Which osmoregulatory challenge increases most dramatically when a freshwater fish migrates to saltwater?

💡 Explanation

When migrating to saltwater, a freshwater fish experiences a hypertonic environment that forces water out of its body through osmosis because the external salt concentration pulls water across the membrane. Therefore, water inflow represents a major challenge, rather than ion uptake/excretion adjustments which become secondary after osmotic pressure is dealt with.

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