There is growing evidence that future fish needs for human consumption must be covered, in progressively higher proportion, by aquaculture and thus a sustainable development of this activity is needed.

One of the main quality indexes in farmed fish for human consumption is the lipid amount and fatty acid (FA) profile stored in the main edible fraction, the muscle. Some n-3 PUFAs have been regarded as essential for human health (WHO, 1977) for their role in preventing and treating a wide variety of disorders. Experts in human nutrition and health agree that fish included in the daily diet helps prevent certain diseases such as cardiovascular ones (Pirini et al., 2010). In this case, the beneficial effect seems to lie in the lipid component of fish and more specifically in the presence of certain inhibitors of platelet aggregation (Nasopoulou et al., 2007) and certain MUFAs and PUFAs, mainly those of the n-3 series.

Numerous studies have demonstrated the influence of the dietary FAs on their profile in fish tissues (Lie, 2001; Yildiz et al., 2008), but other factors could be also involved. The fish FA profile is certainly influenced by season (Leger et al., 1977, Shirai et al., 2002; Hassan et al., 2010) since the biochemical composition of marine organisms can undergo seasonal changes in this respect (Ackman, 1995). Also, the maturation degree significantly alters the FA profile of different fish tissues (Hussain et al., 2011).

Meagre (Argyrosomus regius) is receiving increasing attention for its reported good growth rate and easy adaptation to fish-farming practices. This species has been described as a particularly slender fish, even when it is cultured intensively, and even on a high-fat diet it provides high-quality commercial products. This fish has a high percentage of muscle, low fat, and healthy lipid content, so that it has a low index of atherogenicity and thrombogenicity (Poli et al., 2003). The organoleptic properties show that farmed meagre has no accumulation of perivisceral and muscle fat compared with other cultivated species. Also, it can be cool stored for long periods of time without sensory characteristics being affected (Hernández et al., 2009). Studies on the same species show that it provides low-fat meat even under intensive farming conditions (Piccolo et al., 2008). The low range of muscle lipids observed among dietary treatments implies that meagre, like other species, has a restricted ability for lipid deposition in muscle.

Recently, Chatzifotis et al., (2010) have observed that a 17% lipid diet was the most suitable for meagre in terms of growth, compared to 13% and 21%. Proximate analysis indicated that the lipid content of whole body and muscle was affected by the diets, whereas of the liver remained unaffected.

The aim of the present work was to evaluate the influence of the growth stage on the time course of muscle fatty acid composition of meagre, over a 500-day period of their production cycle in a fish farm based in off-shore cages. This experimental period was designed to complete the evaluation of the possible effects of different culture factors on total muscle lipids and fatty acid content.

Regarding the lipid content of the main edible portion of fish (muscle), Table 2A shows that, over the period monitored, lipids steadily increased during the first year of caging (samples 1 to 8), after which the proportion of lipids in muscle stabilized or slightly fell.

The FA profiles of muscle lipids are shown in Tables 2A and 2B. In first samplings, the most abundant FAs were palmitic (PA, 16:0) among saturated (SFA), OA, 18:1n-9 among monounsaturated (MUFA), and docosahexaenoic (DHA, 22:6n-3) among polyunsaturated (PUFA). This situation persisted throughout the trial. For all groups, SFA and MUFA had a similar proportion of approximately 30%, while PUFA were in the top of the range, with 39%. In terms of the n series, n-3 clearly predominated, accounting for almost four times the content of n-6 at the beginning of the experiment.

Throughout the trial, there were some statistically significant changes in this profile, although many of them can be considered physiologically minor or irrelevant. The most relevant changes included the significant increase in LA, 18:2n-6 among PUFAs and OA, as well as, to a lesser extent, MUFAs. Among the SFA the changes observed were minor, with no clear trend detected.

