Here is a wonderful new study that demonstrates that the antifreeze substances in notothenioid fish are not produced by the liver as was believed for decades and taught in Comparative Physiology courses. Instead, it is produced in two places: most of it in the exocrine pancreas, and somewhat less in a portion of the stomach at the entry of the esophagus:
…..AFGPs are secreted into the intestinal lumen where they protect the intestinal fluid from being frozen by ice crystals that come in with seawater and food. Internal fluids in notothenioids are about one-half as salty as seawater. While seawater reaches its freezing point at -1.91 degrees Celsius, fish fluids freeze at about -1 degree Celsius. These species dwell in water that rarely rises above the freezing point and is regularly filled with ice crystals.
From the intestine, the AFGPs are, apparently, absorbed into the blood. This hypothesis is based on the near-identical composition and abundance of AFGPs found in the fish serum.
They also looked at other groups of Arctic fish, in some of which liver does produce antifreeze susbtances. In those species, the antifreeze was now also found to be secreted by the stomach and pancreas as well.