I've hunted Sika on Chincoteague NWR in VA which basically is the southern end of Assateague. What I found was that if you're seeing whitetails you're not deep enough in the wet thick stuff. Sika get right down in the reeds and the nasty stuff. If you're not wet and muddy then you're hunting the wrong place.

Sika aren't very big so I used to just bone them out and put them in a cooler full of ice. A big stag may go 70 lbs live weight and a mature cow maybe 50. Since I always found them in the thick stuff I used a 12 gauge and buckshot since they were in stuff where the shots were moving. Kind of like jump shooting cottontails.