Views: Only show 3 largest values and their sum per user
If you don't want to build a custom module I would think Rules is your best bet. Also install Conditional Rules as what you will be doing will requires logic a bit difficult to accomplish with Rules alone. 

I will create a Result content type that will hold the aggregated result for the day per angler, as well as entity references to the fish content type. Then, after the angler uploaded a new entry, the rule will cycle through the related fish types and do the Maths and store the result in the result field.