Fish that go in schools are seldom born in rivers; they are raised in the lakes, and this is how they behave: when the desire of spawning comes on them, they swim out to sea in schools, the males leading, and throwing out their milt, while the females come after and swallow and conceive from it.

When the females have grown heavy in the sea, then all the fish swim back to their own haunts. But the same no longer lead; now the leadership goes to the females. They go before in a school as the males had, and now and then throw off some of their eggs (which are like millet-seeds), which the males devour as they follow. These millet-seeds, or eggs, are fish.

The fish that are reared come from the eggs that survive and are not devoured. Those fish that are caught while swimming seawards show bruises on the left side of their heads; those that are caught returning, on the right side.

This happens because they keep close to the left bank as they swim seawards, and keep to the same bank also on their return, grazing it and keeping in contact with it as well as they can, I suppose lest the current make them miss their way.

When the Nahr an- Nil [31.1,30.166] (river), AfricaNile begins to rise, hollow and marshy places near the river are the first to begin to fill, the water trickling through from the river, and as soon as they are flooded, they are suddenly full of little fishes.