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Within GAPI, energy consumption is divided into two categories: Industrial Energy and Ecological Energy. Industrial Energy is energy as we commonly know it—resources such as petroleum and hydroelectric power that are used by aquaculture producers to support fish-farming activities. Ecological Energy is not as familiar a concept, however. It is a measure of how much energy, or net primary productivity (NPP), is imbedded in the feed that is consumed by farmed fish.
Fish meal and fish oil continue to play an integral role in fulfilling the nutritional needs of aquaculture species, particularly for carnivorous marine finfish species (Deutsch et al. 2007; Kristofersson and Anderson 2006; Naylor et al. 1998; Naylor et al. 2000; Tacon and Metian 2008). While the Sustainability of Feed (FEED) indicator measures the amount and sustainability of the wild fish that goes into farming marine finfish, the Ecological Energy indicator (ECOE) focuses specifically on how much ecological energy these systems take out of the environment (i.e., NPP). Values for NPP can be easily obtained by converting the amounts of feed ingredients consumed by each species into grams of Carbon (g C) consumed per kilograms (kg) of farmed fish (Tyedmers 2000).
So, what is ECOE measuring exactly? Energy is converted up the food chain, from solar energy (sunlight) into forms that are biologically consumable. The ECOE indicator measures the magnitude of photosynthesis diverted from the ecosystem and appropriated by an aquaculture production system. Conversion of energy up the food chain is not 100% efficient, however. As energy is converted from plants to successively higher positions on the food chain (i.e., trophic levels), there is always a net loss of usable energy (second law of thermodynamics). In wild systems, roughly 10% of the energy contained in the biomass of the prey is converted to the biomass of the predator. Thus 90% of the energy is lost in the transfer. The higher the trophic level, the greater the net primary productivity needed to produce the fish and the higher the ECOE score. For example, the production of one kilogram of adult cobia requires the equivalent of approximately 10,000 kg of photosynthetic plankton.
While the case can be made that fishery by-products, by-catch, and discarded fish are more sustainable feed alternatives than using wild fish meal and fish oil directly, all of these sources still require the removal of wild fish from the ecosystem and have associated ecological ramifications. Thus, GAPI includes within the ECOE calculation all wild sources of fish feed and oil, no matter how the fish are caught or the initial destination.
It is also necessary to account for the net primary productivity of the agriculture and livestock components of feed. GAPI uses the NPP of poultry as a proxy for the NPP of all livestock, since chicken is a major protein input of feed and typically displays similar feed conversion rates to swine and slightly higher than cattle (Tyedmers 2000). For the plant proportion, a composite value is used, derived from NPP values for wheat, corn, and soy.
ECOE = Net Primary Productivity of Feed InputsNet Primary Productivity of Feed Inputs (NPP) = (m/9) x 10(T-1) (Tyedmers 2000), where 9 represents a conservative 9:1 conversion ratio from wet weight to carbon conent
m = Mass of feed components (mT)
T = Trophic level of the feed components
Units: NPP (in mT Carbon)
Sample Normalised Calculation: Atlantic Salmon from Chile, 2007
GAPI employed the following decision rules to treat gaps in ecological energy data: