These evolutions are reflected in the condition factor K, where K = (Fish weight/fish length^3)*100), through the year (see Figure). High condition factors are associated with plump fish, while low condition factors with lean fish. The gametogenesis influences positively the condition factor, as the gonads represent around 14% of the total body weight in December (Chaoui et al. 2005). The authors conclude that in nature, gilthead seabream has a seasonal feeding pattern and its intense feeding activity in summer allows the gilthead seabream to store energy and nutrients in the liver which is subsequently transferred to the gonad when feeding activity is decreased. This strategy is not unique to gilthead seabream, as storing nutrients in the liver during an intensive feeding period, in preparation of later remobilization for gametogenesis, are key factor for reproductive investment and maternal output in several marine fishes (Murzina et al., 2012). However, in the case of gilthead seabream culture, feed management oriented toward growth and containment can negatively interact with metabolic processes aligned with a migratory pattern.
As seen above, a pale and friable liver is a main symptom of the “winter disease”. It is worth mentioning that the liver is a central organ in fish homeostasis: It plays an important role in energy storage and lipid metabolism, but it is also involved in diverse vital functions such as digestion, blood cleansing, detoxification, energy metabolism, reproduction, immunity etc. Impaired liver functions will impact the overall capacity of the fish to cope with its environment, making it more susceptible to the surrounding threats.
Scientists (Ibarz, Antoni & Padrós, 2010) reported that the damaged liver of Gilthead seabream suffering from “winter disease” is linked to a rapid accumulation of lipids coming from adipose storage in the muscle and the viscera to the liver. If fat is essential, any excess can be detrimental. Added to the metabolic slowdown caused by temperature drop, and which cannot be mitigated by fleeing to deeper and warmer water, in the culture environment it can be “too much” for the gilthead seabream. Excessive lipid deposition alters other liver functions such as listed above, leading to overall unhealthy fish condition. This metabolic collapse is the essence of “winter disease”.
If cage culture does not allow the industry to follow the natural behaviour of seabream, it is possible to adjust practices to mitigate the impact of the “winter disease”. Nutrition and by extension feed management are the main parameters that can be adapted to reduce the risk of steatosis and match the nutritional profile to the evolving winter requirements.
Gilthead seabream is known to be “voracious fishes”: Mussel farmers in the south of France nicknamed them “Piranhas” in a local newspaper, referring to their tendency to form schools and perform raids or “razzia” on mussel colonies. Their metabolism is adapted to consume high amounts of energy in summer but this ability is drastically altered by temperature drop: It is important to adapt the nutritional intake accordingly.