Since the OA increase was offset by the decline in other figures of this group (e.g., palmitoleic acid, PA, 16:1n-7) the overall MUFAs presented a slight oscillation over time, i.e. around a total of 30% with a peak in the sampling 4 that coincided with that of OA.

The total content of PUFAs varied markedly in the first samplings, reaching over 45% in the sampling 6 and showing a similar behaviour until the end of the experiment. Within this group, the increase in n-6 series, due largely to LA, was accompanied by a decrease in n-3 that affected mainly eicosapentaenoic acid (EPA, 20:5n-3). These simultaneous changes in n-3 and n-6 led to a fall in the n-3:n-6 ratio from initial values of 3.8 to values similar to and in some cases lower than 1.0. In relative terms, the greatest reduction in this ratio occurred between the samplings 3 and 4, after which a slow but incomplete recovery was detected.

The muscle-lipid content increased as the fish grew in the farm from 0.65% to 1.7–2% in wet matter. Total FAs varied over the experiment between 60–70% of lipids, in agreement with previous results on fish muscle and liver (Kandemir and Polat, 2007; Guil-Guerrero et al., 2011). Overall, lipid-content values proved clearly lower than those reported for other farmed fish (Grigorakis et al., 2011) and fully justify the common name (meagre, from maigre in French, meaning “lean”) used for this species. The initial fattening of younger fish can be explained by the transition from a feeding based on high-protein/low-fat diets (hatchery diet and Microbaq) to a higher-fat diet. Both, Piccolo et al., (2008) and Chatzifotis et al., (2010) detected a higher proportion of lipids in meagre muscle when the fish was fed on high-fat diets (up to 21%), but the highest value recorded (20.7% lipids in diet, fish of 820 g) was only 3.6%. Poli et al. (2003) had previously reported meagre muscle fat-content values from 2.06% (fish sampled in November) to 2.93% (fish sampled in July). Grigorakis et al. (2011) found 1.06% content in 1280-g meagre sampled in December and fed with a commercial feed of 10% fat. In all cases, these low values confirm the high capacity of this fish to maintain reduced fat accumulation in muscle, despite being fed fat-rich diets, apparently reflecting a peculiar metabolism which bears further study.

Not only the total fat content of a fillet but also its FA profile concern human consumers. At the beginning of the experimental period (sampling 0) the juveniles exhibited muscle with a FA pattern having the following main characteristics: a) PA, OA, and DHA as the figures most abundant in their respective groups of SFA, MUFA and PUFA; b) each of these groups represented approximately one third of total FA; and c) among the HUFA, a clear prevalence of the n-3 series with respect to n-6 series (ratio n-3:n-6 >> 1).

This situation could be considered representative of marine fish species in general (Lanari et al., 1999; Grigorakis et al., 2002; Yildiz et al., 2006), including meagre (Poli et al., 2003; Piccolo et al., 2008; Grigorakis et al., 2011). The high proportion of HUFAs n-3 is commonly attributed to the prevalence of these FA in the marine food web (Sargent and Henderson, 1995).

Frequently, factors such as water salinity, temperature, and physiological and dietary composition are considered the main determinants of possible changes in the fish FA profile. In this case the environmental salinity did not change appreciably throughout the experiment while temperature fluctuated only slightly (Figure 1). An increase in the proportion of HUFAs in fish lipids has been considered to be part of the process of adaptation to lower water temperatures, although data vary among species and type of lipids (neutral vs. polar lipids) (Greene and Selivonchick, 1987; Henderson and Tocher, 1987). In this case (Table 2A, 2B), samples taken in the warmer season (samplings 2–3) were compared with those from colder one (samplings 5 and 9) showed no consistent differences in total HUFA levels or in EPA, DHA or AA. Data from other Mediterranean fish species (Yildiz et al., 2006; Senso et al., 2007) agree in that seasonal differences in the FA profile are minor or inexistent at least for the individual FA or FA groups cited above. Poli et al., (2003) detected some differences in the FA content of cultured meagre sampled in November and May-July (although the authors reported that water temperature varied from 17.2 to 28.6 °C throughout the experiment, no data on water temperature in sampling days were provided) and thus n-3 PUFA, but also total SFA, were higher in fish sampled in November while total MUFA reduced in that season. The relatively high temperatures, even in winter, in the Mediterranean and adjacent Atlantic areas could underlie this lack of marked seasonal effects.

