Is Fisheries Production within Large Marine Ecosystems Determined by Bottom-up or Top-Down Forcing?

C. J. McOwen, W. W. L. Cheung, R. R. Rykaczewski, R. A. Watson, L. J. Wood, (In Press). Fish and Fisheries.

Understanding the mechanisms driving fisheries production is essential if we are to accurately predict changes under climate change and exploit fish stocks in a sustainable manner. Traditionally studies have sought to distinguish between the two most prominent drivers, ‘bottom-up’ (resource driven) and ‘top-down’ (consumer driven), however, this dichotomy is increasingly proving to be artificial, as the relative importance of each mechanism has been shown to vary through space and time. Nevertheless, the reason why one predominates over another within a region remains largely unknown. To address this gap in understanding we identified the dominant driver of commercial landings within 47 ecosystems, encompassing a wide range of biogeochemical conditions and fishing practices in order to elucidate general patterns. We show that bottom-up and top-down effects vary consistently with past fishing pressure and oceanographic conditions, bottom-up control predominates within productive, overfished regions and top-down in relatively unproductive and under-exploited areas. We attribute these findings to differences in the species composition and oceanographic properties of regions, together with variation in fishing practices and (indicative) management effectiveness. Collectively, our analyses suggest that despite the complexity of ecological systems it is possible to elucidate a number of generalities. Such knowledge could be used to increase the parsimony of ecosystem models and to move a step forward in predicting how the global ocean, particularly fisheries productivity, will respond to climate change.