Resource use, niche width, and trophic position reveal diverse trophic structure in a tidal freshwater zone fish...

The tidal freshwater zone is an aquatic transition zone that links a river to its estuary and provides an important habitat used in the life cycle of resident and migratory fishes. Yet, information on the trophic structure of fishes in this habitat is scarce. To address this gap, we characterize the trophic structure of a fish community in the tidal freshwater zone of the Northwest Miramichi River (New Brunswick, Canada). Stable isotope analyses (δ¹³C, δ¹⁵N, and δ³⁴S) of 17 fish species revealed diverse feeding strategies. Resource use varied across species; some fish relied on either marine or freshwater resources, whereas others integrated resources from both habitats. Fishes varied in their trophic position (range 3.1-4.2) which increased with reliance on marine-derived resources. Species isotopic niche widths varied widely (range 20.4 to 1015.3). Species with intermediate marine resource use (~0.50) had a mix of wide and narrow isotopic niche widths, contrary to wide niches predicted by current literature. Our findings indicate that these narrow-niched species may either act as a sort of tidal freshwater transition zone specialist or assimilate equal proportions of marine and freshwater resources on average. Trophic information from the tidal freshwater zone improves the understanding of fish communities and food web structure where freshwater rivers and marine ecosystems meet.