Be aware that a pond dug out on the side of a hill may have porous soil that does not hold water very well. You are maybe digging a leaker hole. You dig a hole but you build a pond - TWO very different processes. Hopefully you will find a spring that can feed water into the pond to keep it full and the water flushed. Both very good things.
The bigger that you make the pond the more fish you will be able to grow in it.
Fertility. Ponds as with field dirt are able to grow just so many pounds per acre. Nature's Law. The fertility of the soil and or water then determines how much can be grown or raised. Nature's Law. Since you are planning on growing some watercress and goodies in the water these plants will absorb some of the pond dissolved fertility and cause or allow less aquatic food chain to grow to feed the food chain for the fish. These plants 'should' help keep the water clearer. Not feeding the fish pelleted food significantly lessens the amount of fish that can be grown within the pond. Similar to grazing animals on pasture. Leaves from the surrounding trees do at least 2 things: add organic nutrients and degrade water quality by mainly consuming dissolved oxygen (DO) during leaf decay. Large amounts of organic inputs may cause DO sages as the pond ages to cause DO depletion and periodic fish kills. Be aware of these serious potential problems.
From our PBoss Common Pond Q&A Archives
https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=96127#Post96127More Pond Management Info
https://fisheries.tamu.edu/aquaculture/water-quality/Lets look at how many fish pounds this small pond can realistically and naturally grow.
Pond 30x50 = 1500sqft. 0.03 ac when completely full over flowing.
Water chemistry with its alkalinity and soil nutrients are VERY important for aquatic productivity. Water Alkalinity should be above 20 ppm for good plankton development / growth to feed the food chain. For less than 20 ppm (30+ is better) and to get good plankton development for the food chain you will have to regularly add lime to the pond as farmers add lime to fields. Too many crayfish can make the water turbid - pros and cons. Main con is limiting sunlight and plankton production. Some crayfish species create tunnels which can perforate the pond banks and cause leaks or seepages.
I will use a natural medium woodland water fertility estimate of 150 lbs/ac X 0.03ac = 4.5 lbs to a maximum 5lbs of panfish per 0.03 ac. This is the total available of ALL total panfish present. Often a clear water woodland pond has very clear water, low plankton production, and the total fish productivity is less than 150lbs panfish per acre.
Normally for a sustainable fishery the harvest is said to be around 30% of the total amount present. Thus 5 lbs in 0.03 ac X 30%(0.30) = 1.5 lbs harvestable fish. This accounts for leaving in a few breeders and sub-harvest sizes for reproduction and the next harvest. You can back calculate these numbers for what size of pond it would take to achieve a harvest of 8 medium BG twice a month. Reference BG standard weights 5" = .09lb (1.44oz); 6" = 0.16lb(2.56oz); 7" = 0.27lb = 4.32oz; 8" = 0.42lb = 6.72oz.
Fertilizing the water and feeding the fish with good aeration, which are not in your plan, can increase this panfish production up to 10X - depending - numerous factors about this complex topic.
Bluegill (BG) are not the best fish IMO to achieve your harvest goals of a tiny 1500 sqft pond. All Natural management for a regular fish harvest is difficult in a tiny 0.03 ac pond. Other species or Tilapia may be a better choice for a tiny fish production pond. Numerous Pros and cons here also.
