Estimating enhanced fish production on restored shellfish reefs using automated data collection from underwater videos

Shellfish reefs across the world have been heavily degraded by mechanical harvesting, disease and declining water quality. In southern Australia, where substantial losses have occurred, government and non-government efforts to restore functioning reefs are now underway with a strong focus on enhancing fish productivity. However, the capacity of these restored reefs to enhance fish production remains unknown, hampering estimates of return on investment. We quantify the density differences of newly recruited juvenile fish and other nekton on these restored reefs, relative to those on unrestored, unstructured habitat. Fish were surveyed at three paired reef-unstructured locations using 169 unbaited stereo video deployments during three periods over 12 months (2022–2023). We used automation software, FishID, to automatically identify, size and count fish in videos. We subsequently applied known growth and mortality parameters to model enhancement of fish productivity. Sixteen species occurred as new recruits, with all but two found at higher densities on reefs than unstructured habitat. Enhancement of fish production from subtidal restored shellfish reefs from a single year's cohort is estimated to be, on average, 6186 kg ha⁻¹ year⁻¹ (SD 1802) after enough time has elapsed for all species to have matured. Species harvested commercially or recreationally contributed 98% of that production (6083 kg ha⁻¹ year⁻¹, SD 1797). Enhancement varied greatly among locations, ranging from 12,738 kg ha⁻¹ year⁻¹ (SD 2894), which is the highest yet recorded anywhere, to 1.4 kg ha⁻¹ year⁻¹ (SD 0.9). The lack of juvenile fish at the location with the lowest estimated enhancement might be explained by the impact of overfishing on recruitment of key species or by an abundance of alternative habitat for juvenile fish. Synthesis and applications. The combination of underwater videos with automated data extraction provides a reliable, cost-effective method for surveying fish on oyster reefs. By quantifying enhanced fish productivity on reefs, we provide estimates that will underpin calculations of ecological, social and financial benefits, supporting the business case for scaling-up restoration efforts.