Differences in connectivity and transport of Toxoplasma gondii in marine environments due to sensitivity in model setup

Terrestrial parasites enter marine habitats through freshwater runoff, posing a threat to marine mammals. Toxoplasmosis, caused by Toxoplasma gondii , causes 40% of adult female Hawaiian monk seal (HMS) mortalities in the main Hawaiian Islands (MHI). While HMS infection occurs through consuming infected prey or seawater, oocyst transport in marine settings remains poorly understood. This study is the first to characterize T. gondii oocyst connectivity and transport in Hawaiian waters, emphasizing how ocean conditions influence oocyst distribution. We employed a particle tracking model with the Regional Ocean Modeling System (ROMS), comparing 2 configurations—unnested ocean model with coarse horizontal resolution (4 km), and nested (200 m) setup utilizing ocean currents in a specific region at finer horizontal resolution—to evaluate the impact of ocean model resolution on oocyst transport. Two particle input scenarios were tested: constant and variable, with the latter based on output from an oocyst hydrological model for O‘ahu. ROMS grid resolution had a greater impact on connectivity than initial particle input, indicating that finer ocean models simulate reduced connectivity and increased retention in nested simulations by capturing complex bathymetry and local physical conditions. We observed consistent retention for all islands and simulations, with limited connectivity between Ni‘ihau, Kaua‘i, and other MHI, suggesting local infection sources. Potential high accumulation areas include west Kaua‘i and east, south, and southwest O‘ahu, driven by interannual wind and associated eddy variability and steep nearshore bathymetry. This research highlights the role of particle transport models in evaluating parasite distributions and aiding marine mammal conservation.