Technical Abstract: Severe muscle deterioration is a physiological response to the energetic demands of fish spawning. This response represents a suitable model to study mechanisms of muscle degradation in fish where the typical tetra-pod methods, such as muscle unloading, are not applicable. Activities and mRNA accumulations of genes in major proteolytic pathways; cathepsins, calpains and the multi-catalytic proteasome, were measured in the white muscles of rainbow trout (RBT) during spawning and post-spawning seasons of gravid fish for comparisons to sterile fish. Fertile fish at spawning had less muscle tissue and less muscle protein compared to sterile fish and post-spawning fertile fish. Muscle deterioration of the fertile fish during spawning was associated with elevated activities and greater mRNA accumulation of cathepsin-L. Concurrently, cathepsin-D, the calpain regulatory subunit and the proteasome catalytic subunit alpha showed increased accumulation of mRNA without a corresponding increase in enzymatic activity. Additionally, to describe the role of the apoptosis in RBT spawning-induced muscle atrophy, caspase-9 cDNA was identified and molecularly characterized from RBT. During spawning, fertile fish had elevated activity and increased mRNA accumulation of caspase-9 but not caspase-3. The present study indicates that cathepsins mediate protein catabolism of the fish during spawning by a cascade involving activation of the apoptosis mediator, caspase-9, but not the apoptosis executioner, caspase-3.