Surplus Production Implications Surplus production (Y) is the production of biomass beyond what is needed to maintain current population size If a fishery only catches the surplus production, then the population size remains If a fishery catches more, then the population shrinks If it catches less, then the population grows
Surplus Production Implications Surplus production has a maximum at about half of unexploited population size B ∞ Keeping a population at 0.5 B ∞ allows catching MSY forever A population is kept stable if the fishing rate F equals the intrinsic growth rate r t MSY is reached at ½ B ∞ and ½ r max Thus, MSY is reached at F msy = ½ r max
The Law of the Sea The Law of the Sea (UNCLOS 1982) requires all signatories to maintain fish stocks at levels that can produce the maximum sustainable yield MSY Thus, the fishing rate F may not exceed F msy
Time series of F in Europe Froese & Proelß (2010)
Time series of Biomass in Europe Froese & Proelß (2010)
Status of European Fish Stocks 14 Landings from ICES 2010, MSY from Froese and Proelß 2010
Status of European Stocks Move stocks here. Landings may then be 60% higher Froese et al. (2011)
Why is Management so Bad? Depending on the productivity of the species (r max ), about 20% of the biomass can be taken sustainably every year Therefore, about 80% of the fish have to be left in the water, although they are easily accessible There is only one problem
“Heringszaun”, Kappeln, Germany Fish weirs in rivers typically cover 60-100% of the river width, facing toward the sea, so they catch the fish before they can spawn. And there used to be many of these.
Exercise Go to www.ices.dk, select Community, Advisory Process, Latest Advicewww.ices.dk Search for a stock for which ICES gives F msy Use graphs to discuss trends in biomass and catch as a function of F / F msy What happens if F > r max ?