Sonar, so good: Tree-reefs drive net gain in fish size, abundance, and diversity

Global land use changes have greatly reduced the flow of driftwood to the sea. This driftwood is critical habitat for many sessile species that require hard substrates, and provides shelter, spawning, rearing, and foraging areas for mobile species. (Re)placement of large wood as a habitat restoration measure is established in rivers and freshwater lakes but has seldom been done in estuaries and marine environments where sunken driftwood occurred in the past. In the subtidal Dutch Wadden Sea, we placed 32 ‘tree-reefs’ to mimic this historical sunken wood to assess the effects on mobile (i.e. fish and crustaceans) faunal biodiversity over a medium-term timescale of 2.5 yr. Because of the challenging environment, where traditional monitoring methods are limited due to low visibility, strong tidal currents, wave action, and unstable sediments, we modified a Garmin Panoptix Livescope (a fishfinder used by sport fishermen) to monitor pelagic fish populations around the reef sites. These efforts were supplemented by benthic fish traps. Within 2.5 yr of deployment, trap fishing showed that at reef sites, fish were 6 times more abundant, larger in size, and more diverse, while crab foraging activity was 77% lower compared to control sites. Sonar showed 215-359% greater pelagic fish abundance at reef sites compared to control sites across 3 size classes of fish monitored. Results indicate that (1) sport-fishing fishfinder technology is a valuable tool to monitor fish in shallow turbid waters at moderate costs, and (2) tree-reefs can be a cost-effective, scalable, natural, and biodegradable approach to restore marine biodiversity and trophic complexity.