STRUCTURAL AND CHEMICAL-CHANGES IN THE MUSCLE OF CHUM SALMON (ONCORHYNCHUS-KETA) DURING SPAWNING MIGRATION

The white muscle of migrating chum salmon (Oncorhynchus keta) becomes increasingly soft and watery as the fish matures and progresses toward s its spawning grounds. Flesh colour, measured by HunterLab parameters a and a/L, was a better indicator of maturity grade than muscle fat, protein or moisture. In the transition from grade 1, silver-bright, to grade 4, spawning, chum muscle protein decreased from 17.9% to 15.8%, fat decreased from 4.9% to 1.0% and moisture increased from 74.2% to 79.5%. Compositional changes and concurrent textural degradation of th e flesh may be attributed to the physical stress of migration, starvat ion, and hormonal changes during the development of the reproductive s ystem. The structural basis for loss of firm texture was investigated with light and transmission electron microscopy (TEM) by comparing the immature (silver-bright) and mature (spawning) salmon. Light microgra phs revealed an apparent loss in myofibrillar material within muscle c ells as the salmon matured. With TEM, local or focal areas of degenera tion were visible in both the spawning and silver-bright fish, but cha nges were more severe in the mature fish. Given a series of micrograph s of muscle, judges could distinguish between the two groups using a s coring system based on defined criteria of myofibril 'splitting', 'sme aring', 'thinning', and 'inter-myofibril spaces'.