstrating that not only these reproduction parameters were not affected negatively by the fasting, but in fact they increased during and after the fasting period. This might have been because the fasting period started early in the spawning season (February), when fecundity was still in an increasing trend and the nutritional status of the females was still great, and the number of vitellogenic oocytes in their ovaries still high. Based on these surprising results, in the main study that was carried out the next year, fasting begun later in the year, around the middle of the spawning period, and also consisted of a longer fasting period of 54 days. The results, as discussed below, were again similar, in that no negative effects on spawning performance were observed, except from a gradual reduction in fecundity.
The spawning kinetics of gilthead seabream during both years were typical of cultured fish (Mylonas et al. 2011). Spawning began in early January, soon after the shortest day of the year, with an initially increasing trend in daily fecundity, while fertilization success was very high almost from the beginning of the spawning season. The values and variation from day-to-day obtained were typical of the species under culture conditions (García-Fernández et al. 2018), although the slight reduction in daily fecundity right before the onset of the fasting period observed in both monitored broodstocks in the main study (year 2) was unexpected. However, egg production and quality did not decrease during the 43 and 54-day fasting periods of the preliminary and the main study, respectively, suggesting that even long fasting periods during the spawning season do not affect the production of good quantity and quality eggs in gilthead seabream. In addition, 5-d larval survival was stable during and after fasting. The decreases in fecundity and hatching percentage observed during the post-fasting period of the main study, could be due to the approaching end of the spawning period, rather than the effect of fasting. As mentioned above, the fasting period in the main study started later than in the preliminary study, and it was closer to the end of the spawning season. Reduced egg quality towards the end of the reproductive season is common in aquaculture and has been shown both in gilthead seabream (Jerez et al. 2012) and in other sparids, such as the red porgy (Pagrus pagrus) (Mylonas et al. 2004). In fact, gilthead seabream fecundity in our facilities exhibits a decreasing trend for at least three weeks to a month before the end of the spawning period (Karamanlidis et al. 2017). Restriction or deprivation of feed in other species during vitellogenesis has been suggested to result in improved, instead of deteriorated, gamete quality (Reading et al. 2018). Also, in prepubertal male European seabass (Dicentrarchus labrax), it has been shown that feed restriction six months before reaching puberty, led to increases, instead of decreases, in some sperm motility parameters (Escobar et al. 2014). On the other hand, in the female European seabass restricted diets with half rations compared to controls, fed to fish 6 months before spawning led to decreased fecundity and hatching percentage, without affecting vitellogenesis and vitellogenin plasma levels (Cerdá et al. 1994a). The negative results of feed restriction in the latter study could be due to the restriction in feeding during the very important period of vitellogenesis in that species, and not simply during the spawning season.
Spawning is an energy and nutrient demanding process and observations of loss of weight after spawning indicate that feed alone cannot cover the energy and nutrient requirements of spawning fish (Tocher 2010). The monitored gilthead seabream were fed to apparent satiation on a commercial broodstock diet, and presumably they had built up adequate nutrient reserves prior to being subjected to feed deprivation. Feed deprived gilthead seabream continued to spawn without any significant negative effects on fecundity or egg quality. The significant reduction of crude protein concentration in maternal muscle and crude lipid concentration in liver coupled with the lower HSI indicate that muscle protein and liver fat were the main energy and nutrient sources utilized in fasted gilthead seabream during spawning. Perivisceral and peritoneal fat, which were not measured in the present investigation, could be additional sources. On the other hand, maternal muscle fat and liver proteins appeared to cover a minor portion of the demands. The reduction in the concentration of muscle crude protein and liver crude fat was counterbalanced by an increase in moisture concentration to maintain cellular volume. In a similar study, gilthead seabream fed on main ration during spawning did not deplete muscle proteins (Harel and Tandler 1994) and apparently, protein demands were covered by the diet; further, fat concentration in muscle decreased, which was not observed in the feed deprived seabream of the present study. It appears that gilthead seabream utilizes different nutrient reserves for maturing oocytes and for maintenance depending on their nutritional status, body reserves and the availability of feed. It is likely that in our study the fish undergo the third phase of fasting, a shift to protein mobilization as the main energy source, after a first short transient phase and a second fat oxidation phase (Bar 2014). Fasting had no effect on crude ash concentration in muscle, liver and gonads, apparently due to its minor influence on the mineral pool and the uptake of minerals from the rearing water. In general, the fatty acid profiles of muscle and liver lipids were similar prior to and after fasting. A reduction in Σω-6 fatty acids in the muscle which is in line with the observation in sharpsnout seabream (Diplodus puntazzo) (Rondán et al. 2004) and an increase in Σω-6 fatty acids in the liver were observed. Despite the statistically significant differences, their numerical differences appear to be small to have a biological significance.
