various members of the superclass nucleocytoplasmic large DNA viruses (which includes iridoviruses) are generally associated with integumentary disruption in sturgeon, including lesions. I would start with some PCR.

Woow, poor fish!!! this lessons are very extended on the body of sturgeon or only specific sites? How many animals was affected? and how old are they?. 

I am assuming these fish are being raised in the slightly saline area of the Caspian Sea. Some wet mounts of skin scraps examined under a phase-contrast microscope will help establish whether there is any bacteria in the lesions. Scrapes can also be plated if you have the facilities to do so. Potassium permanganate, copper sulfate and methylene blue or formalin baths are general external treatments for small fish but generally the bacterial or fungal infections I would be suspicious of occur in freshwater and don't necessarily do well in saline water.  Bath treatments of large sturgeon in net pens or ponds is not practicable. Thus, reduction of stressors (such as crowding and handling) and improvement of water quality are options which may reduce the incidence of these lesions. Medicated feed is also an option but some sort of preliminary diagnosis of the causative agent would be prudent.  We do see similar looking lesions occasionally in large white sturgeon raised here in freshwater but these are typically due to rough handling while being transported and exacerbated by poor water quality and crowding stress.  These large fish typically recover from these perturbations without intervention of medicated feed.

The white sturgeon iridiovirus (WSIV) present here in North America affects fry and fingerlings and appears refractory in adults for the most part.