Nice thought, but I believe a hygrometer would be too insensitive to detect a useful value for you. Admittedly, I have never understood why some people add any salt to ponds. However, I assume that the added concentration would be far less than ocean levels.

Normal saline which is essentially ocean concentration is 0.9% NaCl. That is 0.9 grams in 100 ml of water. That ocean concentration has a sp. gr. of 1.0046. If, for example, you were to add one tenth ocean salt concentration, that is 0.09 g/100ml, I believe the change from pure water would be from 1.0000 to 1.0005. That would not even be on the scale of a hygrometer. Twice that conc would be 1.001 or barely readable. You would do much better by math. Now, if you have a friend who works in a clinical lab like I used to do, you could get an accurate sodium reading if your friend was willing to slip a sample in. Not very likely I suspect.

Do you want to do this because your pond is such an irregular shape and depth that it makes it difficult to get exact gallons?

yes i have an hour glass shape pond.what i was thinking if i added 160 lbs. of salt and it brought the sp. gr. to 1.003,how many gal of water would there be.

Check out the Marine section of your LFS or online. You can buy Hydrometers that you drop water on then it's like a spy glass you look through and it tells you the Specific gravity. They work very well with low levels too. I think there about ã50 to buy but are very accurate.

i found a formula and will try it.

Vol. of water=120 x lbs of salt /salinity
i have a hydrometer and will let you know how i make out.