Farming up Mediterranean food webs.

One of the effects of fisheries on ecosystems is that they tend to remove the larger, higher-trophic-level species and thus progressively lower the mean trophic level (TL) of the landings. This effect is known as “fishing down the food web” (Pauly et al. 1998; but see also Essington et al. [2006] for addition of low-TL fisheries instead of decline in high-TL fisheries, termed “fishing through marine food webs”). At the same time, however, mariculture efforts, originally devoted mainly to low-TL invertebrates, such as mussels or oysters (see Bardach et al. 1972), are increasingly concentrating on producing high-TL fish (i.e., “farming up food webs”; Pauly et al. 2001). Indeed, the mean-weighted TL of mariculture products in countries such as Chile, Canada, Norway, and the United Kingdom increased since 1970 (Pauly et al. 2001). Here, we used Food and Agriculture Organization (FAO) mariculture (brackish aquaculture was excluded) production data (Aquaculture Production: Quantities 1950–2004 database for mariculture; FishStat Plus, version 2.31) and estimates of the fractional TL of the farmed species as given in FishBase (www.fishbase.org; Froese & Pauly 2007) and Stergiou and Karpouzi (2003) to assess the trophic signature of the Mediterranean marine aquaculture industry. In aquatic ecosystems TL can have values of between 2.0, for herbivores and detrivores, and >4.5, for piscivores and carnivores (see www.fishbase. org for methods for its estimation). In 2004 (the most current statistics available), mariculture production of farmed species in the Mediterranean was dominated by the Mediterranean mussel (Mytilus galloprovincialis) (38%), gilthead seabream (Sparus aurata) (29.5%), seabasses (Dicentrarchus labrax and D. punctatus) (27%), and, to a lesser extent, oysters (Ostrea spp. and Crassostrea spp.) (3.5%). The production of herbivorous species (TL = 2), such as bivalves (mussels and oysters), increased from approximately 2,000 t in 1970 to over 100,000 t in 2004. At the same time, the mariculture of intermediate predators (TL = 3.1–4.0; e.g., S. aurata,