You may not want to use a portal, but you will have to in order to show a different record in each tab. Your layout can be based on sites and two portals can be placed on each tab, one for the Transect and one for the Transect Points associated with that Transect.
Portal filters can limit each portal to just one Transect record or just the Transect Points records for the tab panel's Transect.
The draw back to this approach is that you can't ever have 12 or more Transects for a site without modifying the design of your layout accordingly. Other options do not have those limits
1) you don't actually have to create the transect records at the same time that you create the site record. These can be created automatically as part of the process of recording data into a field of the transect record on a given tab.
2) see my earlier comments on portal filters that can be used to limit the portal records on each tab to those for a specific Transect associated with the current site.