Effects of Feeding Levels on the Pathological Alterations in the Digestive System and Mortality of Larvae and Juveniles of Pagrus major

The effects of two feeding levels on the larvae and juveniles of red seabream Pagrus major (also known as madai) were examined beginning with the onset of exogenous feeding until 29 d posthatch. The red seabream were fed enriched live feeds, Brachionus rotundiformis and Artemia spp., at two feeding levels (L1 and L2). At the L1 level, the initial density of the rotifers was 3.0 individuals/mL, and the feeding density was gradually increased to approximately 20.0 individuals/mL up to 29 d posthatch. With Artemia spp., the initial density at L1 was 0.1 individuals/mL (18 d posthatch), increasing gradually up to 1.1 individuals/mL up to 29 d posthatch. The level L1 is the conventional regime adapted by some hatcheries in Japan for red seabream larva and juvenile rearing. The L2 level was the same composition as L1, but the levels of rotifers and Artemia in L2 were five times those of L1. Red seabream larvae and juveniles responded to application of higher feeding levels by ingesting greater amounts of feed that resulted in greater growth but with adverse effects on survival. Histopathological alterations in the digestive system detected by light and transmission electron microscopy included extensive adhesions among the visceral organs to the peritoneal wall, blebbing of the mucosal cells and vacuolar degeneration in the gastric glands of the stomach, necrosis and sloughing of mucosa into the lumen, and submucosal necrosis of the anterior intestine. In addition, atrophy and necrosis of the pancreatic acinar cells and glycogen reduction and fatty degeneration in the hepatocytes of the liver were detected. No appreciable changes were observed in the skin, gills, heart, posterior intestine, kidney, spleen, or central nervous system of fish fed at L1 or L2. It is concluded that higher feeding levels increase the growth rate of larvae and juveniles but act as a stressor causing histopathological alterations in the stomach, intestine, pancreas, and liver that finally lead to death of the larvae and juveniles. In addition to feeding levels, feed enrichment procedures may also have contributed to the observed mortality.