DIETARY BENTONITE (A NATURAL AND LOW-COST CLAY) SUPPLEMENTATION MITIGATE THE IRON TOXICITY IN RAINBOW TROUT

Aquaculture is the fastest growing food-producing sector and contributes over half of the total food fish consumed globally. United States is the eighth-largest producer of rainbow trout (Oncorhynchus mykiss) worldwide, and Idaho is the top producer among all states. However, in Idaho the water source for fish culture normally acquired from ground water aquifers drilled in basalt, which contains high level of iron. Although, iron is an important chemical element required by fish for several metabolic processes, elevated level of iron in fish culture units can cause growth retardation and damages to vital organ. Currently, there is no promising therapeutic agent that can efficiently mitigate this water borne issues in trout farming. Hence, finding an effective and safe compound to prevent iron build-up as well as ameliorating their toxicity on fish is a demand in the aquaculture industry. Bentonite, a natural clay with superior adsorption properties, has been suggested as a good chelating agent for different (heavy) metals including iron. Therefore, the current study was designed to evaluate the dietary effect of bentonite clay in rainbow trout following exposure to elevated iron containing diet.

Six experimental diets were formulated to contain  0% bentonite (D1: CWF/CF), 2% bentonite (D2; CLB), 4% bentonite (D3; CHB), 0.25% iron as FeSO₄ (D4; CFe), 0.25% FeSO₄ +2% bentonite (D5; CFeLB), and 0.25% FeSO₄ + 4% bentonite (D6; CFeHB). In this feeding trial D1 diet was fed to 2 groups of fish viz., group 1 without filter (CWF) and group 2 with filter (CF). Experimental system was used as flow through system and natural ground water contains 3 - 5 ppm iron therefore water filer was used for group 2 (CF) to remove the iron from natural ground water.  Additional 6 groups tanks were containing 3-5 ppm iron in the water throughout the feeding trial. Each diet was fed to four replicate groups of rainbow trout (2.3±0.01 g initial weight) stocked with 20 fish per tank. Fish were hand fed to apparent satiation twice daily for eight weeks.

At the end of the feeding trial, the highest weight gain (%, Fig. 1) and specific growth rate was found in fish fed control diet -reared in filter water (CF) but no significant difference was noticed in the feed conversion ratio among the various groups. Interestingly, weight gain (%) in both CFeLB and CFeHB was significantly higher than the CWF and CFe. No significant variation was noticed among the fish fed either low or high bentonite with iron and those fed with bentonite alone. Based on the growth performance indices, it can be concluded that dietary supplementation of bentonite at a dose of 2% can enhance the growth of rainbow trout when challenged with high dietary iron.