Marine ecosystems operate as pyramids wherein the primary production generated at trophic level (TL) 1.0 is moved up toward the higher trophic levels, with a large fraction of that production being used-up in the process of maintenance, reproduction and other activities of the animals in the systems (Pauly and Christensen, 1995). Thus, notwithstanding humanities general preference for catching and consuming large predators, deliberately fishing down should enable more of an ecosystem’s biological production to be captured by fishing. However, to avoid waste here as well, any decline in the mean trophic level of the fisheries catches should, in this case, be matched by an ecologically appropriate increase in the amount of these catches, the appropriateness of that increase being determined by the transfer efficiency between trophic levels. The default transfer efficiency value is 0.1 (i.e., 10% of production is transferred to the next trophic level), which is the mean from a number of marine ecosystems (Pauly and Christensen, 1995), but can be replaced by any realistic value between 0.01 and 0.30.

Thus, a Fishing-in-Balance (FiB) index can be defined that has the property of increasing if catches increase faster than would be predicted by trophic level declines, and that decreases if increasing catches fail to compensate for a decrease in trophic level. This is due to the fact that, in the absence of geographic expansion or contraction, and with an ecosystem that has maintained its structural integrity, moving down the food web should result in increased catches (and conversely for increasing trophic level), with the FiB index remaining constant.

Examination of various case histories (e.g., Bhathal, 2005; Pauly and Palomares, 2005) shows that the FiB index increases where geographic expansion of the fisheries is known to have occurred. Spatial expansion is one of the masking factors which, until recently, have shrouded the decline of global fisheries (but see Swartz et al., 2010), and thus the potential importance of being able to quantify this process, even if crudely. This is facilitated, on the Sea Around Us Project website, by the presentation of an expansion factor (i.e., the antilog of the FiB index) being included in the table output. Further research into approaches to incorporate spatial expansion into estimation of fishing down indices is ongoing.