Effective enough that we've been known to park our Ula-class submarines under Russian Udaloy-class ASW destroyers and follow them for days. Effective enough that during transit to Syria, the Russian flotilla was followed without contact or intercept from Russian navy attack submarines. Effective enough that we'd have put Piotr Velikij and Kuznetsov on the ocean floor had war broke out, and that's with naval escorts and ASW helos flying during transit.
Effective enough that during blue-on-blue naval engagements during NATO exercises our Ula-class submarines were told to sit out drills because ASW ships and defenders simply couldn't find them.
ASW ships work, but they work best in a unified battle-group, not on their own. Unfortunately naval tactics tend to put them on the outside of a battle-group as ASW pickets where they deploy their towed sonar and weave vigorously back and forth trying to locate a contact in a wide swath. By themselves they're as vulnerable as undefended merchant ships.
Our Nansen class frigates were build as ASW warships. It's why their air-defence and surface warfare suites are so limited, despite the high visibility of the Spy-1F mast. They have massive bow sonars, carry torpedoes and depth bombs and AUVs, towed sonars and ASW helicopters.
They work in conjunction with Norwegian Navy minehunters and sweepers who specialize in finding low signatures submerged contacts with AUVs and other advanced underwaters sensors, and alongside Norwegian Air Force P-3C/Ns and soon P-8Ns. Underwater contacts, mines or submarines are routinely found and we've a lot of experience, experience stretching back decades, in locating and tracking Russian submarines using ASW ships, aircraft and sensors.
Two WWII-era naval mines were found during sonar sweeps during this years Trident Juncture exercises by Norwegian Navy minesweepers and hunters.
During wartime our ASW ships would end up like this:
That's Ula-class submarine KNM Utsira taking a kill-shot against a Nansen class ASW frigate during exercise Joint Viking.
Comparitively, MPA are more effective once a contact has been found as their sensors work best for tracking, not locating a contact. Their MADs register an unholy amount of false positives from natural magnetic sources and they're of limited use against non-magnetic submarines like the German Type 212. Hydrocarbon sensors can find AIP submarines, even those with recycling systems that limit hydrocarbon production and dropped sonobuoys remain the most effective means of finding or tracking a contact. MPAs can also cover a much larger area of space due to their increase speed over ASW ships while their sensors are just as capable of tracking objects over vast distances. They're also much less noticeable for submarines versus ships which produce thermal, acoustic and magnetic signatures and noisy wakes. Due to noise being difficult to transfer from air to water, a loitering ASW helicopter or MPA isn't going to be noticeable for a submarine until they drop a buoy or torpedo, and by then it's simply too late.
Here's a SONAR sweep on a Norwegian P-3C during operations off the coast of Somalia.
Sensors like Synthetic Aperture Sonars further improve the effectiveness of ASW vessels and AUVs. Kongsberg, who I work for in the Sub-Surface Warfare division, makes such sensors that offer superb clarity in locating underwater objects. Enough clarity that I can tell a Yasen from and Akula just by looking at it, no acoustic or other metrics needed. We use them on our AUVs.
ASW warships must, must be supported as part of a larger battlegroup to be effective. And even then they're nowhere near the efficacy they need to be. Combining ASW ships with MPAs, AUV and underwater sensors like SOSUS and they've a much more complete and effective solution to a submarine problem. That's what lets NATO track Russian submarines or India track Chinese submarines.