When we were kids my dad showed us how to catch leaches. Tank an empty coffee can one end removed and the plastic lid in place. Use a punch and from the outside of the can put a bunch of holes similar to the size of the leach or smaller all around the can. Punch a hole to the very bottom and attach a string. Remove the plastic lid an put a couple pieces of bloody meat. put the lid back on. With the string in hand sling the can into the water and attach the string to the shore. Then wait say 24hrs. Retrieve can and look inside for leeches. This will tell you if they are blood suckers or not.