by Jean Peignon, Aquaculture Nutritionist, Aquanéo, France


Gilthead seabream: A mythical Mediterranean fish

Gilthead seabream (Sparus aurata) or 'The golden javelin' in Latin, is an emblematic fish from the Mediterranean Sea. Renowned for its delicate mild, white meat, gilthead seabream is present 'in portion' on the plates of every Mediterranean seafood restaurant. Its consumption is very old in the area, as attested by the mosaics from Ancient Rome, showing detailed pictures of gilthead seabream or archaeological evidence of dried seabream trade 3,500 years ago from Bardawil lagoon in Egypt to the Middle East (Sisma-Ventura Guy et al, 2018.)

Traditionally, gilthead seabream was raised in extensive coastal lagoons, called 'vallicoltura' in Italy or 'hosha' in Egypt, in which the humans exploit their seasonal migrations from the sea into the lagoons by preventing any travel back to the sea. Thanks to its powerful jaws and teeth, the species is well adapted to extensive aquaculture as it is able to feed on various trophic niches, from soft prey like polychaetes or fishes to protected crustaceans or molluscs and even benthic plants (Chaoui et al. 2005).

In nature, young fish join the low water lagoons in spring, where the natural production and temperatures are high, and return in winter to the calm deep waters to reproduce. Nowadays, gilthead seabream is one of the two main species cultivated in Mediterranean, along with European seabass (Dicentrarchus labrax). In 2017 the aquaculture production in Mediterranean, based on sea cages, reached around 110,000 million tonnes (mt)

Cold temperatures: The gilthead seabream's Achilles' heel

Mediterranean farmers know it: compared to the European seabass, gilthead seabream is more sensitive to temperature drops. In nature, this sensitivity can be tackled by migratory behaviour. When temperatures drop, gilthead seabream leave their coastal areas to move to warmer and more stable deep waters, where they reproduce. Spawning occurs between December to January and juveniles join the coastal area to feed on more productive biotopes. Gilthead seabream is a protandrous hermaphrodite, meaning that they are functionally male in the first two years of their life before switching to females. Sex reversal occurs in around the second or third years and impacts the migratory behaviour: with age they tend to stay in deeper water.

Aquaculture in sea cages makes migration impossible and, therefore, gilthead seabream are facing challenging conditions during winter time as surface water parameters fluctuate with the weather conditions: temperature drop and unstable salinity generates a metabolic disruption characterised by an array of symptoms called the 'winter disease' or 'winter syndrome'.

The term 'disease' can be misunderstood: 'Winter disease' is not caused by a pathogen, it is a multifactorial pathology leading to metabolic disruption, organ dysfunction and immunosuppression. Pathogens can take advantage of the weakened fish, but they are not the cause of it. Scientists (Ibarz, Antoni & Padrós, 2010), describe two types of syndromes: chronic mortality from December to February, when temperatures are below 12°C and a second associated with the presence of infections by Pseudomonas anguilliseptica, characterised by acute mortality, usually in early March and April when temperature recover to 15-17°C. The symptoms are diverse: pale gills, corneal opacity, or distended gut tract can be noticed, but the main one is a pale and friable liver, a sign of steatosis (Ibarz, Antoni & Padrós 2010).

Studies of wild populations in Algeria (Chaoui et al. 2005, Chaoui et al. 2006, Chebel, Fateh & Mezedjri, 2018) bring valuable informations to understand the metabolic changes occurring with temperature variations:

  • Starting from November, when temperature goes below 15°C, the feed consumption decreases, ending to a fasting period in December when temperature is around 11°C
  • The period from October to December is characterised by intense gametogenesis
  • The hepatosomatic index, the liver's weight to bodyweight ratio, increases from August in response to the intensive feeding activity and decreases concomitantly with gametogenesis.

These evolutions are reflected in the condition factor K, where K = (Fish weight/fish length^3)*100), through the year (See Figure 1). High condition factors are associated with plump fish, while low condition factors with lean fish. The gametogenesis positively influences the condition factor, as the gonads represent around 14 percent 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, allowing 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 intensive feeding period, in preparation of later remobilisation for gametogenesis, are key factors 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, which cannot be mitigated by fleeing to deeper and warmer waters, in a culture environment it can be considered to be too much for the gilthead seabream to manage. Excessive lipid deposition alters others liver functions such as those 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 'winter disease'. Nutrition and, by extension, feed management are the main parameters that can be adapted to reduce the risk of steatosis and match nutritional profiles to the ever-evolving winter requirements.

Gilthead seabream are 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.

Aquanéo's approach: A three-pronged strategy

Nutrition needs to follow the metabolic rhythm of fish and not the opposite. For that reason, Aquanéo, the aquaculture brand of Techna - specialists in animal nutrition- , has developed an holistic approach to reduce the incidence of 'winter disease' and help farmers to improve their performances:

  • The first aspect recommended is to rethink the nutrition of gilthead seabream according to their seasonal feeding pattern: with two distinct requirement specifications for summer and winter. In commercial conditions, starving during long periods is not desirable, Aquanéo proposes to readjust nutrients to cover the specific needs and remove any excess
  • Another important aspect is energy delivery, in relation to water temperature and metabolic rhythm of fish. Balancing a diet is one thing, but if this is over distributed, then the whole benefit disappears. Taking this into account, Aquanéo has developed specific feeding tables
  • Finally, some plant extracts have the ability to promote lipid metabolism and protect the liver. Aquanéo has developed a special additive to support gilthead seabream metabolism and help them to go through the winter season.

'Winter disease' is a complex problem. Aware that it cannot be managed with a simple solution, Aquanéo mobilised its expertise in nutrition and feed additives to submit to the aquaculture industry a multifaceted approach with synergistic actions. With these three pillars (feed formulation, including additive use and feeding tables) Aquanéo will help feed millers to create a full feeding programme designed to answer this issue.

The relationship between gilthead seabream and inhabitants of the Mediterranean basin is very old, whilst modern aquaculture, on the contrary, is very young. Aquanéo is willing to contribute to the general effort made by the aquaculture industry to develop sustainable practices and, with respect to gilthead seabream metabolic particularities, provide final customers the healthy and delicate 'golden javelin' they enjoyed eating for centuries.

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