How to farm sole

Identity

Solea solea Quensel, 1806 [Soleidae]

FAO Names: En - Common sole, Fr - Sole commune, Es - Lenguado común

Biological features

Body oval. Blind side of head covered with numerous small hair-like fringes; upper eye separated from dorsal profile of head by a distance distinctly greater than its diameter; anterior nostril of blind side surrounded by a small ridge but not enlarged, distance from this nostril to head profile contained 1.5 to 1.8 times in distance from nostril to mouth cleft; anterior nostril on eyed side with tube directed backwards, not reaching anterior margin of eye. Dorsal fin with 72 to 95 rays, its origin on dorsal profile of head before the eyes. Anal fin with 53 to 80 rays. Pectoral fins equally well developed on both sides, with 7 to 10 rays, the fin on eyed side asymmetrical in shape. Base of caudal fin united by a membrane to last ray of dorsal and anal fins, but caudal peduncle still distinct.

Lateral line with 116 to 163 tubed scales, its supratemporal prolongation describing a smooth curve on head. Colour eyed side greyish brown to reddish brown; blind side white. Pectoral fin of eyed side with a black blotch restricted to distal end of fin; hind part of caudl darker than rest of fin.

View FAO FishFinder Species fact sheet

Biological features Solea senegalensis

Flatfish with an oval and asymmetric body (eyes on the right side). Blind-side of head is covered with numerous small hair-like fringes; upper eye is separated from the dorsal profile of the head by a distance distinctly greater than the diameter of the eye; anterior nostril of blind-side is surrounded by a small ridge but not enlarged, distance from anterior nostril to head profile is 1.5–1.9 times the distance from nostril to mouth cleft; anterior nostril on eyed-side with tube directed backwards, not reaching anterior margin of eye. Dorsal fin, with 73–86 rays, originates on dorsal profile of head anterior to the eyes. Anal fin has 61–74 rays. Pectoral fins are equally well developed on both sides, with 7–10 rays, the fin on eyed-side is asymmetrical in shape. The base of the caudal fin is united by a membrane to the last rays of dorsal and anal fins, but caudal peduncle is still distinct. Lateral line with 116–165 pored scales, with its supra-temporal prolongation describing a smooth curve on head.

Colouration of eyed-side varies from greyish-brown to reddish-brown, with large and diffuse dark spots; blind-side is white. Pectoral fin of eyed-side has a black blotch on the distal end; hind part of tail darker than rest of fin. Average adult size is 30–40 cm, but can reach up to 70 cm size (standard length).

Solea solea - versus - Solea senegalensis

There is a high similarity between the two species in terms of morphological and biological features. Morphologically, Senegalese sole (S. senegalensis) can be distinguished from common sole (S. solea) by the black colouration of its interradial membrane on the pectoral fin on the eyed-side. On the other hand, S. solea shows a compact black spot near the margin of the pectoral fin of the eyed-side.

This fact sheet covers two cultured species, common sole and Senegalese sole. Both are farmed with similar aquaculture technology.

Images gallery

Solea specimens growing-out in a fiberglass tank, China (Zhou, X.)	Solea culture in recirculating aquaculture system farm, China (Zhou, X.)
Specimens of marketable size, China (Zhou, X.)	Head detail (Zhou, X.)

Profile

Historical background

Solea solea and Solea senegalensis are two common sole species, found naturally in Atlantic and Mediterranean waters, and are considered potentially important species for marine aquaculture owing to their high market value and consumer demand. The first evidence for S. solea culture was reported by Fabre-Domergue and Biétrix in the 19th century, based on a survey of fish production in France between 1885 and 1905. During the early 20th century until the 1960s, both research on and production of sole entered a slow and persistent decline, despite the encouragement of the French government towards the production of new fish species. There was a resurgence of S. solea production in France and the United Kingdom during the early 1970s, and since then there has been a significant increase in studies and research for the improvement and intensification of sole production.

S. senegalensis is better adapted than S. solea to the warmer waters of temperate climates, and therefore is more suitable for production along the southern coast of Spain and Portugal. During the 1980s, it was cultured extensively in earthen ponds, which often were former salt production ponds.

Since then, numerous research projects in Portugal and Spain have studied methods to improve production. A turning point towards the success of S. senegalensis production was achieved in 1991, when the broodstock kept in the facilities of Instituto de Investigación y Formación Agraria y Pesquera (IFAPA, Cádiz, Spain) began to spawn in a natural and controlled way. Additionally, in 1999 it was reported by the research group from the Centre of Marine Sciences (CCMAR, Portugal) that they achieved continuous and stable spawnings from wild-caught broodstock, resulting in the production of fertile and viable embryos, without any environmental or hormonal manipulation.

