Once a fish, or more likely two, are fired by a sub against target , preferably to attack the target from different directions, at least one wire-guided depending upon the range of the target, the target has few options.The most common are noise-makers; acoustic decoys that simulate as accurately as possible the signature of the vessel.
Next are depth charges to explode at set depths.Our old Nilgiri frigates had a twin-barelled Bofors mortar forward of the 4.5" main turret.Sov. and Ru warships have the MBUs .They have varied rounds as mentioned above and have a fighting chance of destroying or sending a torpedo off course by a series of violent explosions underwater throwing the incoming torpedo off course thus protecting the vessel.
But what do you do with wake-homers? Acoustic decoys could fool the fish, but as said with the new fish under dev. which make repeated attacks, only a hard- kill solution suffices.Here too the MBU has its value.The latest development is that of mini-torpedoes which can detect and attack incoming fish. But they require both integral sonar seekers to detect an incoming projectile, as well as input from the warship if poss.,whose sonars would be far more powerful .This would necessitate communications between the warship and mini-torpedoes, the mini- torpedo best requiring wire-guidance.
In the case of an incoming being a 200kmh Ru Shkval short range shot, the best defence is to hide under a table, put your head between your legs and kiss your ar*e goodbye!
PS: The reason I prefer the MBUs forward is because the higher deck level closer to the superstructure/bridge ,allows for lengthier missiles to be accommodated at that location which would be the old "B" and "C" turrets in the WW2 era.Thus both BMos and B-8 Can be packed in in larger numbers.Given the cost and size of B-8s, there is a need for a smaller QR SAM with a range of around 20km as the last ditch missile defence, which can also be fired in multiple rounds.Final defence,gatlings.With more supersonic /anti- ship missiles being fielded each year, both anti-torpedo and anti-torpedo missile defences have to have multiple defensive suites.