The use of hatchery-reared fishes to enhance wild stocks (hereafter “restocking”) has attracted increasing interest (Cowx 1994, Munro and Bell 1997, Bartley and Bell 2008) due to dramatic decreases in yield and even the collapse of some fisheries (Wilby et al. 2009). A steadily growing number of species and countries have been involved in restocking programmes over recent years (Born et al. 2004). The annual budget invested in rearing hatchery fish and releasing them to the wild is not known precisely, but it is in the range of billions of dollars (Brown and Day 2002). Restocking has been criticized because it may be neither effective nor economically viable (Bell et al. 2006). Therefore, an appropriate restocking-based management strategy should employ a number of pilot studies to assess the actual effects of restocking on stock enhancement and to develop strategies that maximize the profit of stocking while minimizing its environmental impact and economic cost (Blankenship and Leber 1995).

The success of restocking programmes depends on how well the hatchery-reared fishes can adapt to natural habitat conditions and survive once released (Ireland et al. 2002). However, post-release survival may be low due to the inexperience of released animals in wild conditions (Iglesias et al. 2003, Sparrevohn and Støttrup 2007). Thus, releasing a very large number of juveniles is often needed to ensure the success of restocking programmes (Bartley and Bell 2008), which implies high cost. Moreover, post-release survival is mediated by the size of individuals at the time of release (Tsukamoto et al. 1989, Svåsand and Kristiansen 1990, Leber et al. 2005), and releasing undersized fish seems to be largely ineffective (Kristiansen et al. 1997). However, the size at which fishes are released is also strongly governed by economic constraints. Most of the budget of restocking projects is invested in production costs (Ungson et al. 1993). These costs include the maturation, hatchery, nursery and grow-out phases (Leung et al. 1993, Kam et al. 2002). Therefore, the longer the fish remain in captivity, the greater the cost to feed and house them (Tominaga and Watanabe 1998, Brown and Day 2002), and fewer fish may be released within a fixed budget. Consequently, the optimum release size would result from a trade-off between size-dependent survival rates, growth rates and production costs (Leber 1995).

In addition to fish size, maximizing survival of released fish depends on biological quality (Leber 1995, Tominaga and Watanabe 1998, Tsukamoto et al. 1999, Le Vay et al. 2007). Quality not only refers to the release of disease- and deformity-free fish but also implies particular physiological and nutritional characteristics. Physiological welfare can be evaluated by body condition, and nutritional quality is directly assessed by tissue composition (Pepper et al. 1992, Le Vay et al. 2007). Biological quality has been directly related to the quality of the diet used to feed the larvae or juveniles (Le Vay et al. 2007). Therefore, an optimal diet allows the production of quality fish and also provides good growth rates.

Here we examine the meagre, Argyrosomus regius (Asso, 1801). This fish, belonging to the family Sciaenidae, is distributed along the eastern Atlantic coast (from Norway to Congo) and throughout the entire Mediterranean (Chao 1986). In Mediterranean waters, the abundance of meagre has decreased alarmingly (Quéro and Vayne 1987, Sadovy and Cheung 2003). Indeed, this species is considered extinct in the Balearic Islands, where it was frequently captured by the artisanal fleet only a few decades ago (Mayol et al. 2000). Consequently, a consortium composed of policy makers from the local government (Balearic Government), researchers from LIMIA (Laboratori d’Investigacions Marines i Aqüicultura, Balearic Government) and IMEDEA (Instituto Mediterráneo de Estudios Avanzados, CSIC-UIB), and local fishermen (associations of artisanal fishermen and recreational fishing clubs) have launched a restocking programme.

As meagre is considered an emerging species in European aquaculture (Monfort 2010), numerous studies related to rearing conditions and growth have been conducted in recent years with a view to improving production (Piccolo et al. 2008, Roo et al. 2010, Estévez et al. 2011, Vargas-Chacoff et al. 2014, Velazco-Vargas et al. 2014). In contrast, the main objective of this study was to determine the optimal rearing protocol for obtaining juveniles suitable for restocking, considering both biological and economic aspects. To this end, growth, physiological quality (body condition indices), nutritional quality (tissue composition) and cost of seven different diets were compared to identify the optimal diet for producing juveniles of a desired release size at the lowest cost, thus facilitating the release of the greatest number of specimens.

Meagre were spawned in May at LIMIA facilities using protocols developed by LIMIA (Pastor and Grau 2013). Eggs were obtained by hormonal induction of meagre breeders captured in the Bay of Cádiz. During the hatchery phase, the larvae were reared under controlled conditions and fed a diet composed of rotifers (day 2-15) and Artemia sp. (day 8-30). The fish fry were fed commercial feed (Skretting^®, Burgos, Spain; day >30) and were maintained in tanks and later in small sea cages during the nursery and pre-grow-out phases.

