Wow, great looking pond! Is that sand all over the bottom or clay? Hard to tell.
If you have boulders available, I would order a variety of sizes and a few loads. As mentioned, limestone would be beneficial. Then rent/hire a skid-steer to place them around the pond basin making huts, cavities, and caves (aka Love shacks). They should become excellent places to dangle a hook.
I didn't have the ability to move stones larger than 2' without a skid-steer, but of the "love shacks" I made, the fish are always around them or in them. I built one that had about a 5cu ft interior with my son that wound-up being about 12' under. I snorkel down to it and find perch in there all the time staring back at me. They love it! The FHM on the other hand used to think it was theirs, not the best home for them any more.
