DIETARY METHIONINE REQUIREMENT OF LARGEMOUTH BASS Micropterus salmoides L.

The use of alternative protein sources in aquafeed formulations increased faster than amino acid requirements have been defined for most cultured fish species. Methionine can be the first limiting amino acid in alternative-protein based diets and its deficiency leads to reduced fish performance and increased nutrient load into receiving waters per unit mass of fish produced. Production of largemouth bass (LMB) for food-fish markets has increased but information on amino acid requirements for this species remains limited. Thus, we conducted a 9-week feeding trial to assess the dietary methionine requirement of LMB.

A semi-purified basal (Basal) diet was formulated to contain 40% crude protein (CP), 12% lipid, 0.5 g/100 L-methionine (Met) and 0.23 g/100g L-cysteine (Cys). Crystalline Met was supplemented to the Basal diet in additive amounts of 0.15 g/100g originating six additional diets with Met ranging from 0.65 to 1.4 g/100g. Each diet was fed twice daily to apparent satiation to triplicate groups of 20 LMB juveniles (5.9 g/fish initial weight) stocked in 110-L glass aquaria operating as a recirculating system. Water quality was maintained within acceptable ranges for LMB and a 12h photoperiod was maintained using fluorescent lights controlled by timers. Fish in each aquarium were group-weighed at 5 and 9 weeks of feeding.

Survival of LMB fed the Basal diet was 83.3% and that of fish fed the other diets ranged from 90 to 98.3%. A four-parameter saturation kinetics model (SKM) was used to fit the resulting data from the feeding trial and the dietary Met concentration producing 95% of maximum theoretical response was depicted as the requirement. In the presence of 0.23 g/100g Cys, the dietary Met requirement for 95% of maximum weight gain and FE of LMB was estimated to be 0.77 and 0.74 g/100g (Fig. 1), corresponding to 1.83 and 1.76 g/100 of dietary CP, respectively. The dietary Met requirement value determined in this study will aid in the formulation of nutritionally-balanced and cost-effective diets for LMB.