The success of a restocking programme depends on complex tradeoffs, but in practice it could be considered a two-fold process. First, the post-release survival rate of hatchery-reared specimens is primarily dependent on the size/age and the quality of specimens at the time of release (Tsukamoto et al. 1999, Leber et al. 2005); larger fish tend to be more resilient. Second, success may depend on the number of fish released. However, both processes are limited by economic factors. The economic factors of restocking programmes have rarely been studied (Kellison and Eggleston 2004, Leber et al. 2005, Patrick et al. 2006), but play important roles in the recovery of several species.

While the size-dependent mortality rate of restocked meagre in the wild has been analysed elsewhere (Gil et al. 2015), here we focused on the methods for maximizing the number of fish to be released.

Growth of meagre is high during the grow-out phase and feed conversion rates (i.e. the proportion of food that is converted into biomass) are good (Duncan et al. 2013). Recently, meagre growth has been widely studied in different culture systems (Pastor et al. 2002, El-Shebly et al. 2007, Chatzifotis et al. 2010, Vargas-Chacoff et al. 2014) and with different diet compositions (Piccolo et al. 2008, Estévez et al. 2011, Chatzifotis et al. 2012, Velazco-Vargas et al. 2014) due to the increasing interest in meagre for diversification of aquaculture.

To compare the growth rate of juveniles that were fed with different diets was challenging in this study because fish showed a clear shift in growth rate following the winter (Fig. 1). As in most marine organisms (Pauly 1990, Alcoverro et al. 1995, Coma et al. 2000), the growth of A. regius appears to be influenced by seasonal changes. The growth rate clearly decreases at temperatures less than 16ºC (Quéméner 2002, El-Shebly et al. 2007). For this reason, the conventional von Bertalanffy growth model did not adequately fit the growth pattern of the juvenile meagre on the diets tested in this study. The observed growth pattern was better described by introducing a seasonal oscillation rather than using the conventional formulation (Somers 1988, García-Berthou et al. 2012, Gil et al. 2014b). This method yielded season-independent growth rates that were fully comparable among diets.

The diet experiments conducted with juvenile A. regius revealed that Diet G provided the best growth, because semi-moist diets appear to have better palatability than dry pellets. The second-best growth rate was obtained with Diet A (dry commercial pellets), but this diet minimized the cost of growing fish for any desired size at release (see below). The experimental diets (C, D and E) clearly underperformed when compared with the others. This result supports the need for completing pilot studies for comparing different rearing protocols.

The expected fitness of fish released into a natural environment is a result of both fish quality and size (Tsukamoto et al. 1999). There is a long history of research on hatchery technologies aiming to produce high-quality juveniles not only for aquaculture but also for stock enhancement (Fushimi 2001, Shields 2001). Studies on red sea bream, Pagrus major have shown that the quality of hatchery-reared juveniles depends primarily on the nutritional quality of the diet (Nakano 1996, Le Vay et al. 2007). Similarly, the current study showed that the biological condition of the fish (estimated from the relationships between length and total weight, liver weight and mesenteric fat weight) and the biochemical composition of the entire fish body were influenced by the diets used to feed juvenile meagre.

The biochemical composition of the fish body (specifically, crude fat and protein) was similar and optimal for diets A, B, F and G but deficient for diets C, D and E. The proximate composition of cultured fish is affected by endogenous and exogenous factors. The levels of protein and ash are primarily related to fish size (endogenous factors), whereas fat content depends on exogenous factors, such as diet (Shearer 1994). Although exceptions occur, muscle with relatively low lipid content is common in many demersal species. These species are more sedentary than pelagic carnivores, which typically show burst swimming behaviour and thus sustain higher lipid levels in muscle tissue to fuel this response (Sheridan 1988).

Weight-length relationships were used for comparing body condition based on the assumption that heavier fish of a given length are in better condition than thinner fish (Froese 2006). Poor conditions may lower the chances of survival because fish are more susceptible to predation and to a variety of environmental stressors (Lloret et al. 2002). According to these criteria, fish on Diet G exhibited the best condition, followed by those on Diet A. Note, however, that diets A, B, F and G all yielded a very similar total weight for a size at release of 30 cm. These total weights differed by only a few grams (Table 4). Additionally, note that the within-diet differences were large in comparison with the among-diet differences (Table 4).