Fasting reduced the crude protein concentration in gonads, as well as gonad size in terms of absolute weight and GSI. At the same time, the fecundity and proximate composition and quality of eggs were not affected. A confounding effect of time and fish physiology may exist as the reproduction season was advancing. Apart from the reduced crude proteins and subsequent water increase, no substantial effect of fasting was observed in crude lipids or fatty acids in gonads. Minor differences were observed in the gonadal fatty acid profiles of starved and fed fish, indicating adequate lipid reserves and non preferential use of any fatty acid. Since a fully-fed control group throughout the experiment was not considered necessary to examine the main hypothesis of the study, we do not have available data to confirm the above claims and exclude any confounding effects of the reproductive cycle of gilthead seabream. It is evident from the fasting and post fasting comparison, however, that gonadal development cannot be arrested by fasting, and the produced eggs are of similar quality in terms of fertilization, embryonic development and early larval survival.
The egg proximate composition was maintained unchanged without any apparent effect of feed deprivation in crude protein, crude lipid, crude ash, moisture and energy density. The reduction of crude protein in the ovaries did not influence the crude protein concentration in the eggs. This may imply that not all ovary tissues are directed into eggs or if there was an effect this was not detectable due to the big difference in crude protein concentration between ovaries (c.a. 20%) and eggs (c.a. 3.8%), potential homeostatic mechanisms may have maintained the crude protein concentration in eggs despite the reduction of crude protein in the ovaries. In general, fish maintain egg quality under limiting resources. In red seabream (Pagrus major) and Nile tilapia (Oreochromis niloticus), the protein content of the diet did not affect egg quality (Watanabe et al. 1984) while in three-spined sticklebacks (Gasterosteus aculeatus) the ration size (Fletcher and Wootton 1995) and in Nile tilapia feed deprivation also did not affect egg quality (Lupatsch et al. 2010). Still, it is generally agreed that diet composition affects the quantity and quality of eggs produced, and these reproduction parameters are highly influenced by nutrients such as essential fatty acids and α-tocopherol (Bell et al. 1997; Izquierdo et al. 2001; Watanabe and Vassallo-Agius 2003). Fish with long vitellogenic periods, e.g. salmonids or moronids such as European seabass, need to receive an adequate feed for a longer period prior to spawning, compared to fish with short vitellogenic periods, such as gilthead seabream. On the other hand, the latter may by more sensitive to qualitative and quantitative changes during the spawning season. Yet, it appears that gilthead seabream can build up adequate energy and protein reserves during the ovarian growth phase, and feed restriction for many weeks within the spawning period has no detectable effects on egg production and quality. Past data reporting that feed consumption decreases around the onset of spawning (Kadmon et al. 1985), may suggest that gilthead seabream can cope well under feed restriction during spawning.
As far as feed management is concerned, fasting during the spawning phase, if needed for management purposes, might be a better option than the provision of a deficient or imbalanced diet in certain nutrients. A decisive factor in the influence of fasting or feed restriction on reproductive performance is the timing of feed deprivation and the physiological state of fish up to that point. So it is probably more important to provide optimal nutrition throught the year before the reproductive season when the animals are building their body reserves, as well as during the extended process of vitellogenesis, rather than during spawning. Therefore, other husbandry parameters, such as stocking density, water quality, lack of disturbances and other welfare aspects, may be more important to reproductive performance than feeding to apparent satiation during the spawning period.