Strain the fish from the pan - in this instance the liquid in the pan should be considered the braising broth, its purpose was to make the fish taste wonderful and is now largely redundant. However, a couple spoonfuls over the fish, watercress and rice or noodles will provide a good sauce and the seasoning. Taste the sauce, but be aware that our palates are not always used to the sweet and salty intensity of Japanese sauces. It is likely to taste quite strong so further reduction will increase this effect. Make a judgement; reduce, leave as it is, dilute with chicken stock, white wine or water.