In fish, liver is considered a major fat and glycogen storage organ, and can provide an additional way to estimate nutritional state (Adams and Greeley 2000). Accordingly, correlations involving liver weight and fish length are calculated to estimate condition in fish (Benejam et al. 2010). In the current study, the measurements of fish condition based on liver weight were highest and almost equal for diets A, B, F and G, indicating a higher accumulation of reserves. Chatzifotis et al. (2006) have suggested that the liver of the sciaenid brown meagre, Sciaena umbra, may serve as a storage organ for energy because of its relatively high lipid content (39-43%). Furthermore, the low lipid content of muscle (1-2.5%) observed in certain species of sciaenids (Poli et al. 2003, Hernández et al. 2009, Grigorakis et al. 2011) indicates the minor role of muscle as an energy storage tissue (Chatzifotis et al. 2006).

The experimental diets (C, D and E) clearly resulted in poorer growth, total weight and liver weight-length relationships. However, the mesenteric fat weight-length relationship yielded greater values than the other diets, indicating a substantial accumulation of mesenteric fat during the experiment. Piccolo et al. (2008) evaluated the effect of two diets with different protein/fat ratios (P/F) and found that, although the biometric traits analysed were not affected by the diet, the amount of mesenteric fat was significantly higher for diets with a lower protein/fat ratio. Too much dietary lipid may result in excessive fat deposition in the visceral cavity and tissues (Lanari et al. 1999). In fish, an appropriate energy-to-protein ratio in the diet contributes to the effective utilization of dietary proteins through the protein sparing effect (Watanabe 1982). Therefore, fish are able to utilize dietary lipids up to a certain level beyond which growth may be retarded because high energy intake depresses appetite before sufficient protein has been consumed to support growth (Ellis and Reigh 1991). However, meagre is a carnivorous species, feeding on Mysidacea, Decapoda and Teleostei in the wild (Cabral and Ohmert 2001, Gil et al. 2014a) (Chatzifotis et al. 2012), and does not appear to require high dietary lipid levels. At lipid levels greater than 17%, Chatzifotis et al. (2010) did not observe a sparing effect on protein. Therefore, the mesenteric fat weight-length relationship should not be viewed as an indicator of good condition.

In summary, fish condition and biochemical composition suggest that all the diets except the experimental ones (i.e. C, D and E) produce fish of similar quality and are fully suited for use in a restocking programme.

Economic effectiveness is one of the most frequently criticized aspects of restocking programmes, although most restocking programmes have yet to be fully assessed because this aspect has received little or no attention (Hilborn 1998). However, economic viability has been demonstrated in certain cases: e.g. release programmes for Pacific salmon Oncorhynchus sp. (Isaksson 1988), Japanese flounder Paralichthys olivaceus and red sea bream P. major in Japan (Kitada 1999, Okouchi et al. 2004, Kitada and Kishino 2006).

In our case, the costs of the grow-out phase showed marked among-diet differences. The high cost of the semi-moist diet and its required cold storage limit the use of Diet G (Kubitza and Lovshin 1997, Kim and Shin 2006). Diet A was the best option for minimizing production costs for any size at release because this diet has both a reasonable price (unlike Diet G) and good growth performance, allowing fish to spend less time in the grow-out phase before attaining the desired size. This last point is decisive because time savings imply reduced personnel costs, which represent the largest component of the total cost (see Table 5).

Despite the favourable results obtained with Diet A, the absolute cost of the grow-out phase was higher than the costs obtained in other studies of the entire production process (Svåsand et al. 2000, Leber et al. 2005, Patrick et al. 2006). However, it is important to note that LIMIA facilities are designed for research purposes. Continuous improvement in hatchery technology minimizes production costs (Sproul and Tominaga 1992, Ungson et al. 1993). so it is expected that productivity will be largely improved at facilities specialized in producing fish for restocking. Nevertheless, production capability is somewhat limited and producing large numbers of large specimens is unreliable given, for example, the spatial limitations of hatcheries (Kellison and Eggleston 2004). Ultimately, the annual budget imposes a limit to the number and size of the fish that can be produced.

In conclusion, Diet A was the best option for the production of juvenile meagre for a restocking programme. This diet was capable of producing quality juveniles while minimizing production costs for any possible size at release and maximizing the number of juveniles that could be produced on a fixed budget. However, other factors that need further study include the magnitude of the release (i.e. the critical number of released fish), the survival of released fish (Gil et al. 2015) and the economic value of the fish (fisher profits). All of these factors must be considered for a suitable assessment of the cost-effectiveness of a restocking programme. Adapting and improving the programme in light of the results of pilot studies and incorporating the opinions of different stakeholders into decision making may successfully facilitate the design of restocking programmes that are socially acceptable, economically viable and environmentally sustainable (Bartley and Bell 2008).