European sea bass in Mediterranean sea cages experience temperature extremes in summer and winter that require active management to prevent welfare harm and mass mortality.
Summer hyperthermia in surface-sited cages causes reduced oxygen solubility, increased pathogen growth and heat stress in fish unable to seek cooler water. Winter cold stun events cause immune suppression and disease outbreaks in fish weakened by low temperature. Seasonal management including cage depth adjustment, reduced stocking density in summer and emergency plans for extreme temperature events are welfare-positive interventions that also protect production.