During the commercial slaughter of farmed turbot (Scophthalmus maximus), a total of 67 fish were, on six occasions, removed from their rearing conditions at 14°C and put, as is standard commercial practice, into chilled seawater (-1.5 to -0.8°C) to monitor behavioural, muscular, osmoregulatory and respiratory responses during chilling time (90 min). Results show that a thermal insult alters the iso-osmotic balance, leading not only to an Na+ influx and an intracellular release of Ca2+ and K+, but also to a disturbance of respiratory function, leading to acidosis as a result of H+ and CO2 accumulation, increased pCO2 and reduced HCO3− in the blood. Once the internal temperature dropped below 1°C, the muscles contracted (cold shortening) and, although the fish were still alive, they reverted to a state of rigor, leading to a complete breakdown in their ability to move or ventilate and resembling an unconscious condition or death. Remarkably, the fish were able to prevent themselves undergoing hypoxia as pO2 remained within acceptable limits. No changes in muscle pH were observed and, thus, no noted effects on textural properties. We conclude that live chilling from 14°C to approximately -1°C is a highly questionable practice. It causes physical and physiological changes that are generally associated with stress and, in the case of observed forced muscle contractions, could lead to severe pain. Furthermore, we conclude that cold shortening associated with chilling can be easily mistaken for rigor mortis and, as such, should be subject to further attention in future research on quality.
