In the thermal range of the Pacific waters, known for the geographical distribution of Sardinops sagax caeruleus (Jenyns, 1842) (Pacific sardine), could be exposed to a stressing temperature environment with dilated effect. This work examines the liver and kidney of sardines acclimated at different temperatures and exposed to the lethal temperature and critical thermal maximum trials. The liver and kidney tissues of Sardinops sagax caeruleus acclimated for 25 days at 19, 21, 23 and 25 °C were anatomically examined after exposure to acute heat stress (AHS) caused by increasing the water temperature at a rate of 1 °C per min, and the chronic heat stress (CHS) effect by abruptly exposure to constant water temperature different from that of acclimation (AT). We observed in fish exposed to AHS that the liver tissue had vacuolated or necrotic hepatocytes and infiltration of inflammatory blood cells (25 °C) and the kidney tissue showed degenerative changes in the glomeruli and renal tubules and increased melanomacrophage centers. The CHS effect in liver and renal tissues produced damage signs of pyknosis, apoptosis, necrotic areas, and an increase in melanomacrophage centers as well as outbreaks of bacterial infection. The results demonstrate that S. sagax caeruleus did not tolerate an abrupt thermal change of more than 4 °C, independently of the ATs, over 50 % died. The consequences of the experimental acute and chronic thermal stress were histopathological alterations of liver and kidney. It was expected that the chronic stress temperature could produce in fish conspicuous histological changes, and indeed it was the most deleterious.
KEY WORDS: Sardinops sagax caeruleus; Temperature stress-induced; Histopathology.