If so, it has suffered cold storage damage. An extension of the hardness test is to hold out a hand vertically and drape the fillet over it. An unfrozen fish will be completely flexible and the fillet will touch both the back and the palm of the hand. Again best with thinner fillets and there is some difference in drape among species. As the fish deteriorates in frozen storage, the fillet becomes stiffer and does not drape as much, or at all.
Tests based on aggregation of protein again measure storage deterioration.
No aggregation is detected in fish stored under good conditions. One type of aggregation is induced by formaldehyde, which is formed from TMAO, but I believe TMAO contents in salmon are low to negligible. Richard Chivers referred to the Torry meter. This measures electrical properties of fish muscle - combination of resistance and capacitance. During icing the cell membranes disintegrate, the fluids equilibrate and the meter readings decrease.