Dietary electrolyte balance of Atlantic salmon (Salmo salar) freshwater feeds: Impact on osmoregulation, mineral metabolism and performance in seawater

Antony J Prabhu Philip,Per Gunnar Fjelldal,Sofie C Remø,Chandrasekar Selvam,Kristin Hamre,Marit Espe,Elisabeth Holen,Kaja H Skjærven,Vibeke Vikså,Saravanan Subramanian,Johan W Schrama,Nini H Sissener

doi:10.1016/j.aquaculture.2021.737305

Antony J Prabhu Philip, Per Gunnar Fjelldal + Show 10 more

Open Access

https://doi.org/10.1016/j.aquaculture.2021.737305

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Abstract

Dietary electrolyte balance is the equilibrium of monovalent cations and anions that influence the acid-base balance of the feed (dEB = Na + K − Cl, mEq kg−1). Dietary electrolytes/minerals can influence the physiological changes during smoltification in Atlantic salmon. In this context, we aimed to study if the dEB of the freshwater feeds can be used to pre-adapt the hypoosmotic functionality and the associated effects on mineral metabolism. The dEB of commercial freshwater Atlantic salmon feeds in Norway varied from −9 to 400 mEq kg−1 feed. Three experimental feeds were formulated to study incremental levels of dEB reflecting the low (L-dEB, −50 to 0), median (M-dEB, 200–250) and high (H-dEB, 350–400). Triplicate groups of Atlantic salmon parr (36 g) were fed one of the three feeds for 8 weeks in freshwater at 12 °C. The fish were transferred to full strength seawater in indoor tanks and fed a commercial diet for 6 weeks. Growth was not differentially affected by dEB levels, neither in the freshwater phase nor in the seawater. Plasma electrolytes (Na+ and Cl−) and gill mRNA expression of sodium potassium ATPase (NKA a1b, seawater isoform) were significantly lower in L-dEB fed fish. In the intestine, carbonate precipitates 24 h after seawater transfer was higher in fish fed both L-dEB and H-dEB feeds compared to the M-dEB fed fish. Whole body and plasma mineral levels were significantly affected by dEB levels in freshwater feeds. Interestingly, the carryover effect of dEB in freshwater feeds was significant after 6 weeks in seawater for plasma and whole-body Zn status, with the H-dEB fed fish showing significantly increased body Zn status compared to L-dEB and M-dEB fed fish. The study revealed that mineral metabolism and intestinal response to seawater transfer can be pre-adapted by modulating the electrolyte and/or mineral balance in freshwater feeds in Atlantic salmon. Further, dEB did not affect long term development of cataract or vertebral deformities.

Highlights

Atlantic salmon has an anadromous life cycle migrating between freshwater and marine environments
Given the increasing evi dence on the osmoregulatory importance of mineral precipitation in the intestine of marine teleosts (Perry et al, 2011; Salter et al, 2018), and in Atlantic salmon (McKay et al, 2020), its dietary regulation is of signif icance in Atlantic salmon aquaculture
Our preliminary data from the Norwegian fish feed surveillance program suggested that commercial Atlantic salmon feeds in Norway might have a large variation in dEB (Sissener et al, 2013)

Summary

Introduction

Atlantic salmon has an anadromous life cycle migrating between freshwater and marine environments. The smoltification process is under the regulation of various endocrine and environmental factors (Zaugg, 1982; Brauer, 1982; McBride et al, 1982). Dietary min erals have a role to play in the development of hypoosmotic ability of fish moving to seawater (Zaugg, 1982). Little is known about the impact of dietary salt induced regulation of osmoregulatory responses in the intestine. Given the increasing evi dence on the osmoregulatory importance of mineral precipitation in the intestine of marine teleosts (Perry et al, 2011; Salter et al, 2018), and in Atlantic salmon (McKay et al, 2020), its dietary regulation is of signif icance in Atlantic salmon aquaculture

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