Ok so my dad has had his pond for almost 10 years now and it was dug out at 18 feet. Well my cousin brought his depth finder and said he finder reads at 13 feet. Do you guys think it’s time to dig the silt out of the pond? And if it is what is the best way to do it? Drain the pond or could you do it without draining the pond? Also the pond is about 2.5 surface acres of water.

Is keel off-set programed into depthfinder?? "most" units when calibrated are between 2 and 3 ft off on the shallow end OR... the deep end. I have done some testing for Navico and it is nearly impossible to be exact unless you use a rope to verify your depth and set Manual off-set accordingly.
I'd verify with a rope unless you have someone that knows how to calibrate a transducer, or do both at the same time.

bass67, it is often much cheaper to raise the dam rather than dig out. Don't know if that is possible in your case, since it depends upon topography, but I do know that moving mud is very expensive.

I'm sure no expert but 5 feet of muck in 10 years seems extreme to me. I'd guess the reading is off as said above. If normal ponds filled with much that muck that quick, the 8 foot deep ponds dug around here would only be 2 feet deep in 3 years but many are decades old.

Maybe it wasn't really 18 feet deep to begin with. Maybe they didn't subtract the height of the transit from the rod.

Your numbers are likely off.
I got a good idea of the muck depth in my pond by probing the bottom. I put together a length 1" PVC and lowered down till I felt resistance, then leaned into it, till I felt solid ground.
Not high tech, but it seemed to work repeatably.

The best way to reduce the muck is aeration. It keeps the good bacteria that digest muck alive. A good way to boost the process is adding Muck digesting pellets. This is a better way to go than digging out the pond.

Could you give me more information about those muck pellets. Like how much per surface acre of water i would need and also were to get it from?