Normally it is not.
When using a knife to cut kosher food there are two requirements.
- That the knife is completely clean.
- That the knife has not been used since it was cleaned/kashered for 24 hours.
Normally, a person might want to argue that the Fish is cold and so none of these are really issues. However, Sushi is normally NOT halachically cold.
There are two types of "heat" when dealing with kashrut.
- Physical heat, such that your hand would not want to touch it.
- Spicy "heat", which has a strong flavor.
Under spicy "heat", the common example is an Onion. If the knife cuts an onion, and before cleaning off the knife cuts squid or cooked eel, or a number of other unkosher foods found at a place that serves Sashimi, or more likely, even cuts them at the same time, the knife is no longer kosher. In a restaurant that serves sashimi there is a good chance that Wasabi, Soy Sauce, Radish, Rice wine Vinegar, Ginger, or many other vegetables, all of which count as "hot" will have treifed up the knife less than 24 hours before your sashimi was cut.
I have heard of people who have their own knife which they have given to a sushi chef in places where kosher agencies are not around.
Edit: I see the need to quote a source here so:
Furthermore, davar charif is not limited to imparting tastes into
food; it can also impart a taste into a utensil. One scenario would be
when a mixture of meat and sharp spices is being chopped with a blade.
The sharpness of the spices combined with the pressure of the blade
will cause the meat taste to become absorbed into the chopper.
This stringency is also relevant to cutting boards. The Chochmas
Odom discusses cutting a piece of salty herring (a davar charif) on
a non-kosher plate. He states that the combination of the sharp taste
and the pressure from the blade will draw absorbed issur out from the
plate and impart it into the fish. Similarly, Rav Shlomo Kluger
writes that if an onion was cut with a fleishig knife on top of a
milchig plate, both the onion and the knife will absorb a combination
of meat and milk tastes, thus rendering them both non-kosher.