In general a filet of fish needs to have visible scales to prove its kashrus unless there is a reliable hechsher. However salmon is unique in its pink coloring, and no non-Kosher fish looks the same. Experts at the OU have researched all the possibilities that non-Kosher fish could be dyed etc, and so far have found no risk of fraud. If the salmon has been cooked or has other ingredients added, it certainly would require a recognized hechsher.
Meat or chicken should not be cooked in the same oven as fish simultaneously, unless one of them is covered. If one mistakenly did cook them together the Rema rules it may be eaten.