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Ecosystem regime shifts can have severe ecological and economic consequences, making it a top priority to understand how to make systems more resilient. Theory predicts that spatial connectivity and the local environment interact to shape resilience, but empirical studies are scarce. Here, we use >7000 fish samplings from the Baltic Sea coast to test this prediction in an ongoing, spatially propagating shift in dominance from predatory fish to an opportunistic mesopredator, with cascading effects throughout the food web. After controlling for the influence of other drivers (including increasing mesopredator densities), we find that predatory fish habitat connectivity increases resilience to the shift, but only when densities of fish-eating top predators (seals, cormorants) are low. Resilience also increases with temperature, likely through boosted predatory fish growth and recruitment. These findings confirm theoretical predictions that spatial connectivity and the local environment can together shape resilience to regime shifts. This study finds that habitat connectivity can increase resilience to ecosystem regime shifts. The authors used >7,000 fish samplings from the Baltic Sea to study a spatially propagating shift from an ecosystem dominated by predatory fish to one dominated by their prey, also finding that fish-eating seals and cormorants increased the risk of a shift.

Interest in coastal restoration measures is increasing, but information about subsequent ecosystem recovery processes is limited. In Björnöfjärden on the Baltic Sea coast, Stockholm archipelago, a pioneering case study to reduce coastal eutrophication led to improvements and initially halved phosphorus levels. Here, we evaluate the effects of the restoration on the local fish assemblage over one decade after the measures. The study gives a unique possibility to evaluate responses of coastal fish to nutrient variables and abatement in a controlled natural setting. Cyprinid abundance decreased and perch partially increased with decreasing turbidity levels, while mean trophic level increased over time in the restored area. Responses were overall weak, likely attributed to an attenuation of the eutrophication abatement effect over time. The results suggest that nutrient reduction gives slow responses in fish compared to alternative measures such as fishing closures.

Areas closed to fishing year-round (no-take zones, NTZs) or during spawning time (spawning closures) are used as a management tool to increase declining fish populations. We evaluated the effects of a 147 km(2) NTZ and a 3980 km(2) spawning closure on whitefish populations in the northern Baltic Sea, and also accounted for fish consumption by seals and cormorants. Fish monitoring with multimesh gillnets in 2011-2016 showed a significant increase in catch per unit effort (CPUE) of mature whitefish (> 30 cm) both in the spawning closure and the NTZ compared with the reference area open to fishing. The rate of increase was significantly higher in the NTZ than in the spawning closure. Our results suggest that NTZs may strengthen coastal fish populations in temperate regions and that also seasonal closures under a critical period of the life cycle may benefit the populations.

Ecosystem regime shifts can have severe ecological and economic consequences, making it a top priority to understand how to make systems more resilient. Theory predicts that spatial connectivity and the local environment interact to shape resilience, but empirical studies are scarce. Here, we use >7000 fish samplings from the Baltic Sea coast to test this prediction in an ongoing, spatially propagating shift in dominance from predatory fish to an opportunistic mesopredator, with cascading effects throughout the food web. After controlling for the influence of the driver (increasing mesopredator densities), we find that predatory fish habitat connectivity increases resilience towards the shift, but only when densities of fish-eating top predators (seals, cormorants) are low. Resilience also increases with temperature, likely through boosted predatory fish growth and recruitment. These findings confirm theoretical predictions that spatial connectivity and the local environment can act together to shape resilience towards regime shifts.