Report of the Workshop on the Impact of Zooplankton on Cod Abundance and Production (WKIZC)

Evidence of large scale changes in planktonic ecosystems and consequences of this for other trophic levels, including fish, has greatly improved. Relationships between indicators of plankton variability and fluctuations in recruitment or growth are in most cases supported by information about the underlying processes. The effects of physical and biological forcing on cod are not necessarily either linear or additive, and reductions in stock biomass due to inten-sive fisheries may have increased their sensitivity to climatic fluctuations.
Many research groups are currently engaged in efforts to build coupled biophysical models including the spatial and temporal interactions between zooplankton and larval cod, i.e. the integration of general circulation models with biological formulations of growth, feeding and behaviour of larval fish. These models are the main tools to integrate research from laboratory studies on sensory ecology, environmental effects on feeding and growth processes with large-scale fluctuations in oceanography and productivity of marine ecosystems driven by climatic forcing. The models are maturing, but still have important limitations, e.g. in the representa-tion of small scale predator-prey interactions, the distribution of prey at a sub-grid scale and in realistic representation of larval behaviour and physiology.  
Important results are also emerging from simpler models. For instance, comparisons between lab-derived growth models and field data suggest that surviving larvae grow near their physio-logical maximum (limited by temperature).
The preferred prey-size of larval cod seems to be proportional to larval length, deviations are due to absence of large prey items such as Calanus, and for cod 15-25 mm long the presence of large prey may be crucial. Both larvae and zooplankton appear to be concentrated in fronts. Although Calanus finmarchicus is a major component of the larval diet in most stocks, Pseu-docalanus occupies this role at the warm end of the species range and particularly in the Bal-tic.
Recruitment to the Faroe Plateau cod stocks appears to be governed by a tightly coupled  tro-phic chain, from nutrients through phytoplankton and zooplankton to forage species, such as sandeels.  In other areas, such as the Baltic, the processes are considerably more complex, with large-scale, long-term changes in the physical and chemical environment causing inter-related fluctuations in the populations of cod and the two pelagic fish species.  
It remains difficult to pull the detailed, process information which has emerged from large scale national and regional GLOBEC programmes into a form which finds a use in ICES fish stock assessments. The approach in such assessments is to use fixed stock-recruit relationships as a guide to short and long-term management, with some sensitivity analysis to explore the consequences of alternative stock-recruit relationships i.e environmental variability is treated as noise.