Eaten tons of vac-packed fish over the years.

The one thing I see in every bag upon thawing is juice.... juice that should be in the meat, but instead is lost due ice crytals rupturing microscopic cell membranes.

Jack Whelan states in his book Smoking Salmon and Trout, long considered the bible on the subject, that even though a piece of frozen fish still feels frozen after being out of the freezer, it has begun to thaw at a crystalline level.

He states this even happens in your own home freezer when you load it up with new unfrozen stuff. The "heat" of the unfrozen goods will be absorbed by the frozen goods in the freezer.... causing then to partially "thaw" on a microscopic level even though they still feel frozen. The greater the load of unfrozen goods, the more pronounced the effect.

As the freezer comes back to fully freeze all the goods inside, new ice crystals are formed in the partially thawed goods, causing significant cell damage, unbeknownst to the owner.