When a hook penetrates deep into a fish, it can cause severe damage and death. This research found that the death rate can be as high as a third:
"Deep-hooking (hooking in the gills or gullet) causes up to 35% mortality."
The heart, stomach and the liver are the most likely areas to suffer most:
DAMAGE TO HEART, STOMACH AND LIVER
"Mortality is at its highest when fish are deep hooked. Necropsies performed on gut-hooked fish in a study by Grover¹, found that the majority had sustained major internal damage to the heart, stomach or liver."
¹ Lucy JA and Studholme AL, editors. Catch and release in marine recreational fisheries. Bethesda: American Fisheries Society; 2002. 250p.