Setting fish quotas based on holistic ecosystem modelling including environmental factors and foodweb interactions – a new approach Lars Ha˚kanson* and Andreas Gyllenhammar Department of Earth Sciences, Uppsala University, Villav. 16, SE 752 36 Uppsala, Sweden; *Author for correspondence (e-mail: Lars.Hakanson@geo.uu.se) Received 27 November 2003; accepted in revised form 8 March 2005 Key words: Aquatic systems, Baltic, Fish production, Fish quota, Foodweb modelling, Recovery, Thresholds Abstract 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 abi- otic/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 con- nected to overfishing and changes in environmental conditions, factors regulating recovery and methods for setting optimal fish quotas using this modelling approach. Introduction and aim There are numerous reports on declining fish stocks of several species from many parts of the world (e.g., cod in the Baltic Sea, the North Sea and along the Atlantic coast of Canada; Hutchings 1996; FAO 2001; Anonymous 2002). The traditional method to set fish quotas using fish catch statistics is highly uncertain (uncer- tainties up to a factor of four have been re- ported) and the quotas are also often violated (Karagiannikos 1996; Anonymous 2002). Many persons (about 1600 according to informal sources) are also directly and indirectly involved in setting fish quotas in Northern Europe for different species and areas. For example, the permitted quota for cod in the Baltic Sea in 2002 was 75 kt (75,000 tons) but initiated scientists and experts affiliated with the fishing industry have stated that the overfishing may be 50–100% of the legal quota. The fish production evidently depends on the survival of the roe and young Aquatic Ecology (2005) 39:325–351 Ó Springer 2005 DOI 10.1007/s10452-005-3418-x