How salmon find their river

Atlantic salmon lay their eggs in gravel in clean, flowing rivers. The young fish, called fry and then parr, live in the river for one or more years. They then turn silver, become smolts, and move down to the sea.

At sea they feed and grow, often for more than a year. The ocean stage is the rich one. The river is the nursery and, later, the spawning ground.

From gravel to sea

When they are ready to spawn, adults move back towards the coast and into rivers. Experiments that blocked the nostrils of fish have shown how important smell is. Each river has a chemical signature from its rocks, soils, and plants, and the fish learned that signature as juveniles.

Once they are in the right river they still have to swim upstream, often against a strong flow, and sometimes up falls. They do not eat much on this final journey. The body they built at sea fuels the climb.

The trip upstream

The female cuts a nest in the gravel, called a redd, with her tail. She lays eggs, the male fertilises them, and both adults cover them. Many Atlantic salmon die after spawning. Some survive and may return again.

Dams, pollution, and warm, low rivers all interrupt this route. A fish pass is a channel or series of pools that lets migrants get past a weir. The biology of the return only works if the river is still reachable.

From gravel to sea

Many salmon grow up in a river, live at sea, and come back to fresh water to spawn. Smell is a large part of the return. Atlantic salmon lay their eggs in gravel in clean, flowing rivers. The young fish, called fry and then parr, live in the river for one or more years.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How salmon find their river needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

The trip upstream

The young fish, called fry and then parr, live in the river for one or more years. They then turn silver, become smolts, and move down to the sea.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

They then turn silver, become smolts, and move down to the sea. At sea they feed and grow, often for more than a year.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

At sea they feed and grow, often for more than a year. The ocean stage is the rich one.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

The ocean stage is the rich one. The river is the nursery and, later, the spawning ground.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

The river is the nursery and, later, the spawning ground. When they are ready to spawn, adults move back towards the coast and into rivers.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

When they are ready to spawn, adults move back towards the coast and into rivers. Experiments that blocked the nostrils of fish have shown how important smell is.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

Experiments that blocked the nostrils of fish have shown how important smell is. Each river has a chemical signature from its rocks, soils, and plants, and the fish learned that signature as juveniles.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.

What has to come first

Each river has a chemical signature from its rocks, soils, and plants, and the fish learned that signature as juveniles. Once they are in the right river they still have to swim upstream, often against a strong flow, and sometimes up falls.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How salmon find their river needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.