Salmon's need to lay many eggs because the eggs are very fragile. These eggs are not very likely to survive and hatch. Lots of eggs need to be laid to guarantee the survival of at least some of the eggs.
The subordinate clause is "When the delivery man arrives" because 1. it is introduced by "when" and 2. we can ask question to the main clause about this one.
The answer in the past simple
They set up their own forts in governent granted lands.
