Frozen fish stored several months at about −20°C may, after cooking, become tough, chewy, rubbery, stringy, or fibrous. This is accompanied by a loss in functional characteristics of the muscle proteins, mainly solubility, water retention, gelling ability, and lipid emulsifying properties. Freezing and thawing may result in lysis of mitochondria and lysosomes and a change in distribution of enzymes (Karvinen et al., 1982). A gradual decline in the activities of various muscle enzymes has also been observed during storage at freezing temperatures. The loss of ATPase activity, both in meat homogenates and in protein solutions, may reach 50–80% (Buttkus, 1967). The total solubility of proteins in neutral 5% NaC1 solution may decrease to about 30%, whereby the main loss regards the contractile proteins, mainly myosin heavy chain, M-proteins, tropomyosin, and troponins I and C in descending order (Owusu-Ansah and Hultin, 1992). It was shown by Jarenback and Liljemark (1975b), that the myosin microfibrils in fresh cod muscle could be almost totally extracted, whereas myosin in frozen muscle after prolonged storage was resistant to extraction. Significant changes in the sodium dodecyl sulfate—polyacrylamide gel electrophoresis (SDS—PAGE) and HPLC profiles of cod sarcoplasmic proteins due to frozen storage have been shown recently by LeBlanc and LeBlanc (1992b).