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 in an unstructured habitat. Enhancement of fish
production from subtidal restored shellfish reefs from a single year’s
cohort is estimated to be, on average, 6,186 kg.ha-1.yr-1 (SD 1,802) after
enough time has elapsed for all species to have matured. Species harvested
commercially or recreationally contributed 98% of that production (6,083
kg.ha-1.yr-1, SD 1,797).  Enhancement varied greatly among
locations, ranging from 12,738 kg.ha-1.yr-1 (SD 2,894), which is the
highest yet recorded anywhere, to 1.4 kg.ha-1.yr-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.