But within that portal will be a nested portal.
Sorry but nested portals are not possible in FileMaker. You'll need to figure out a different layout design in place of that.
On possible approach is to set up a list view layout based on Nets with fields from Lake placed in the header. Then a portal in the body can show the catch for each net.
In place of different tabs for each "outer portal" you'd have a different list view layout based on that portal. In some fileMaker databases, developers have been able to place buttons in the header of such layout that look like file tabs but that actually change layouts--creating the illusion that the user is interacting with a tab control when they are not actually doing so.