Histological Changes and Reduced Gastric Epithelial Proliferating Cell Nuclear Antigen Immunoreactivity During Spawning Migration in Wild Pink Salmon (Oncorhynchus gorbuscha)

Anadromous salmonids cease feeding during spawning migration and rely on stored energy reserves for migration and reproduction. Although gastrointestinal changes associated with spawning migration have been described in salmonids, detailed histological and quantitative information remains limited for semelparous Pacific salmon, which die after reproduction. In this study, we examined histological changes in the stomach and intestine of wild pink salmon (Oncorhynchus gorbuscha) between the oceanic feeding migration and riverine spawning migration phases. Gastric folds were narrower, the gastric mucosa was thinner, and gastric pits and glands appeared reduced in size during spawning migration. Intestinal mucosal fold height was also reduced, whereas goblet cell density increased. Eosinophilic and periodic acid–Schiff-positive brush borders were retained on the intestinal mucosal folds during both migration phases. Immunohistochemical analysis further revealed a marked reduction in the density of proliferating cell nuclear antigen (PCNA)-immunoreactive cells in the gastric mucosal epithelium during spawning migration. These findings demonstrate regression of the gastrointestinal tract associated with the transition from feeding to reproductive migration in wild pink salmon and suggest that PCNA immunoreactivity of the gastric mucosal epithelium is strongly reduced as this semelparous species approaches its terminal reproductive phase.