Suchergebnisse

Over three million tonnes (t) of tilapia, mostly Nile tilapia (Oreochromis niloticus, L.), are produced annually making it the second most abundantly produced freshwater fish (FAO, 2010). Tilapia are mouthbreeders that often produce stunted populations under pond conditions; one means of prevention is to produce all-male fish with the additional advantage that males usually grow faster than females. All-male populations can be achieved by supplementing feed with androgens such as 17-a-Methyltestosterone (MT) during days 10–25 posthatch (Pandian and Sheela, 1995). However, MT is considered to be carcinogenic (Velazquez and Alter, 2004), and Hulak et al. (2008) also showed that effluents of systems in which carp were fed diets containing MT caused masculinization of female fish. Furthermore, in aquaculture the application of hormones to fish destined for human consumption is prohibited in the European Union under directive 96/22/EC, article 5, which also prohibits import of animal products produced with hormones. Kwon et al. (2000) showed that Fadrozole, a non-steroidal compound, caused masculinization in tilapia by inhibiting aromatase, which is the enzyme responsible for the conversion of endogenous androgens to estrogens. Steinbronn et al. (2004) were able to show that a dose of 2000 ppm Quillaja saponins (Sigma S-2149) inhibited reproduction of tilapia after dietary application for 32 days to first-feeding fry, suggesting saponins as a possible alternative to MT. These secondary plant compounds consist of either a steroid or triterpenoid basic structure (aglycone or sapogenin) plus one or more sugar side chains (Francis et al., 2002a). In a previous experiment a saponin fraction from the soapbark tree (Quillaja saponaria M.) inhibited aromatase in vitro (Golan et al., 2008). The fenugreek plant (Trigonella foenum-graecum L), widely cultivated in the Middle East and Asia, also has a high saponin content. The experiment was therefore conducted to test whether saponin fractions from Q. saponaria and from T. foenum-graecum were able to influence the sex ratio and gonad histology of Nile tilapia.

Since 2006, hormones and sub-therapeutic antibiotics have been prohibited in the European Union. However, they were the most efficient feed additives promoting growth and nutrient utilization. Saponins, naturally occurring plant metabolites, could possibly fill the role of alternative feed additives. These compounds were previously known only as anti-nutritional factors that had to be inactivated before plant meals like soybean meal could be fed to fish, but in several experiments it was shown that low levels of Quillaja saponaria (South American soap bark tree) saponins had various positive effects on common carp, Cyprinus carpio L. and Nile tilapia, Oreochromis niloticus (L). Improved growth and reduced oxygen consumption per unit protein gain have been observed in both species. In Nile tilapia, a sex ratio skewed towards a higher percentage of males, and a tendency for females to produce fewer eggs, have been found. Experiments have been conducted to evaluate the effects of fractionated saponins from fenugreek (Trigonella foenum-graecum), and from the South American soap bark tree (Quillaja saponaria), on growth, feed, and nutrient utilization, GH and IGF-1 gene expression, oxygen consumption, sex ratio, gonad development, and reproduction. Saponin fractions from fenugreek did not improve growth, feed, or nutrient utilization, or oxygen consumption significantly. In all experiments examining a single fraction, eluted with 40% methanol, produced poorer results compared to the control. In a preliminary study, the results of the effects of saponin fractions on sex ratio were encouraging but in a larger scale repetition, the observed effects could not be confirmed. No effect on sex ratio was observed during the long term experiment in Jericho, where, due to limited amounts of available fractionated saponins, commercially available Q. saponaria saponins were used. Data from these experiments do not support the application of the tested saponins and saponin fractions to promote growth, improve metabolism, or as a substitute for methyltestosterone to produce male monosex tilapia. The observed effects on growth and nutrient utilization support use of saponins or their fractions in animal nutrition. However, more research on the specific nature, and effects of saponins in fish, both at a molecular and at a macro level, is imperative.