This might be a strange question, but why do sea turtles lay eggs on land and not for example dig up holes inside the ocean? They live their whole lives in the ocean, so why do they lay eggs on land? Why travel so far just to lay eggs?
Same goes for some salmon, why do they leave the oceans and lakes, and go upstream on rivers and not lay their eggs where they live?
It is probably something to do with protecting their offspring, but it seems to me that they still have predators that hunt their offspring fairly easily where they hatch/lay their eggs, so maybe there is another reason as well?
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Good answers, but there is even more cool reasons in regards to eel migration. Eels are an old species that originated around where Indonesia is today. Because of continental drift waterways were opened westward. Some eels moved to live there (in future Europe and America) and continued to travel to “Indonesia” for spawning, but even more drift closed the gap towards the Pacific and the Atlantic eel branch was established (some in N America and some in Europe). Initially both groups had similar distance to their new spawning ground – the Sargasso sea, but as continental drift continued westward, the American eel can chill, while the European eels today have to travel 5000-6000 km to their spawning grounds. Also this separation resulted in the speciation of two distinct species.
Easily digestible [source](https://worldoceanreview.com/en/wor-5/coastal-dynamics/on-the-origin-and-demise-of-coasts/evolution-of-the-eel-a-matter-of-continental-drift/)
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