The USGS gages in Hackberry show that the water temp has already dropped to 46 deg F, that the lake level is down two feet, and the salinity is low at 12 ppt.
There is very little risk under these conditions of opening the gates. Water will most likely flow out of the marsh and any backflow (into the marsh) is relatively low salinity. Since the temperature has already dropped to 46 deg F, there is a good chance of a fish kill, and strong motivation to open the gates.
See:
http://waterdata.usgs.gov/usa/nwis/uv?08017095