Models reveal potential synergies in management actions for bull kelp forest resilience and restoration

Heatwaves are a growing threat to marine ecosystems. Kelps are particularly vulnerable because they require cool, nutrient‐rich water to thrive. Heatwaves can also trigger increased herbivory by sea urchins. Management interventions—including re‐seeding kelp spores and manually removing urchins—could mitigate heatwave damage or aid recovery. However, we do not yet know the optimal timing of these management actions or whether combining them enhances their effectiveness. We used a stochastic, stage‐structured model of kelp‐urchin population dynamics, which included an urchin‐grazing behavioural switch and was parameterized to represent bull kelp ( Nereocystis luetkeana ) forests along the coast of Oregon, USA. We simulated a 2‐year heatwave and considered two urchin predator scenarios: one in which the sunflower seastar Pycnopodia helianthoides was driven to local extinction by seastar wasting disease during the heatwave (the historical reality), and the counterfactual where seastar densities remained constant. All management interventions individually were notably more effective when seastars were present. When each was enacted alone, kelp seeding was less effective than urchin removal. Both interventions improved kelp forest persistence most when begun before or during the heatwave, but were not effective if started and completed before the heatwave began. The effectiveness of both interventions increased with increasing effort (% biomass removed or added), but with diminishing returns. Combining urchin culling and kelp seeding was highly effective, and even more so if urchin culling started first, with large synergistic gains starting at low intensities of kelp seeding (5% of pre‐heatwave biomass) and higher intensities of urchin removal (25% of pre‐heatwave biomass). Those synergies can be achieved when actions start after a heatwave has begun, easing the need to anticipate heatwaves. Synthesis and applications . We found that urchin culling and kelp seeding interventions can be effective at rebuilding kelp forests, but require intensive effort (e.g. removing 25%–50% of urchins over multiple years), especially in the absence of predators. Our results provide recommendations for the magnitude and timing of culling and seeding that would be needed to provide desired levels of kelp forest resilience.