On the other hand, it is difficult to define with precision the possible effects of water temperature upon the fish FA profile, when the corresponding studies are based on successive samplings from wild and/or farmed fish, since several biological variables such as fish age, reproductive status, changes in dietary composition (natural prey or artificial feed) could mask strictly abiotic environmental ones such as temperature.

In our assay, the most significant change over the whole period was perhaps the marked reduction in the n-3:n-6 ratio from an initial value close to 4.0 to a ratio of only about 1.0. This resulted from both a reduction in n-3 PUFA, mainly EPA, and a rise in LA content, the latter increasing by a factor of more than 3. Other marked changes affected the MUFA group, with a reduction in POA, 16:1n-7 and an increase in OA content. As a general trend, these changes were noticeable during the first part of the experiment, with a certain trend to stabilize thereafter.

Once again, the transition from the Microbaq diet, especially rich in n-3 HUFA (based on fish oil, as in commonly used hatchery diets), to the second diet used here (Alphamar), which incorporated higher levels of vegetable oils (as deduced from its high OA and LA levels, and the extremely low n3:n6 ratio, Table 1) was undoubtedly the main cause of the observed changes.

Figures 2–4, which display the time course of the fish-to-diet ratio (FtoD) for several representative individual or grouped FA, clearly show the convergence between FA composition of fish and diet for LA, 18:2n6 (the main figure in a lot of vegetal oils) and n-3 HUFA (the most abundant in fish oil). Both circumstances have reduced the FtoD n-3:n-6 ratio by a factor of close to four (Figure 4); a clear reduction in this ratio has also been detected in muscle of fish fed on diets with high proportions of plant oils in sea bass and sea bream (Grigorakis, 2007). Concerning individual n-3 HUFA, it is evident that fish have preserved the muscle levels of DHA at about two-fold higher that the dietary ones, while EPA FtoD showed a notable reduction to values close to one. This situation could be indicative of a specific greater need for DHA that could be accompanied by a conversion from EPA to DHA when the dietary supply of DHA was insufficient to meet the fish metabolic requirements. The practical disappearance of the initially reduced levels of other ▵6 desaturated n-3 precursors, such as stearidonic acid (SA, 18:4n-3) and eicosatrienoic acid (ETA, 20:3n-3), was also noticeable. The possible metabolic stress associated with this situation should be considered when formulating diets of low fish/high vegetal oil for fish culture, since not only muscle composition but fish growth and health could be negatively affected.

When the time course of the indices for fish-flesh lipid quality was considered (Table 2B), the situation proved unclear. In fact, FLQ became worse mainly due to EPA reduction, but when other HUFAs were included in the evaluation, such as in IA and IT, the negative effects of n-3 HUFA and n-3:n-6 reductions were partially counteracted by the beneficial effects of both lower SFA and higher MUFA and n-6. Previous studies with this species (Poli et al., 2003; Piccolo et al., 2008; Grigorakis et al., 2011) reported values of IA from 0.38 to 0.69 and IT values from 0.09 to 0.39 in the range of those found in this experiment and found in other cultured marine fish (Grigorakis, 2007).

The data compiled confirm the low fat content in meagre muscle in relation to other marine species, although values increased during culture. On the other hand, the muscle-fat FA content can be considered healthy for human consumption, values being similar to those found for cultured sea bass and sea bream, although when the culture plans include the use of plant-based lipid diets, the mixture of vegetal replacers of fish oil must be carefully designed. The use of high-HUFAn3 ending-diets could also be taken in consideration.