Slide 1
Two waters pose opposite problems
A salmon moving from river to sea encounters a large change in salt concentration. Its body must regulate water and ions—charged particles in dissolved salts.
More to Read
In freshwater, water tends to enter the fish while salts tend to leave it. In seawater, the higher salt concentration reverses the fish's main water-balance problem.
Slide 2
Preparing before the sea
Young salmon undergo smoltification: changes that prepare them for life at sea. This transition affects how their bodies work, as well as their appearance and behaviour.
More to Read
Hormonal signals help coordinate changes in organs including the gills. Readiness is therefore a developing state, not simply a fish deciding to swim into saltier water one morning.
Slide 3
Gills do more than breathe
Specialized gill cells help move salts between the fish and surrounding water. Their activity changes as salmon adapt to marine conditions.
More to Read
The kidney and intestine also help control water balance. The gills are important, but they do not manage the transition alone.
Slide 4
An estuary offers a transition
Some young salmon spend time in estuaries before entering the ocean. They can feed while adjusting to changing water conditions.
More to Read
Residence time varies among species and populations. The estuary is not a mandatory waiting room with one fixed duration, but a habitat whose opportunities can affect the migration.
Slide 5
Migration needs a working body
A connected river route matters only if the fish can survive the conditions along it. Physiological preparation and habitat access work together.
More to Read
Temperature, water quality and obstacles can affect the journey. Explaining migration only as a map of distance misses the bodily changes that make movement between environments possible.
Sources
- Salmon Life Cycle and Seasonal Fishery Planning
NOAA Fisheries
- Osmoregulatory actions of growth hormone in salmonids
US Geological Survey