Earlier quoted context omitted.
Humans mostly synthesise Vitamin D. Plus foods like eggs contain them and milk in a lot of countries is fortified with vitamin d. Seafood is not the only source of that essential vitamin.
Vitamin D3 is not synthesised properly for many leading to several bone related issues, I'm one of them. In natural sources it is found only in animal products especially in fish.
One can consider a supplement. It's not strictly "natural", but cooking animal products instead of eating them raw isn't exactly "natural" either..