A scientist made three observations: (1) in the world' s temperate zones, food is more plentiful in the ocean than it is in fresh water  (2) migratory fish in temperate zones generally mature in the ocean and spawn in fresh water  and (3) migratory fish need much nourishment as they mature but little or none during the part of their lives when they spawn  On the basis of those observations, the scientist formulated the hypothesis that food availability is a determining factor in the migration of migratory fish
Subsequently the scientist learned that in the tropics migratory fish generally mature in fresh water and spawn in the ocean
whether in the world's temperate zones, the temperatures of bodies of fresh water tend to be lower than those of the regions of the oceans into which they flow