You can read paper with title: Osmoregulatory physiology and rapid evolution of salinity tolerance in threespine stickleback recently introduced to fresh water, by Divino et al., 2016.

It is written there that a major site of osmoregulation is the vascularized gill epithelium, where specialized ion transporting cells called ionocytes buffer against osmotic perturbations by taking up ions from the environment (hyper-osmoregulation) or secreting excess ions from circulating plasma (hypo-osmoregulation) to compensate for diffusive ion losses in fresh water or gains in seawater (Edwards and Marshall, 2013). Detailed models of teleost branchial ionocyte function in fresh water and seawater have mapped suites of membrane-bound transporters that control the intake or expulsion of ions. The basolateral Na+/K+-ATPase pump (NKA) is required for both functions because it produces electrochemical gradients across the cell membrane that drive the coordinated work of secondary cotransporters and exchangers (Edwards and Marshall, 2013). However, NKA is often upregulated in seawater exposure because it facilitates paracellular extrusion of sodium and also powers the Na+/K+/2Cl− cotransporter (NKCC), which is involved in chloride secretion (for reviews, see McCormick, 2001; Kaneko and Hiroi, 2008).

FYI: Professor McCormick is one of top researcher about Environmental and endocrine control of osmoregulation, growth, migration, development and reproduction in teleost. You can read a lot from his paper. Good luck!.

Best regads,

Munti

Thank you very much Mrs. Munti Sarida for your answer.

I have read couple of Prof. McCormick's papers and its well explained about the salmonids osmoregulation. I think I need to read a lot of his papers because, apparently, each salmon has different capabilities to survive when dealing with an osmoregulatory challenge. Like what I found in Hasegawa et.al. paper (1987) where Chum Salmons (Oncorhynchus keta) reared in freshwater for prolonged periods still able to survive even after they directly transferred into seawater.

Thank you for your answer, I hope you have a nice day.

Sincerely,

Muchammad Nurul Fajri