Multidisciplinary approach to monitor eelgrass (Zostera marina) and green crab (Carcinus maenas) dynamics in Placentia Bay, Newfoundland and Labrador, Canada

Eelgrass (Zostera marina) meadows are ecologically significant coastal habitats that have declined globally due to a variety of anthropogenic and natural pressures. In Placentia Bay, Newfoundland and Labrador, Canada the arrival of the invasive European green crab (Carcinus maenas) in the mid-2000s was followed by widespread eelgrass loss and a sharp decline in associated fish abundance and biomass. In this thesis, I use a multidisciplinary approach to reconstruct eelgrass dynamics across spatial and temporal scales and evaluate the impact of green crab presence and restoration efforts. Data sources include satellite imagery, remotely piloted aircraft systems (RPAS), underwater snorkel transects, and local ecological knowledge (LEK). Results show that eelgrass extent declined dramatically following the green crab invasion, up to 87% at some sites. However, during the 2017-2022 restoration period, which included green crab mitigation fisheries and eelgrass transplanting, modest signs of eelgrass recovery were observed. RPAS and snorkel transect data confirmed increases in eelgrass cover near some restoration sites. LEK survey data aligned with both documented green crab spread and eelgrass loss. Furthermore, results suggest a potential threshold in green crab density (catch per unit effort, CPUE) below which eelgrass recovery is more likely to occur; however, more research is needed as this value is likely site specific. This study demonstrates the value of combining community knowledge with scientific monitoring to understand habitat changes. Findings support the continued need for invasive species management, active habitat restoration, and highlight the importance of maintaining green crab populations below a certain threshold to allow eelgrass recovery.