I commercially fished Black Cod for 20 yrs with traps, 100 traps per string, 10 strings of gear. Usually, we would anchor our shallow end in about 250 - 300 fathoms. Lay the string out over the edge into anywhere from 500 to 600 fathoms or deeper, run along the edge then bring the 2nd end of the string back up into the shallows to anchor.
Depending on the time of year, the Black Cod would be concentrated in a certain "section" (depth) of the string. We would take note and lay most all the 10 strings in that depth, with the ends anchored in the shallows.
In the late spring-early summer you would find more Black Cod, and a larger average size up in the shallows 150 - 200 fathoms with a high percentage of mature females.
If you want to catch big Black Cod, you have to go deep. As stated, 150 - 200 fathoms is shallow for mature Black Cod.
That's why it is difficult to raise Black Cod in farms. It's very difficult to reproduce the pressures produced at the depths that Black Cod thrive in.
Catching the odd incidental runt happens now and then but, I'm afraid that it will be rare to get a big Black Cod on the hook unless you are very deep.
Edit: For bait we would use frozen squid in a mesh bait bag hanging in the trap then throw a 2lb or 3lb block of frozen hake in the trap to get a feeding frenzy going on.