Setting fish quotas based on holistic ecosystem modelling including environmental factors and foodweb interactions – a new approach.

We present a new approach to set fish quotas from holistic aquatic foodweb modelling (the LakeWeb-approach). This modelling includes changes in environmental conditions (nutrients, salinity, temperature, oxygen), process-based mass-balance calculations of nutrient concentrations from inflow, internal processes and outflow, calculations of how changes in nutrient concentrations affect primary production, how such changes influence secondary production and how this influence fish production and biomass. This approach gives dynamic, quantitative responses to alterations in driving variables and abiotic/biotic feedbacks. We have applied this approach for preliminary simulations of the cod biomass in the Baltic. We also show that this approach adds a new dimension in setting fish quotas, which in the future could complement, rather than compete with, the more established methods used today based on fish catch statistics and models based on other presuppositions. Our preliminary results indicate that under present environmental conditions (2003), the cod is likely to be extinct if the annual catch is between 95 and 100 kt. The present fish quota is 75 kt/yr in the Baltic, but the overfishing may be 35 kt/yr. We discuss cause–effect relationships regulating fish production, key factors influencing thresholds and points of no return connected to overfishing and changes in environmental conditions, factors regulating recovery and methods for setting optimal fish quotas using this modelling approach.