2.1.1 Released carbon from ingested food
According to the bioenergetics model for fish (Warren & Davis, 1967), ingested food carbon will be allocated to physiological compartments such as defecation, respiration, excretion and assimilation (Figure 1). Specifically, part of the ingested carbon is absorbed across the fish gut wall after digestion. The unabsorbed food carbon is transformed and defecated as feces. The absorbed carbon is first allocated to the metabolic pool (e.g., blood and liver), which has a fast turnover rate. Then part of the absorbed carbon is further assimilated and incorporated into the fish body (e.g., white muscle), i.e., the structural carbon pool, which has a slow turnover rate. The unassimilated part of the absorbed carbon in the metabolic pool is directly catabolized and excreted. Thus, the assimilation efficiency (AE) is the fraction of ingested food that is incorporated into the body. The unassimilated food carbon is released into water as DOC, CO2, and PC.
Therefore, the carbon budget model for ingested food is simplified toC = FC release + A, where C is the ingested food carbon, A represents the assimilated carbon, andFC release represents the carbon released from ingested food, i.e., the food carbon release.