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Abstract:Life cycle assessment is used in this study to evaluate the potential environmental impacts of producing gilthead seabream in an offshore sea farm exposed to storms in the Spanish Mediterranean Sea. The farming methods used can be considered as "basic" since no centralized or automatic food distribution system exists and there is no control system. This study aims to identify the main hotspots under these conditions and to propose and compare viable alternatives to them. Contribution analysis found that the component of the system with the greatest potential environmental impact (48% of the overall impact) was fish feed, especially the raw material used. Other contributory factors were the fuel consumed by the vessels operating in the farm (35%), the dumping of N and P in the environment due to the metabolism of the fish (12%), the cages and their anchorage system (5%). A sensitivity analysis showed that a significant reduction in potential environmental impact can be achieved by increasing feeding efficiency. Feed formulation in raw materials is also an important factor and could serve to diminish overall adverse effects. A balance needs to be found between productive performance (growth, survival and feed conversion rate), feed price and its influence on production costs and the overall environmental consequences.

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"Taxocene surrogation" and "taxonomic sufficiency" concepts were applied to the monitoring of soft bottoms macrobenthic assemblages influenced by fish farming following two approaches. Polychaete assemblage evaluation through multivariate analysis and the benthic index BOPA were compared. Six fish farms along the Spanish Mediterranean coast were monitored. Polychaete assemblage provided a suitable picture of the impact gradient, being correlated with total free sulphides. BOPA did not support the impact gradient described by the polychaete assemblage, providing erroneous categorizations. The inclusion of several polychaete families, which were locally identified as indicative of affection to recalculate BOPA, resulted in an improved diagnosis and correlation with the impact gradient. Nevertheless, frequent misclassifications occurred. These results suggest that the structure of polychaete families, sulphides and granulometry conform an appropriate strategy for fish farming monitoring. Biotic indices need to be specifically designed for concrete activities, and regionally validated, because of the environmental plasticity of benthic invertebrates.

Settling velocity and total ammonia nitrogen leaching from commercial feed and faecal 50Several studies under laboratory conditions reported that 250-300 g kg -1 of ingested food is 51 voided as faeces (Butz and Vens-Cappell, 1982). Just as with food losses, a continuous 52 4 improvement in diet elaboration has led to a gradual reduction of faecal discharges to about 53 100-250 g kg -1 of ingested food (Cho et al., 1994; Talbot and Hole, 1994 Watling, 1994; Elberizon and Kelly, 1998; Chen et al., 1999a,b; Chen et al., 2003) MRAs for feed pellets (Table 3) showed that water temperature and pellet density had no (Table 5). Constant k was significantly higher at 25 ºC (P < 0.05) and in smaller feed pellets 215 (P < 0.05), while t a was significantly larger at 15 ºC (P < 0.05) but also for smaller pellets (P 216 < 0.05) ( 246The influence of pellet weight was also identified by other authors (Elberizon and Kelly, 247 1998; Chen et al. 1999b, Sutherland et al., 2006. In this study, we not only found that initial 248 pellet size determined settling velocity but also, unlike Chen et al. (1999b) authors state, the range of parameters studied was not so critical to the settling velocity. 270Despite this, we observed greater but non significant settling velocities at low temperature, as 271 Chen et al. (1999b) noted. These authors suggested that this could be due to the influence of 272 temperature on pellet density, although in our experiments we found that seawater 273 temperature did not affect pellet density.