The functioning of marine ecosystems: a fisheries perspective.

Abstract

There is considerable evidence that environmental variability plays a major role in controlling abundance and distribution of marine populations and that fisheries alter ecosystem functioning and state. This overview documents emergent, i.e. visible to us as observers, ecosystem-level ecological patterns and addresses important questions regarding the exploitation of marine resources. Do marine ecosystems function differently from terrestrial systems? Do multiple stable states exist in marine ecosystems? Does removal of top predators in marine ecosystems result in fundamental changes in the plankton communities (top-down 'trophic cascades'), as observed in lakes? Alternatively, are marine ecosystems characterized by bottom-up control such that fishing predatory fish does not disturb community structure and function? Does heavy exploitation of forage species, such as anchovies and sardines, cause changes in the functioning of upwelling ecosystems? The key to answering these questions and exploring whether general principles apply lies in understanding the energy flow within the ecosystems. The chapter reviews different types of energy flow in marine ecosystems, i.e. bottom-up control (control by primary producers), top-down control (control by predators) and wasp-waist control (control by numerically dominant species). No general theory can yet be ascribed to the functioning of marine ecosystems. Ecological understanding and models of ecosystem functioning are provisional and subject to change, and common sense is not sufficient when studying complex dynamic systems. However, tentative and partial generalizations are proposed, namely that bottom-up control predominates; top-down control plays a role in dampening ecosystem-level fluctuations; trophic cascades seldom occur; and wasp-waist control is most probable in upwelling systems. Moreover, alternation and large-scale synchronized fluctuations in fish stocks, stability of fish communities and emergent features such as size spectra are potentially important patterns when assessing states and changes in marine ecosystems. New and meaningful indicators, derived from our current understanding of marine ecosystem functioning, can be used to assess the impact of fisheries and to promote responsible fisheries in marine ecosystems.