Hypoxia, but not an electrolyte-imbalanced diet, reduces feed intake, growth and oxygen consumption in rainbow trout (Oncorhynchus mykiss)

Abstract

Oxygen limitation and dietary imbalances are key aspects influencing feed intake (FI) and growth performance in cultured fish. This study investigated the combined effects of hypoxia and dietary electrolyte balance on the growth performance, body composition and nutrient utilization in a rainbow trout (Oncorhynchus mykiss) isogenic line. Fish were fed ad libitum two experimental diets: electrolyte-balanced or -imbalanced diets (DEB 200 or 700 mEq kg^-1, respectively) and exposed to normoxia or hypoxia (7.9 or 4.5 mg O₂ l^-1, respectively) for 42 days. DEB did not affect FI, growth performance or body composition. Nevertheless, hypoxia had a negative impact, reducing FI (6%), growth rate (8%), oxygen consumption (19%), energy (5%) and lipid (42%) contents. Digestible energy intake and heat production were higher in normoxic fish (40% and 23%, respectively), retaining 64% more energy in lipid or protein. Hypoxia reduced the apparent digestibility of dry matter, ash, protein, lipid, carbohydrates and energy. Trout fed DEB 700 diet were energetically less efficient, reflected in higher heat production and energy requirements for maintenance. FI was inhibited by low dissolved oxygen levels, but not by electrolyte-imbalanced diet, in spite of the higher energy requirements for maintenance. This study highlights the importance that dietary-electrolyte content and DO levels have on energy balance and growth performance when fish are fed to satiation.

Figure 1

Effects of dietary electrolyte balance (DEB) and dissolved oxygen levels on (A) feed intake expressed per unit of metabolic body weight (FI_MBW); (B) growth rate expressed in metabolic body weight (GR_MBW); (C) hepatosomatic index (HSI); (D) feed conversion ratio (FCR); (E) oxygen consumption (OC) and (F) oxygen consumption to growth rate (OC/GR_MBW) of rainbow trout. Further details are provided in Methods. Values are mean ± SEM (n = 3 tanks, except for HSI where n = 9 fish). Different letters indicate differences among treatments (α = 0.050).

Figure 2

Effects of dietary electrolyte balance (DEB) and dissolved oxygen levels on (A) hematocrit (HCT); (B) haemoglobin (Hb); (C) mean corpuscular haemoglobin concentration (MCHC); (D) white-blood cell volume (WBCV) and (E) spleno-somatic index (SSI) of rainbow trout 2 or 6 h after feeding. Further details are provided in Methods. Values are mean ± SEM (n = 9). Different letters indicate differences among treatments, while the asterisk indicates differences between sampling times (α = 0.050).

Figure 3

Effects of dietary electrolyte balance (DEB) and dissolved oxygen levels on (A) heart blood pH; (B) caudal blood pH; (C) chyme pH, and (D) dry matter content in the stomach of rainbow trout 2 or 6 h after feeding. Further details are provided in Methods. Values are mean ± SEM (n = 9). Different letters indicate differences among treatments, while the asterisk indicates differences in sampling times (α = 0.050).