Möllmann, Professor for Fisheries Science at Hamburg University, and Quaas, Professor of Economics at Kiel University, teamed up to reflect the nature of their work: bringing the ecological and economic together in studying the phenomenon of large persistent changes in ecosystems known as regime shifts.
The presenters' perspective coupled the approach taken by ecologists – identifying evidence and early warning signals of regime shifts – with the model-based technique used by economists. Despite being widely documented, regime shifts are often overlooked in fisheries management, and Möllmann and Quaas idea is to improve knowledge for this end goal.
Many Atlantic stocks have shown signs of regime change, where environment and fishing interacts. Möllmann outlined evidence for some of these shifts, both by analysing linear models where certain recruitment and biomass thresholds have been reached and by looking at more complex non-linear feedback dynamics.
Quaas proceeded to outline economic perspectives, making key points including that shifts can happen when the value per unit of fish catch is acknowledged to not be a constant. He emphasized that when a fishery generates economic welfare, the system may show these non-linear feedbacks and regime shifts.
Case studies of eastern and western Baltic cod stocks were then provided. For the former, a linear model overestimated the prediction of spawning-stock biomass, which caused too much optimism in the stock's recovery. Quaas and Möllmann expressed how such models can be misleading and that using a multi-model approach can be more effective in reducing uncertainty.
The talk wrapped up with the presenters stating that market forces need to be taken into account alongside ecological ones and that sustainable management should go beyond short-term economic interests, stressing the importance of keeping track of regime shifts for ecosystem-based management.
Christian Möllmann, left, and Martin Quaas at ICES ASC 2018 in Hamburg.