Collie Jeremy S.
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PreprintEnd-to-end foodweb control of fish production on Georges Bank( 2009-05-06) Collie, Jeremy S. ; Gifford, Dian J. ; Steele, John H.The ecosystem approach to management requires the productivity of individual fish stocks to be considered in the context of the entire ecosystem. In this paper, we derive an annual end-to-end budget for the Georges Bank ecosystem, based on data from the GLOBEC program and fisheries surveys for the years 1993-2002. We use this budget as the basis to construct scenarios that describe the consequences of various alterations in the Georges Bank trophic web: reduced nutrient input, increased benthic production, removal of carnivorous plankton such as jellyfish, and changes in species dominance within fish guilds. We calculate potential yields of cod and haddock for the different scenarios, and compare the results with historic catches and estimates of maximum sustainable yield (MSY) from recent stock assessments. The MSYs of cod and haddock can be met if the fish community is restructured to make them the dominant species in their respective diet-defined guilds. A return to the balance of fish species present in the first half of the 20th century would depend on an increase in the fraction of primary production going to the benthos rather than to plankton. Estimates of energy flux through the Georges Bank trophic web indicate that rebuilding the principal groundfish species to their MSY levels requires restructuring of the fish community and repartitioning of energy within the food web.
PreprintBalancing end-to-end budgets of the Georges Bank ecosystem( 2007-05-09) Steele, John H. ; Collie, Jeremy S. ; Bisagni, James J. ; Gifford, Dian J. ; Fogarty, Michael J. ; Link, Jason S. ; Sullivan, B. K. ; Sieracki, Michael E. ; Beet, Andrew R. ; Mountain, David G. ; Durbin, Edward G. ; Palka, D. ; Stockhausen, W. T.Oceanographic regimes on the continental shelf display a great range in the time scales of physical exchange, biochemical processes and trophic transfers. The close surface-to-seabed physical coupling at intermediate scales of weeks to months means that the open ocean simplification to a purely pelagic food web is inadequate. Top-down trophic depictions, starting from the fish populations, are insufficient to constrain a system involving extensive nutrient recycling at lower trophic levels and subject to physical forcing as well as fishing. These pelagic-benthic interactions are found on all continental shelves but are particularly important on the relatively shallow Georges Bank in the northwest Atlantic. We have generated budgets for the lower food web for three physical regimes (well mixed, transitional and stratified) and for three seasons (spring, summer and fall/winter). The calculations show that vertical mixing and lateral exchange between the three regimes are important for zooplankton production as well as for nutrient input. Benthic suspension feeders are an additional critical pathway for transfers to higher trophic levels. Estimates of production by mesozooplankton, benthic suspension feeders and deposit feeders, derived primarily from data collected during the GLOBEC years of 1995-1999, provide input to an upper food web. Diets of commercial fish populations are used to calculate food requirements in three fish categories, planktivores, benthivores and piscivores, for four decades, 1963-2002, between which there were major changes in the fish communities. Comparisons of inputs from the lower web with fish energetic requirements for plankton and benthos indicate that we obtained reasonable agreement for the last three decades, 1973 to 2002. However, for the first decade, the fish food requirements were significantly less than the inputs. This decade, 1963-1972, corresponds to a period characterized by a strong Labrador Current and lower nitrate levels at the shelf edge, demonstrating how strong bottom-up physical forcing may determine overall fish yields.
PreprintComparing species and ecosystem-based estimates of fisheries yields( 2011-06) Steele, John H. ; Gifford, Dian J. ; Collie, Jeremy S.Three methods are described to estimate potential yields of commercial fish species: (i) single-species calculation of maximum sustainable yields, and two ecosystem-based methods derived from published results for (ii) energy flow and for (iii) community structure. The requirements imposed by food-web fluxes, and by patterns of relative abundance, provide constraints on individual species. These constraints are used to set limits to ecosystem-based yields (EBY); these limits, in turn, provide a comparison with the usual estimates of maximum sustainable yields (MSY). We use data on cod and haddock production from Georges Bank for the decade 1993-2002 to demonstrate these methods. We show that comparisons among the three approaches can be used to demonstrate that ecosystem based estimates of yields complement, rather than supersede, the single-species estimates. The former specify the significant changes required in the rest of the ecosystem to achieve a return to maximum sustainable levels for severely depleted commercial fish stocks. The overall conclusion is that MSY defines changes required in particular stocks, whereas EBY determines the changes required in the rest of the ecosystem to realize these yields. Species specific MSY only has meaning in the context of the prey, predators and competitors that surround it.