Salem

Which is the best thing to put on shellfish traps to catch these things

1) Oysters
2) Crawdad
3) Crab

?

(4: Maybe there is some thing that will catch these 3 above +- in a equal proportion?)

And how many of them can I expect filling the 15 slots of the shellfish trap with the food in question?

I have the shellfish traps closely together on a lake

That's a complicated question . You'll need a lot of testing to find it , and from my experience there's a lot of RNG involved ; and we don't even know if location and food are linked : if shellfish likes the same thing everywhere ; or everytime (because fish/shellfish used to "move") ; or everywhere AND everytime.

I have been using pinecone scales lately. It seems to me they like them more than meat shreds.

Hm... I'll try with pinecone scales. The farmed stuff I put there did not bring good results too

The white cabbage leafs (with 12% purity) worked better than anything else on my lake.

Mayhaps it's also influenced by biome?

I may easily be wrong but I don't think the game differentiates between water in different biomes. The biome is "water"

salt water for lobsters, does the yellow-sand lakes count as salt water?