Halibut Stock Assessment

International Pacific Halibut Commission uses hook and line surveys to assess
halibut stock. It is important to know the hooking success of halibut by size.
This frame supports four baited hooks (16/0 Mustad circle hooks baited with
¼ lb of chum salmon) and is placed on the ocean floor at depths between
20 and 100 fathoms (36 – 182 meters). It is repeatedly placed on the ocean
floor for approximately one hour then brought to the surface to assess what
was caught. The DIDSON monitors the device on the bottom to determine
how many times fish (as a function of size) attack the hook but are not caught.
The advantage of a DIDSON over optical systems is that DIDSON can monitor
the device at the desired depths without regard to ambient light.

In this shot, the shark was right side up, so we are seeing the shark's
ventral side. Note the mouth. It appears as if the shark is right above
the halibut and they react as if it is; it's actually about 2 meters up.
We are looking at the 2-d layer cake made out of a 3-d image.

Courtesy of Steve Kaimmer, International Pacific Halibut Commission