Evaluating spatial and temporal dynamics of cold- and warm-adapted fish in a changing Long Island Sound

Long Island Sound (LIS) fish communities were once dominated by cold-adapted species in the spring and warm-adapted species in the fall. However, recent decades have seen a transition to dominance by warm-adapted species year-round. In this study, using spatial indicators and data from a comprehensive fishery-independent survey, we quantify the spatiotemporal distribution and abundance of summer flounder (Paralichthys dentatus ), a warm-adapted species, and winter flounder ( Pseudopleuronectes americanus ), a cold-adapted species, in the LIS to compare trends between warm- and cold-adapted species with similar morphologies. Spatial indicators were evaluated using changepoint analyses, and environmental occupancy analyses were conducted to compare environmental conditions associated with positive- and zero-catch tows through time. Results showed that winter flounder spatial distribution contracted, with declining abundance and positive area in both seasons, while center of gravity estimates indicated reduced dispersal across the LIS. In contrast, summer flounder expanded in abundance and distribution, with increasing positive area and inertia, indicating greater dispersal across the LIS. Major transitions in the late 1980s, early 2000s, and mid-2010s were identified, suggesting periods of change in ecosystem structure. Environmental occupancy analyses indicated that winter flounder remained associated with cooler and generally deeper habitats, while summer flounder were more frequently associated with warmer environmental conditions. These findings suggest that spatial indicator trends were associated with shifts in habitat occupancy in addition to changes in abundance. This study highlights the need for species-specific monitoring and adaptive management to account for shifting distributions and population trajectories in the LIS.