56 JUNE 2018 • WORLD AQUACULTURE • WWW.WAS.ORG 6-8 m. Storm conditions are mostly associated with currents and waves from the southwest and southeast. Waves from the southeast increase the influence of Mediterranean waters, which generally means an increase in salinity and seawater temperature. Upwelling events play also an important role on variation of environmental conditions on the Algarve coast and are responsible for the emergence of cold and productive waters that occur usually between April and October (Leitão et al. 2005). Mussel Production Method and Mussel Growth Mussel cultivation in the APPA is carried out on a series of semi-submerged longlines with an average length of 400 m (Fig. 1). Each longline consists of 250 headlines, each 12 m long, with a distance of 1-1.5 m between each. Headlines are set about 5 m below the surface. The position and buoyancy is maintained by a system of buoys and anchors. Each headline represents an annual average production of 100 kg of mussels (Fig. 2). Seed collection is carried out during spring and autumn on larval fixation ropes. When mussel seeds reach 2-3 cm, they are placed on socking loops and around one year later most mussels begin to reach commercial size (total length of 50 mm). The sock biodegrades in 2-3 weeks, leaving mussels tightly attached to the rope. After that mussel lines are lifted up periodically for sorting and harvesting (Fig. 3). Mussels less than 50 mm are inserted into new socks and submerged until they reach commercial size. Mussels produced offshore of Algarve have an average growth rate of 4.36 mm (total shell length) per month. However, there is a great disparity in growth due to increasing overlap of mussel beds, being difficult for the whole population to access food. Presence of small, slow-growing individuals is very significant. Periodic sorting and harvesting allows smaller individuals to obtain a new and more advantageous position, where access to food is easier (Fig. 4). Growth rate also depends on environmental factors such as water temperature and food availability, which can fluctuate seasonally and annually. Seasonal weight variation depends mainly on weight variation of gonads (Villalba 1995). In general, the condition index of mussels is greatest between March and May, during the spawning season. After the spawning period, the condition index drops back, recovering later during the summer months. Another peak of high condition occurs in October before autumn spawning, dropping dramatically until January. Low temperatures, a decrease of food availability and gonads in the resting phase are the main reasons for the index decline (Fig. 4). In February 2014, mussels produced offshore of Armona were certified by the Marine Stewardship Council as a sustainable seafood product. The longline system was considered to be harmless to endangered marine species and to the seabed ecosystem. Nutritional Profile The fatty acid profile has become a subject of an increasing importance to the scientific community and consumers in general, related to the importance of fatty acids (FA) on human health, especially the beneficial effects of ω-3 polyunsaturated fatty acids (PUFA) on human welfare. Mussels, like most shellfish, contains a rich lipid profile that includes diverse saturated fatty acids, monounsaturated and polyunsaturated fatty acids (Table 1). Variation in the fatty acid profile of bivalves depends on seasonality, gametogenic cycle, temperature and food quality and abundance and the culture site (Baptista et al. 2014). Mussels produced offshore of Algarve have an average of 9.3 percent (total FA) of eicosapentaenoic acid (EPA, 20:5 ω-3) which is higher than sardines Sardina pilchardus. The DHA content (15.6 percent of total FA) of mussels is similar to that of tuna (Peng et al. 2013). Mussels also have a higher content of 18:2 ω-6 (linoleic acid), 18:3 ω-3 (alpha-linolenic acid), 20:4 ω-6 (arachidonic acid) than sardines (Hale 1984). Total fatty acid content decreases during the winter because of the decline of lipid reserves, recovering in early summer. The variation of DHA/EPA ratio is generally related to variation of diatom concentration in the bivalve diet, and higher DHA values are generally found in warm Mediterranean waters. In Armona island, offshore DHA values are generally higher than that of EPA, but a significant increase in EPA values in the spring is generally observed due to the diatom population growth which usually occurs during spring. In general, biochemical and sensory characteristics of mussels produced offshore of Armona have good acceptability by consumers and are able to compete with mussels produced in other EU countries (Fig. 5). FIGURE 4. Seasonal variation of mussel shell total length and condition index (IC). FIGURE 3. Sorting mussels by size aboard. Photo: Pedro Pousão-Ferreira.
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