The sound of resilience? Exploring trophic cascades and positive feedbacks between cod and eelgrass in a trawl-free ecosystem

This study examines The Sound, a strait between Denmark and Sweden protected by a trawl ban since 1932. Consistent with previous research, we show that the area supports greater densities, sizes, and landings of Atlantic cod ( Gadus morhua ) than adjacent trawled waters. Simulations from a bespoke food web model indicate that predation from the cod can suppress mesopredators, thus reducing filamentous algal growth through a trophic cascade, which benefits eelgrass ( Zostera marina ). In line with this, monitoring data show lower mesopredator densities and more extensive eelgrass meadows in The Sound compared with nearby trawled areas, although salinity and hydrodynamic conditions could also contribute. However, despite the trawl ban, adult cod densities are currently at their lowest levels since monitoring started, raising concerns about the ecosystem’s long-term resilience. Model results further demonstrate how preserving eelgrass meadows benefits cod, illustrating feedback mechanisms that highlight the importance of integrated fisheries and conservation management.