Manganese Requirement of Rainbow Trout Fed A Commercial-type, Plant-Based Diet

Fish meal (FM) is the primary protein source used in commercial rainbow trout diets. Demand for alternative, plant-based (PB) protein sources has increased in recent years due to growth of aquaculture production worldwide and reduced availability and high cost of FM for use in aqua feeds. However, there are limits to the use of PB sources in aqua feeds due to high levels of fiber and presence of anti-nutrient factors, such as phytate. Recent findings also show that many PB protein sources contain concentrations of essential vitamins and minerals that differ from that of traditional FM, which suggests that the levels of supplementation in feed will also differ. Current practice in formulation of trout diets is to add a mineral pre-mix to prevent deficiency of certain minerals. Levels of minerals in standard premix were determined and optimized for FM-based diets but may be insufficient for rainbow trout fed a PB diet.

A study was conducted to determine the manganese (Mn) requirement of rainbow trout fed a PB diet. Rainbow trout (22.2±1.2 g) were fed either a FM control or PB diet supplemented with various levels of Mn (0, 0.5, 1, 2, 4, 8, 16, or 32 mg/kg diet) for 12 weeks. Fish were group-weighed every 4 weeks to determine feed intake and growth performance. At the end of the study, 4 fish per tank were sampled for plasma and tissues to measure physiological indices. Trout fed diets with 2, 4, and 8 mg/kg Mn had significantly greater weight gain compared to the other Mn-supplemented diets and similar to the FM diet. Weight gain declined significantly as supplemental Mn increased beyond 16 mg/kg. Several non-linear models were fit to the weight gain data - a rational model (R² = 0.84) suggests that approximately 5 mg/kg supplemented Mn to be optimal for weight gain with this PB diet formulation. Feed conversion (FCR) showed a similar but inverse pattern with Mn-supplementation in diets. Other physiological performance measures will also be discussed.