Effect of dietary lipid sources on the reproductive performance of Nile tilapia Oreochromis niloticus

Abstract

Abstract Traditionally, fish oil (FO) has been used extensively in aquafeeds. The stagnation in global fish oil production coupled with an increased demand for its use in aquaculture feeds, especially salmonid feeds, has greatly inflated fish oil prices. Therefore, in order to sustain the rapid growth of the tilapia industry, the dependence on these commodities in feeds should be reduced through use of cheaper and more sustainable sources of dietary lipids, such as palm oil. This study therefore investigated several, previously poorly understood, effects of palm oil on reproductive performance of the commercial tilapia species, Oreochromis niloticus; which currently ranks as second most popular species in world aquaculture. In the present study broodstock were fed on experimental diets at full and half ration regimes throughout their entire life cycle from exogenous feeding. Studies were conducted in standardised and controlled hatchery conditions, thereby reducing the potential influence of environmental variations. First feeding O. niloticus fry were fed on four diets, cod liver oil (D 1), palm oil (D 2), mixed palm and cod liver oil (D 3) (9:1 ration) and a commercial trout diet as control (D 4) (Skretting, U.K.) on a reducing ration based on fish size. The present study investigated the effect of dietary lipid sources on (1) growth performance, (2) biochemical composition of eggs (total lipid and fatty acid composition), (3) morphological parameters of eggs (total and relative fecundity, egg size, egg weight and EW:BW), (4) larval quality (larval length and weight) and (5) oocyte recruitment and its associated sex steroid hormones. Experimental diets and feeding ration significantly influenced (p<0.05) the growth performance over a period of 120 days. Total lipid and fatty acid composition of eggs originating from broodstock fed on palm oil, mixed palm and cod liver oil (9:1) or a control diet were not significantly different (P>0.05) when fed at either full (3% BWday-1) or half ration (1.5% BWday-1). The present study, however, confirmed that fatty acid composition of fish eggs reflected the fatty acid composition of the diet, although specific fatty acids were selectively utilized or retained in the eggs. The mean inter-spawning interval (ISI) increased with increasing fish size and averaged 14, 19 and 24 days for fish fed on palm oil, mixed palm and cod liver oil or control diets, respectively. The shortest ISI observed was 7 days for fish fed a palm oil diet. Total fecundity ranged from 660 - 820 eggs/clutch. Mean total fecundity was 750, 820 and 660 eggs/clutch for fish fed a palm, mixed palm and cod liver oil or a control diet, respectively, but these differences were not significant (P>0.05). However, relative fecundity and egg weight to body weight rates as a percentage (EW: BW) were found significantly differ (p<0.05) between fish fed the control diet and experimental diets. Mean egg diameter (2.2 mm) was not significantly influenced (p>0.05) by experimental diets. The egg volume, egg dry and wet weight, fertilisation and hatching rate were also not significantly different between fish fed the experimental diets. Oocyte development was classified into distinct stages based upon oocyte size, biochemical properties and structure. The recrudescence to these stages was not significantly influenced by broodstock fed experimental diets either at full or half ration. Steroid hormones and histological analyses provided valuable data concerning the oocyte development and recruitment in this species. Levels of 17ß-oestradiol (E2) and testosterone (T) peaked within 6 days of spawning, suggesting that vitellogenesis began as early as day 2 or 3 post-spawning. By day 6, ovaries were dominated by large late-vitellogenic/maturing oocytes (stages 6 & 7) occupying about 70% of the ovary. Gonadosomatic index (GSI) reached maximal levels by day 6. It is suggested that pre-vitellogenic oocytes are recruited into vitellogenic growth immediately after spawning and complete vitellogenesis on day 6 post-spawning. Finally, the present study investigated the effect of food restriction at two rations (full and half) on broodstock reproductive performance. Oreochromis niloticus were rationed from first feeding and throughout their life-cycle. The dietary regime, full ration (3%) and half ration (1.5%), influenced fish size but despite this variation no significant differences (p>0.05) were detected in total lipid and fatty acid composition in the eggs, total fecundity, egg diameter, total egg volume and larval size. These results suggested that despite large differences in food availability throughout their life cycle, investment in reproduction had remained remarkably consistent. It appeared that during food restriction, O. niloticus sacrificed body weight and growth so as to maintain reproductive investment. In summary, this study provides valuable information using a novel experimental design on the effects of dietary lipid sources on reproductive performance of female O. niloticus. Substituting palm oil for fish oil as the dietary lipid source and reducing ration by half (1.5% BWday-1) had no significant effect on reproductive performance. Therefore it is suggested that under controlled conditions, lipids of non-marine origin, such as palm oil, can be successfully substituted for broodstock diets. Halving feed requirement should also increase profitability of seed production. KEYWORDS: Tilapia; O. niloticus; palm oil; diet; fecundity; spawning periodicity; oocyte recruitment; reproductive performance.