Developing tools to incorporate ecosystem considerations into management (KB-24-005-026)

The project aimed at developing management rules for an “ecosystem approach to fisheries management”. We focused on two ecosystem effects influencing the dynamics of fish stocks : environmental regimes and trophic interaction. The blue whiting / hake complex was taken as case study. The study was based on a multispecies model , GADGET, in which the recruitment dynamics of blue whiting was varying between low productivity and high productivity regimes.

 

Management targets (Fmsy) were derived in three different ways : 1) assuming constant productivity and no interaction between the stocks, 2) assuming 2 recruitment regimes (productivity) for blue whiting and no interaction, 3) assuming 2 recruitment regimes for blue whiting and trophic interactions with hake.

 

Managing the 2 stocks using an adaptive strategy (changing the management target for blue whiting depending on the environmental regime) did not improve management. The only minor benefit was a reduction of the risk for the stocks to fall below Blim (which was already low). There was no clear benefit either in using the multispecies Fmsy values as management targets.

 

For this specific case study, the lack of benefit in implementing an ecosystem based management approach is explained by the very flat shape of the equilibrium catch v.s fishing mortality curves, and the large overlap in the fishing mortality corresponding to pretty high yields in the different scenarios.