Do you have fish in the pond as well? Maybe if you did they will attact the leeches? Here something from another website:

Fill a coffee can (with a plastic lid) with about 1/4 cup of raw meat (ground beef, liver, chicken or turkey giblets).

Poke many 1/8 to 1/4 inch holes in the side of can. The nail holes should leave a sharp burr on the inside of the can.

Put the lid on the can and submerge it completely in your pond and placed a rock on top of the can to prevent it from falling over.

Check the trap a couple of times a week and remove any leeches. DO this until the leech numbers decrease.