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 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 m). 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 m up. We are looking at the 2D projection of a 3D scene. The halibut is below the DIDSON™; the shark is above.
Courtesy of Steve Kaimmer, International Pacific Halibut Commission