How to cook? Laj is a good thing. So, too, can carps be roasted and the heads make great soups.
The blood veins are where the mercury gets stuck at. Eat them and you'll have extra mercury flavor.

You can remove the veins by slicing them off.
I went to a meeting with some elders. They were bragging about such big carps they caught and were now
serving whole roasted on the table. No vein was removed. I knew it wasn't good, but I kept my mouth shut
just so I wouldn't be ruining a meal.