The influence of the dietary inclusion of the alkaloid gramine, on rainbow trout (Oncorhynchus mykiss) growth, feed utilisation and gastrointestinal histology

This study examined the influence of the alkaloid gramine, when included in diets for rainbow trout. Quinolizidine alkaloids have been suggested as a potential anti-nutritional problem with the use of lupin (Lupinus sp.) meals in aquaculture diets. The findings from the present study show that above a critical threshold, the alkaloid gramine does have a strong anti-palatability effect. The effect is noted at a minimum gramine concentration of 500 mg/kg of diet, though not at 100 mg/kg. A continuing strong anti-palatability response is noted at higher inclusion levels and at the highest gramine inclusion concentration examined in this study (10,000 mg/kg), insufficient feed was consumed to even supply maintenance protein and energy demands. No adaptation to concentrations of gramine was observed throughout the 6-week study. No effects on nitrogen, energy or phosphorus digestibility were seen at the 500 mg/kg inclusion concentration of gramine relative to the reference diet, although the inclusion of the yellow lupin kernel meals (both Wodjil and Teo varieties) in the diet did improve the digestibility of phosphorus. Growth, as assessed using a range of parameters including weight gain, growth rate, nutrient and energy retention of fish fed the experiment treatments was largely consistent with feed intake. Survival of fish was significantly reduced at gramine inclusion levels above 1000 mg/kg. Feed conversion ratio (FCR) and feed conversion efficiency (FCE) were also reflective of feed intake and growth levels observed of each treatment. The concentrations of the plasma thyroid hormones tri-iodothyronine (T3) and thyroxine (T4) of fish from each of the treatments were consistent with feed intake (including the controls) suggesting that the concentrations of these hormones are in response to feed intake, not specifically the gramine levels in the diets. However, the inclusion of the Lupinus luteus kernel meals resulted in a significant change in T4 levels, with a degree of independence of the feed intake, suggesting that there may be another mechanism by which these meals are influencing the concentrations of this hormone. In this study, there was an increase in the density of melano-macrophage centres (MMC) with high dietary levels of gramine. However, in the absence of any histological evidence for a toxic effect, it is likely that the increased MMC densities observed in the fish fed high concentrations of gramine are associated with starvation. This study demonstrated that the lupin alkaloid gramine, can have a strong anti-nutritional effect on fish at inclusion concentrations greater than 100 mg/kg, but that its mode of action is primarily through an anti-palatability effect. It is therefore considered unlikely that alkaloid effects would be observed in diets even with 50% inclusion of kernel meals from Australian commercial L. luteus varieties