Interaction between dietary calcium supplementation and chronic waterborne zinc exposure in juvenile rainbow trout, Oncorhynchus mykiss

This study investigated the effects of dietary Ca²⁺ on branchial Ca²⁺ and Zn²⁺ uptake, new and total zinc accumulation in target tissues (gill, liver and kidney), calcium and zinc homeostasis, and acute tolerance to waterborne zinc in fish chronically exposed to waterborne zinc. Juvenile rainbow trout (Oncorhynchus mykiss) were maintained on a calcium-enriched diet [41.2 mg vs. 21.2 mg (control) calcium/g dry wt. of food] and chronic waterborne zinc exposure (2.3 μmol/L), both separately and in combination, for 28 days. Calcium-supplemented diet in the absence of waterborne zinc significantly reduced branchial Ca²⁺ and Zn²⁺ influx rates, and new and total zinc accumulations in target tissues relative to control. However it did not protect against the acute zinc challenge. In contrast, waterborne zinc exposure significantly increased branchial Ca ²⁺ and Zn²⁺ influx rates, new and total zinc concentrations in target tissues, and acute zinc tolerance relative to control. Interestingly, no such changes in any of these parameters were recorded in fish treated simultaneously with elevated dietary Ca²⁺ and waterborne zinc, except acute zinc tolerance which was highest among all the treatments. Thus, we conclude that the interactions between elevated dietary Ca²⁺ and waterborne zinc can protect freshwater fish against waterborne zinc toxicity.