Habitat use by juvenile salmon, other migratory fish, and resident fish species underscores the importance of estuarine habitat mosaics

Interfacing with land and sea, estuaries support a mosaic of habitats that underpin the production of many coastal fisheries. These ecosystems are threatened by multiple stressors, including habitat loss and climate change, but the relative importance of estuarine habitat types for different fish species remains poorly understood since direct habitat comparisons are rare. This knowledge gap is exemplified in temperate estuaries by salmon—ecologically and commercially important species that use estuaries during their migrations to and from the ocean. Here, we tested for species-specific habitat use by sampling fishes in 3 interconnected estuarine habitats (brackish marsh, eelgrass, and sand flat), across seasons and temperature regimes. We quantified fish species richness, community distinctness, and catches (of Chinook and chum salmon, other migratory fishes, and resident fishes) in the Pacific Northwest’s heavily urbanized Fraser River estuary, the terminus of what was once the world’s most productive salmon basin. Overall, eelgrass habitat supported the greatest fish species richness (n = 37) and catches (37402 fish), exceeding that of both the marsh (19 species, 7154 fish) and sand flat (22 species, 6697 fish). However, the majority of salmon were caught in the marsh (61%). These differences, coupled with our finding that at least one unique fish species inhabited each habitat (eelgrass = 15, marsh = 8, sand flat = 1), demonstrate species-specific habitat use and underscore the importance of connected seascapes for biodiversity conservation.