Effects of a dairy–yeast prebiotic and water hardness on the growth performance, mineral composition and gut microflora of fathead minnow (Pimephales promelas) in recirculating systems

We conducted concurrent 8-week feeding trials with fathead minnow (FHM) (Pimephales promelas) fed diets with 0 or 2% dairy–yeast prebiotic in recirculating systems with low (LOW, 35 mg CaCO3 per L water) or moderate (MOD, 141 mg CaCO3 per L water) hardness to determine the effects of the prebiotic and water hardness on general performance, whole-body ash and mineral concentrations, and composition of the gastrointestinal (GI) microflora. Twenty-five fish with mean (± SE) individual weights of 0.34 ± 0.002 g were stocked into each of 4 replicate 110-L tanks per diet per system. Fish were fed to satiation twice daily on weekdays and once daily on weekends. Average individual weight gain (0.89–0.91 g) was higher and feed conversion (2.6–3.1 g) was lower in fish in the MOD system, than weight gain (0.54–0.55 g) and FCR (4.1–4.6) of FHM in the LOW system. Feed conversion was also lower in fish fed the prebiotic diet (2.6–4.1) than the basal diet (3.1–4.6). Survival of FHM (94–97%) was similar among treatments. Whole-body ash was higher in fish from the MOD system (7.9–8.4%) than the LOW system (6.7–7.7%) regardless of diet. Potassium was higher in FHM from the MOD system (9.78–10.07 g per kg diet) than the LOW system (9.45–9.52 g per kg diet), while sodium, iron, zinc and copper were higher in fish from the LOW system. Sodium was lower in FHM fed the prebiotic diet (2.39–2.73 g per kg diet) than the basal diet (2.42–3.42 g per kg diet). The GI microflora contained 626 different species. The Shannon–Wiener (S–W) index of diversity for GI microflora (genus level) was higher in FHM from the MOD system (3.29) than the LOW (2.12) system, but the S–W indices were similar for the basal (2.95) and PREB (2.45) diets. Acinetobacter was more abundant in the microflora of FHM fed the prebiotic diet than the basal diet. The mineral composition of the water had the strongest influence on FHM performance and the GI microflora, and the prebiotic did not increase mineral concentrations in fish fed diets with complete mineral mixes in the LOW or MOD system.