Nowadays, natural spawning of S. senegalensis broodstock is well controlled, and can be performed following standard procedures based in temperature manipulations (Anguis and Canavate, 2004). As a consequence in Spain, mass production of juvenile S. senegalensis started in 1993.

By the early 1990s, Spain had begun to invest in alternative aquaculture species with high market value as a result of market saturation with seabream, seabass and turbot species. Quickly, Senegalese sole became a promising target species for further development. Currently, the state of sole production in Spain is slowly growing, and production is expected to continue to increase; the main producers are located in the regions of Galicia, Andalucía and Canarias. In Portugal, the production of S. senegalensis is mainly conducted by a single company, located in the northern region. Even so, other fish farms across the country are producing low volumes of sole in polyculture systems. Currently, S. solea production is low, primarily owing to its slower growth rate in comparison with S. senegalensis.

The three major bottlenecks for Solea spp. production are 1) high larval mortality rates related to nutrition and growth dispersion 2) sub-optimal larval weaning strategies and 3) inadequate control measures for common diseases. Additionally, the techniques for intensive rearing of sole are yet to be optimized, which limits the final volume of fish production.

The European sole farming began in Portugal in the late 1970s with only 2 tonnes per year. In the mid-1980s, Spain began sole production, by which time Portuguese production was around 35 tonnes. Sole farming continued to expand, fluctuating between 10 and 70 tonnes until 2007. Italy entered the sole market in 2008 with 20 tonnes, but stopped production in 2012. The greatest impulse in sole farming was in 2010 when French production started with 150 tonnes. In that same year, Spain produced 165 tonnes. Currently, France and Spain are the leading sole producers. Sole production has been evolving at a low rate since late 1970s and only reached a total of 347 tonnes in 2010. Since then, a maximum production of 571 tonnes was registered in 2013, but mostly in Europe (FEAP, 2014). Outside of Europe, Solea senegalensis is farmed at least in China however there are no official data relative to production statistics of Solea spp. in other regions. A rough estimation points to a Chinese production of 300–500 tonnes in recent years (unofficial data).

Main producer countries

Main producer countries of Solea solea and Solea senegalensis (FAO Fishery Statistics, 2015)

Habitat and biology

Solea solea

Demersal marine species living on sandy or muddy bottoms, ranging from near shore to 200 m of depth. Adults feed mainly on polychaete worms, molluscs and small crustaceans. Females reach sexual maturity around four years old and total length of 27–30 cm. Spawning periods differ depending on geographical location: in the Mediterranean spawning takes place between January and April, with two peaks in February; in the Bay of Biscay spawning occurs between December and May; and in the North Sea spawning happens between April and June. The optimal temperature for spawning ranges from 8 to 12 °C.

Solea senegalensis

Demersal marine species living on sandy or muddy bottoms, ranging from brackish lagoons and shallow waters to coastal areas up to a depth of 100 m. Adults feed on small benthic invertebrates, mainly polychaetes and bivalve molluscs, and to a lesser extent small crustaceans. Sexual maturity of females occurs at 3 years of age and 32 cm total length. Spawning occurs during the summer between the May and August, with a peak in June (Iberian Peninsula, Bay of Biscay). Spawning is highly dependent on water temperature, which should be between 15 and 20 °C.

Production

Production cycle

Production cycle of Solea spp.

Production systems

Traditional sole farming in natural conditions is conducted in extensive and semi-intensive land-based ponds. Frequently, these ponds are managed using polyculture techniques, raising sole in conjunction with other species, like seabream, to increase profit. Significantly higher production volumes are realized through intensive culture systems using water re-circulation technology in indoor facilities.

Recirculation systems are also used during the first stages of sole life cycle.

Seed supply

Seed supply of sole is produced by hatcheries that are usually associated with ongrowing facilities.

Reproductive adults are caught in the wild during the inter-spawning season (July to December) and acclimatized for at least 12 months before natural spawning occurs. Both species are gonochoric, having separate males and females. Sexual maturation for the females is achieved at 2 years for S. senegalensis and 3 years for S. solea, when the total lengths are 32 cm and 30 cm, respectively (Dinis and Reis, 1995). Broodstock is maintained in circular or rectangular tanks with flat bottoms, with at least 3 m2 of area and a water depth of 50–80 cm. Breeder adults should be fed both natural food, like squid, polychaetes and mussels, as well as inert diets. Salinity should be 33–35 p.p.t. The photoperiod should be natural; Solea species do not use daylight as a cue for spawning in contrast to other flatfish species.

Optimal environmental conditions and stocking density vary between the two species. Natural spawning season is late winter for S. solea (January–March) while for S. senegalensis it occurs in spring (March–June). For S. solea, water temperature should be between 8 and 12 °C, the sex ratio should be 0.5–3 males for each female and densities should be 0.6–3 kg/m2. For S. senegalensis, natural spawning is obtained with temperatures between 16 and 22 °C, the sex ratio should be 2 males for each female and the stocking density should be kept at 1–1.5 kg/m2 (Dinis et al., 1999).

