If you build it and they come, will they stay? Maturation of constructed fish spawning reefs in the St. Clair-Detroit River System

Constructed rock reefs have been used to remediate spawning habitat for Lake Sturgeon (Acipenser fulvescens) and other lithophilic spawning fishes in the St. Clair-Detroit River System, North America. Early projects used a cross-channel design and species-specific metrics (e.g., proximity to historical spawning locations) to guide reef placement. However, the Middle Channel Reefs and portions of other early projects were compromised by fine sediment accumulation. Therefore, geomorphological criteria were considered in siting reefs constructed after 2013 to avoid sediment sources and improve the likelihood of successful reef function. To evaluate the effectiveness of the revised placement process, we quantified physical maturation of constructed reefs using annual side-scan and down-looking sonar surveys beginning in 2014 and underwater video surveys beginning in 2015. Reef areas and hardness were measured from sonar surveys and underwater video was used to quantify surficial sediment composition. Size and hardness of reefs developed using geomorphological criteria decreased with time, but at rates slower than what was observed at the Middle Channel Reefs. Sediment composition of the reefs remained similar through 2017 and prevalence of reef rock was high, except at Hart's Light Reef, where dreissenid mussel shells composed 32% of the surficial substrate by age three. However, accumulation of fine sediments was documented at all reefs in 2018. Despite using geomorphic criteria to identify areas most suitable for reef construction, reef sediment composition has changed, and future reef restoration projects could benefit by incorporating methods for maintenance, in addition to using geomorphic criteria, to identify restoration sites.