The beauty of simplicity in science: Baltic cod stock improves rapidly in a 'cod hostile' ecosystem state

The Baltic Sea ecosystem is hypothesized to have undergone a regime shift during the last 3 decades, altering its functioning and the composition of its zooplankton and fish communities. The new stable state has been considered as 'cod hostile' due to reduced spawning success in cod, as well as increased predation on and declining food sources for cod larvae. Nonetheless, the eastern Baltic cod stock has recently recovered after more than 2 decades of low biomass and productivity. The recovery was mainly driven by a sudden reduction in fishing mortality and occurred in the absence of any exceptionally large year classes. The recovery of the cod stock during a 'cod-hostile' ecological regime indicates that fisheries are the main regulator of cod population dynamics in the Baltic Sea.