For S. solea, fecundity may vary between 10 and 140 g/kg and fertilization rates from 25 to 80 percent. For S. senegalensis, total daily egg collection may range between 0 and 180 g, with fertilization rates between 20 and 100 percent. The length of the spawning period is variable, from 4 to 6 months, however breeders do not spawn every day. Variation in egg size appears to be related with temperature and spawning period for both species.

Nursery

The floating eggs are collected using surface collectors and incubated in the same circuit as the broodstock tanks in order to maintain consistent temperature and salinity as the spawning conditions. Egg size has a significant influence on larvae size. On average, upon hatching S. senegalensis larvae are smaller than S. solea. Incubation of the eggs requires adequate water quality parameters, achieved through gentle aeration and slight water turnover until hatching.

After hatching, the free swimming larvae have a total length of 2.2–2.9 mm for S. senegalensis and 4–5 mm for S. solea. At this stage, larvae are stocked in cylindroconical tanks at an optimal density of 40 larvae per litre. Normal larval development is observed from 10 to 16 °C for S. solea larvae and 16 to 23 °C for S. senegalensis larvae. At first feeding, usually 2–3 days after hatching (DAH), larvae should be fed with enriched rotifers for 3 days, followed by Artemia nauplii. At 9–10 DAH, Artemia meta-nauplii (24 h) are introduced, but they should be enriched with n-3 polyunsaturated fatty acid, also known as omega-3, to guarantee better larval development and survival rates. S. senegalensis metamorphosis begins 9 DAH and is completed by 19 DAH. After metamorphosis, larvae are transferred to raceways or flat bottom tanks, at stocking densities between 500 and 1 500 larvae per m2. At this stage, the fry may be fed frozen Artemia (24 h meta-nauplii) until weaning. Inert diets of crumbles may be supplied because the mouths of the larvae open starting 2 DAH for S. senegalensis (Engrola et al., 2009) and 10 DHA for S. solea (Gatesoupe and Luquet, 1982). During the larval stage, Solea growth and ingestion rates are high.

Ongrowing techniques

S. senegalensis can be cultivated in earthen ponds, usually in polyculture systems in conjunction with seabream or seabass, most commonly in extensive and semi-intensive systems, mostly found in Southern European countries including Portugal and Spain. Water circulation in these ponds is usually not continuous, instead depending of tidal flow. In polyculture systems, feed supply is generally targeted to the mid-water species, while the diet of the sole is largely comprised of benthic prey.

Stocking densities are variable, although the density is much lower when compared to intensive production. Fish are not calibrated along the production cycle, and therefore sizes may vary greatly at harvest.

Intensive farming uses a combination of shallow water raceway tanks with recirculation aquaculture systems (RAS). This technique achieves optimal growth conditions throughout the production cycle and consequently higher production. The size of the rearing tanks can be variable depending on the production scale. Both species are produced at temperatures between 19 and 21 °C. To ensure optimal growth, the diets usually have high protein content (50–55 percent) on a dry matter basis (DM) and are low in fat (8–10 percent DM) (Borges et al., 2009). During the pre-fattening stage, S. senegalensis juveniles of 5 g are stocked at a density of 2 kg/m2. They are raised until they reach 80–90 g each, at which point they enter the fattening period. The fattening period lasts until the fish reach 350 g (average marketable size), at which time the density may reach 25 kg/m2. The entire ongrowing period, from initial stocking at 5 g until harvest at average commercial size of 350 g, may last between 16 and 18 months.

Harvesting techniques

Fish are submitted to a fast period before harvest to improve flavour and marketability. In earthen pond farming, at the end of the production cycle the ponds are drained and the sole are hand-picked. In intensive indoor production, fish are manually harvested with a net and slaughtered by immersion in an ice-saltwater slurry, and then transported to processing units.

Handling and processing

Harvested fish are packed in polystyrene boxes, covered with a layer of ice and plastic film. Generally, sole are marketed fresh, either whole or as boneless fillets. A small fraction is further processed as frozen fillets.

Production costs

In a recent study (Bjørndal and Guillen, 2014), production costs were calculated based on intensive sole production, assuming ongrowing from 5 g juveniles until 350 g harvest weight after 18 months. An average cost of 10.9 USD/kg was attained. The major cost fraction is the price of the juveniles, accounting for 39 percent of the total cost, followed by the feed fraction accounting for 16 percent. The combined labour, operating and maintenance fraction attributed to 19 percent of the total production cost.

