An Indicator for ecosystem externalities in fishing

Ecosystem externalities arise when one use of an ecosystem affects its
other uses through the production functions of the ecosystem.We use simulations from a size-spectrum ecosystem model to investigate the ecosystem
externality created by fishing of multiple species. The model is based
upon general ecological principles and is calibrated to the North Sea. Two
fleets are considered: a “forage fish” fleet targeting species that mature
at small sizes and a “large fish” fleet targeting large piscivorous species.
Based on the marginal analysis of the present value of the rent, we develop
a benefit indicator that explicitly divides the consequences of fishing into
internal and external benefits. This analysis demonstrates that the forage
fish fleet has a notable economic impact on the large fish fleet, but the
reverse is not true. The impact can be either negative or positive, which
entails that for optimal economic exploitation, the forage fishery has to
be adjusted according to the large fish fishery. With the present large fish
fishery in the North Sea, the two fisheries are well adjusted; however, the
present combined exploration level is too high to achieve optimal economic
rents