Shallow hooking is where fish are hooked in the mouth, whereas deep hooking is where the hook penetrates further into the body.
The University of Maryland (Reiss, 2003) found that deep-hooking causes a third of the fish to die. The majority had sustained major internal damage to the heart, stomach or liver.
The same department (Lukacovic), again using anglers, reported that 5.8% of perch were deep hooked, with a third of them dying. The department investigated complaints of dead and floating striped bass near fishing areas. They discovered angling tackle left in the bodies of many fish and physical trauma caused by hooks.
The Maryland Department of Natural Resources in the USA (Lukacovic, 1999) used anglers, in their research into different hooks, to catch striped bass. They found that a fifth of the fish caught on conventional hooks had been deep-hooked. Of these, over half died. The hooks went to the heart and liver, causing severe internal haemorrhage.
Scientists at an American Fisheries Society Symposium (Prince, 2002) reported their finding regarding deep-hooking of sailfish. Sailfish were caught off Guatemala and Florida. Nearly half of the sailfish caught by conventional hooks had them lodged in their mouth, throat, gill arch, oesophagus, pharynx, or stomach. Many lacerations or bleeding were considered potentially lethal. Several fish were hooked in the eye or eye socket. Hooks also penetrated the brain cavity.
The same American Fisheries Society symposium also received another paper (Skomai, 2002) on hooking. In this study, a third of tuna caught on conventional hooks were deep-hooked. One tuna, aged four, had a circle hook protruding through the lumen of the anterior stomach. The exposed hook point ripped tissue that supports viscera in the abdominal cavity and caused internal bleeding. Another tuna, aged one, had a circle hook that lodged between two gill arches in the pharynx and caused extensive bleeding from gill filament damage.
Hooks in the tuna penetrated the abdominal cavity and gut, tore veins, and damaged livers. There was also scraping of facial tissue, and extensive damage to eyes and their sockets. Blindness could result in death if the fish could no longer feed properly.