Diseases and control measures

In general, cultured Solea are extremely susceptible to a host of diseases that commonly affect other cultured flatfish and fin?sh species. In most cases, the severity of the disease seems to be linked with increasing intensification of production. At the present, photobacteriosis (Photobacterium damselae subsp. piscicida), vibriosis (Vibrio sp.) and flexibacteriosis (Tenacibaculum maritimum) are considered the most important pathogens affecting sole culture and limiting successful expansion. The main disease problems affecting Solea species are described in table below.

Together, these findings highlight the need for effective disease control measures and continuous epidemiological surveillance through all life stages of Solea culture.

A partial list of people involved in research and treatment of diseases affecting S. solea and S. senegalensis is provided below:

• Carlos Zarza , Senior Research Pathologist at Skretting Aquaculture Research Centre, Stavanger Area, Norway
• José Ignacio Navas-Triano , Head of Production Department at IFAPA Centro Agua del Pino, Ministry of Agriculture and Fisheries, Government of Andalusia
• Florbela Soares , Researcher at Aquaculture Unit, INRB/IPIMAR Institute, Olhão, Portugal
• Juan Luis Barja Pérez , Instituto de Acuicultura, University of Santiago de Compostela, Spain

Statistics

Production statistics

The Solea spp. market is dominated by capture fishery production, which accounted 36 367 tonnes in 2012, while in the same year the total sole aquaculture production was below 400 tonnes.

The graph below is referred to Solea solea only.

Market and trade

The European sole market comprises two species, common sole (Solea solea) and Senegalese sole (Solea senegalensis). There is little distinction made between species by consumers (Dinis et al 1999). However, different market values are observed for each species, generally, common sole is more appreciated in Western and Northern Europe while Senegalese sole received a much lower price in these markets. In Southern Europe the opposite situation is observed, where Senegalese sole is more valuable (Imsland, 2010).

Status and trends

Even with a small aquaculture share in the market and the decreasing landings, sole first sale value has been decreasing since the 2006 peak, when it reached 469 550 USD; in 2012 total value was 360 670 USD (Eurostat, 2014).

Sole is mainly sold fresh, either as a whole fish or as fillets. For fillets, the most common dose portion is 350 g, but fillets can also be found in the larger sizes of 600 g as preferred by the hotels, restaurants and café (HORECA) sector. With the decrease of the wild stocks, captured fish will tend to be smaller and be directed to portion size markets (Bjørndal and Guillen, 2014). However aquaculture may adapt their production and aim to larger fish size markets where typically higher unit price is obtained. Price per kilogram varies within the whole sale market based on size: 28 USD for 1 kg fish, 17–19 USD for a 750 g fish, and 11 USD for a fish less than 500 g. Despite the fact that farmed sole have a commercial size range less than 500 g, prices in 2013 were 14 USD/kg, higher than equivalent sized wild fish. The observed trend is for increasing second sale prices since 2010 for fresh sole, both wild and farmed, but lower prices are found for frozen sole (MercaMadrid cited by Bjørndal and Guillen, 2014). The declining fisheries and high market prices are factors that increases the potential of sole farming as a new species for aquaculture.

In China, Solea spp has not prospered as predicted 10 to 15 years ago. One of the major reasons is the faster aquaculture development of similar native species like Cynoglossus semilaevis. In 2013, a Portuguese sole hatchery sold juveniles (7 g weight) to Asia, and increased sales are expected in a near future (Morais et al., 2014).

Main issues

Sole was a promising species for the diversification of aquaculture in the 1980s, however, technical and disease problems were responsible for the main setbacks in the development of sole farming industry (Howell, 1997; Dinis et al., 1999). The efforts applied in research allowed the industry to surpass these difficulties and improve breeding, husbandry, nutrition and disease management.

Expansion of sole farming still faces some challenges. Production costs are mainly dependent on supply of juveniles (39 percent) and feed prices (16 percent) (Bjørndal and Guillen, 2014). Refining breeding programmes, improving the quality of juveniles, designing more suitable diets and controlling diseases are essential steps to reduce production costs and boost production. New markets may also be explored with a larger range of fish sizes and convenience packages coming from the industry sector.

Responsible aquaculture practices

The International Council for the Exploration of the Sea (ICES) assesses the status of common sole and Senegalese sole on a rolling basis (assessments are updated yearly). The farming of Solea spp. in European countries should follow the guidelines and recommendations proposed in the Code of Conduct prepared by the Federation of European Aquaculture Producers in order to maintain a responsible production sector towards the fish it rears, the environment and the consumer.

Portuguese national fisheries policy regarding aquaculture practice is in accordance with those established by the EU in the Common Fishery Policy, and in particular with the 2002 Strategy for the Sustainable Development of European Aquaculture, which promotes environmental, economic and social sustainability. The principles of FAO Code of Conduct for Responsible Fisheries should be followed in respect to both Sole species.

July 2015