The question being investigated by the working group--the effect of climate variability on cod stock fluctuations--is simply stated, but involves many different scientific subdisciplines and scales of investigation, from the effects of small-scale turbulence on encounter rates between individual fish larvae and their prey, to the effects of interdecadal changes in windfields on large-scale circulation and transport of heat and young fish. In spite of the complexity of the processes by which variable physical forcing may affect cod stocks, the effects of climatic variability on fish abundance has been demonstrated for several cod stocks. For example, periods of low temperature cause stock decline at the northern limits of the range (Barents Sea, Greenland); particular hydrographic and wind conditions result in unusual transport of eggs and larvae (Iceland-Greenland) or flush and re-aerate deoxygenated basins where cod spawn (Baltic). These examples combine empiricism, an increasing understanding of ocean/climate variability, and detailed knowledge of processes during the life history (mainly early life history) of cod. They provide reason for believing that the question is not wholly intractable, and that we may be able to predict at least the broad direction of changes in stocks under different physical regimes.
The GLOBEC approach provides a conceptual framework within which studies at different scales can be nested. It is obvious from the reports of work in progress or in planning (for U.S. GLOBEC, see Report by W. Peterson) that a great deal of research is already underway that is very relevant to Cod and Climate Change, because of the focus on cod recruitment and studies of copepod dynamics. The challenge for this working group was to identify themes and approaches which strengthen and facilitate the scientific programs being followed at individual, laboratory or national levels, and to propose and initiate relevant regional or international studies which can be better implemented within ICES and GLOBEC.
A number of unifying themes were discussed relative to the scale--large, intermediate, small--of the physical processes affecting them. Large-scale processes range from global to regional (e.g., Georges Bank, the North Sea) and include long-term changes in atmospheric and oceanic dynamics which affect cod stocks through changes in heat and transport. Intermediate-scale processes include eddies, rings and fronts, which have effects due to localized aggregation, retention and enhancement of plankton production. Small-scale processes include water column stability and turbulence, which affect plankton production and encounter rates of predator and prey. The working group attempted to identify key interactions between physical and biological processes at all of these scales. The intent was to strengthen the rationale for physical and biological oceanographers and fisheries biologists to work together from an early stage in developing models and observation programs. The categorization by scale is not rigid, because, for example, large-scale events, such as anticyclones, are often a major source of variability at smaller scales, and models of intermediate-scale features, such as fronts, may be sensitive to boundary conditions generated at larger scales.
The need to consider cod in the context of the full multi-species ecosystem and possible models and methods for accomplishing this task were discussed. Although variability in cod stocks is the principal issue, there are compelling reasons to include other species in the investigations when their dynamics interact with cod, or if they are more sensitive indicators of ecosystem change than cod.
The working group recommended that two special workshops be held in 1994 to plan and carry out retrospective analyses of physical and biological data, including considering the case for setting up a Cod and Climate database. Other recommendations propose topics for research which should be considered as themes for future workshops, or joint sessions of committees at ICES meetings. There are also a number of proposals for specific action in relation to existing monitoring programs (e.g., the CPR program), or the initiation of new regional programs. Finally, the consensus of the meeting was that the activities of the working group could be carried forward effectively by the special workshops in 1994, and that the group should meet next in 1995 to review progress and carry out further coordination, if required. Copies of the discussions held at this meeting can be obtained from Ms. Amy Freise, GLOBEC.International Secretariat, Chesapeake Biological Laboratory, P.O. Box 38, Solomons, MD 20688, USA (Internet: freise@cbl.umd.edu).
