With Elemenopy"s “cookbook” mention: probably little direct help here; but this thread had me looking out a small but interesting cookbook (published in the UK) which I have, on “Romanian, Bulgarian and Balkan” cuisine. Includes some seventy recipes (a lot of them seeming very enticing): many identified as Romanian or Bulgarian, most others implied as “generic” to the Balkan peninsula – a few identified as from particular parts of the former Yugoslavia (as PPs here figure, seemingly a strong “family” likeness among foods from various Balkan lands).
The only recipe in the book specifically referred to as Serbian, is a rather splendid-sounding meat loaf involving beef, pork, and bacon; though other things mentioned in this thread, appear too: the small grilled skinless sausages, and the eggplant [aubergine] / pepper spread, “ajvar”, though not there referred to by that name.
Re the river fish element: my cookbook as referred to, has a couple of recipes (exact country unattributed) for carp dishes – one of which I’ve tried making, and reckoned good. I feel that there’s a tendency cuisine-wise in some parts of the world, to underrate freshwater fish – dismissing it as lacking in taste / tending to a “muddy” taste, and beset with small bones. There are ways of coping with these traits: assorted recipes can make carp, for example, delicious. (The people of my own surrounded-by-the-sea country tend nonetheless, to conservative tastes overall as regards creatures living in water; and in the main, unfairly dismiss and despise freshwater fish.)