In my opinion a frozen one may be the same, as long as is not a commercial fish.

It will have much more water as ice, also for typical mistakes on freeze chain, deliverie,  storage, etc

You may want to froze fresh fish on your own quickly by using nitrogen.

Hope it helps

For the accuracy, you should calculate the moisture content of a sample based on fresh sample. Fish will loss its moisture during frozen depending on how long the frozen sample is stored before it is analyzed.

First thaw the fish sample to room temperature and then go for DM estimation