Rigor mortis is a result of complicated biochemical reactions which cause muscle fibres to shorten and tighten, and finally the fish becomes stiff. Rigor mortis has many technological consequences. If, for example, the bones were removed prior to rigor mortis the length of the fillet shortens by 30%. At the same time, the fillet becomes wider and thicker because its volume does not change.
This tightness very often causes the connective tissue of individual myomeres to break; this process is termed “gaping” and results in muscle separation which is considered a quality defect. “Gaping” depends on temperature; the higher the temperature of fish at the beginning of the rigor mortis process the greater the gaping of the muscle. Therefore, during rigor mortis fish temperature should be as low as possible. For example, for roach and perch kept at 0° C rigor mortis begins 24 hours after death and lasts for 72-80 hours. When the same species is kept at 35° C it begins 20-30 minutes after death and stops after about 3 hours. The time rigor mortis begins and its duration depend on the fish species (e.g., for carp at 0° C it starts after 48 hours, for roach and perch at 0° C after 24 hours), on the fish catching technique, and on fish temperature. It was also found that fast swimmers, for example trout, undergo rigor mortis faster but for a shorter duration than slow swimmers like carp.
In those fish which are in good condition (well-nourished) rigor mortis is more intensive. Fish put to death just after removal from the water reach a state of rigor mortis later than those fish which died after a long agony. In the case of carp put to death just after capture rigor mortis begins after 48 hours, but if the carp died after a long agony it sets in after 24 hours (at 0° C).
Unnecessary and rough handling of the fish can shorten the time of occurrence and duration of rigor mortis. Such treatment causes stress in live fish.
Fish body temperature is a decisive factor in the onset and duration of the rigor mortis process. The higher the temperature the sooner it begins and the faster it ceases. This is evidenced by enzymatic reactions whose speed increases with increased temperature. At high temperatures it results in greater changes in proteins, the latter causing higher loss of tissue juices, e.g., during processing. Usually, the later rigor mortis begins and the longer it lasts, the longer are the storage life of the fish and its use for consumption.