Sea trout are a fish species that migrate annually between spawning grounds in rivers and the sea. The data show the population of sea trout spawning at two different sites along the river. Two changes occurred through the years. In 1998 a fish ladder was installed allowing fish to pass over the dam, and in 2008 the dam was removed. What is the purpose of installing a fish ladder?
1) To prevent sea trout from migrating
2) To increase the population of sea trout
3) To provide a safe passage for sea trout over the dam
4) To block the migration of sea trout