Prioritising investment in kelp forest restoration: A spatially explicit benefit-cost analysis in southern Australia

• The benefit-cost ratio for restoring all the kelp forests in Port Phillip Bay is 1.10. • Individual sites have benefit-cost ratios varying from 0.33 to 3.4. • By restoring reefs with a net benefit, results in a benefit-cost ratio of 2.10. • This analysis can be used to identify priority areas for restoration. Kelp forests are globally significant ecosystems providing critical ecosystem services, including fish production, nutrient cycling, carbon sequestration, and recreational uses. However, widespread degradation caused by anthropogenic pressures has led to significant declines in kelp forests, necessitating cost-effective restoration strategies. This study performs a spatially explicit benefit-cost analysis of kelp forest restoration in southern Australia to explore how variations in costs and benefits can inform prioritisation of restoration strategies. Costs of ecosystem restoration were calculated based on the time to cull overabundant sea urchins from each location and for active kelp restoration costs. We found that investing in kelp forest restoration at the broad-scale (3,291 ha) returns a positive benefit-cost ratio of 1.10 (where 1.0 is break-even). There was substantial site-specific variation in the benefit-cost ratio (0.33 to 3.4), driven by variation in predicted kelp biomass and thus nitrogen storage benefits ($0 − $105,000 /ha). For culling costs, this varied based on urchin density, the depth (dive time) and travel time to the site. Given this variation, we considered another scenario where only the reefs that returned a positive benefit-cost ration were restored (1,221 ha), which would deliver $92.1 million in benefits, from an investment of $43.9 million and would result in a benefit-cost ratio of 2.10. This research demonstrates how spatial prioritisation can guide investments in marine ecosystem restoration to maximise return on investment. However, while kelp restoration proves beneficial, realising its potential will require robust funding mechanisms (perhaps via market-based incentives), which are currently lacking.