32 DECEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG (trophic level of 3.79). Seabream are found in seagrass beds, sandy bottoms and the surf zone, commonly to depths of about 30 m, but adults may occur to 150 m depth. Seabream are sedentary, either solitary or in small aggregations. Seabream are mainly carnivorous, accessorily herbivorous, feeding on shellfish, including mussels and oysters (trophic level of 3.26). Competition for trophic resources on seagrass meadows will depend on the carrying capacity of natural ecosystems. This situation may arise after a large escape event in an area where wild populations are near the ecosystem carrying capacity. Genetic Impacts of Escaped Fish Most studies on the genetic impact of escapes have focused on Atlantic salmon. There is an overall lack of knowledge regarding genetic effects of seabass and seabream escapees on wild populations. Escapes have occurred since the 1990s, without evidence of deleterious effects on wild populations, at least at present. Fisheries professionals have not raised an alarm over the situation with seabass and seabream catch, inasmuch as escapes are producing a low intensity risk. However the lack of evidence could also be an indication of uncertainty and not the absence of effects. The research effort carried out during the last year has reduced the level of uncertainty to medium. The typology of marine fish reproduction minimizes the genetic impacts derived from escapes because most species have exceptionally high dispersal and migratory capabilities, wide geographic distributional range and very high fecundities with pelagic dispersion. Nonetheless, striking differences in genetic structure have been found in certain cases (CIESM 2007). It is reasonable to expect that wild populations of species such as seabass and seabream will be less sensitive to eventual genetic swamping from the escape of caged fish. This assertion is based on the large population sizes and high dispersal capabilities through the larval stage of seabass and seabream. Genes providing “good” or “bad” fitness are constantly been brought about by long range gene flow, especially in marine species, added or removed locally by strong selection pressures. At present, broodstock are derived primarily from wild populations. The apparent lack of geographically linked genetic structuring within the species decreases the risk for adverse genetic interactions from escaped fish or intentional population displacements (ICES WGEIM 2006). However, selective breeding is one of the key priorities of seabream aquaculture for the European Union and some other countries have selective and domestication breeding programs. This can affect the genetic structure and increase risk for future interbreeding with wild populations. Also, the common practice for farmers to use breeders from different geographic origin could effect changes in allele frequencies and/ or the introduction of non-native alleles into local populations in case of escapes from culture facilities, in particular from floating cages, or restocking programs (Sola et al. 2007). Future concerns may occur if differences between wild and farmed populations are large. Landings of seabream from fisheries at Mediterranean and Atlantic ports is relatively constant year to year, with between 7,000 to 8,500 t (FAO, www.globefish. org). Aquaculture production is 94.4 percent of total seabream production, with 139,925 t in 2010. Seabass have a similar pattern, with a world culture production of 118,931 t (FEAP 2010). The seabass fishery landed 8,000 to 12,000 t per year, only 10 percent of the annual production. Consequently wild populations of seabass and seabream could be much smaller compared to farmed populations, especially in areas where farms are concentrated, with production up to 4,000 t per year in relatively small areas. Escape rate and receiving population size are critical questions for defining environmental impact of escapes. The magnitude of the environmental impact can increase progressively over time as wild populations become more important than wild populations of seabass and seabream. Future Scenarios for New Species: Meagre and Rabbitfish as Case Studies New economic resources are being provided by the European Union for diversification of species and products in European aquaculture. There is a research effort to develop new species to culture because finfish aquaculture in Europe is largely dominated by a few species. Therefore, in the future, potential for growth might also depend on the capacity of the sector for sustainable exploitation of aquatic biodiversity through species diversification. Species selection for aquaculture development often poses an enormous challenge for decision-makers. Often, species selection considers criteria such as aquaculture potential (availability of juveniles or breeders from the wild), adaptation to local environmental conditions, rearing potential, growth rate, consumer acceptance and flesh quality (Quéméner et al. 2002), without considering the potential negative impact of escapes on ecosystem services. The lack of information about how emerging species behave after an escape may pose future problems with escaped individuals, especially if escaped fish represent a species characterized by a high trophic level, large maximum size and a voracious appetite. FIGURE 3. Typical array of cages in a Mediterranean fish farm of seabream and seabass (Photo: P. Sanchez-Jerez).
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