Fish migrating from the ocean to rivers provide marine-derived subsidies as prey for freshwater predators. While salmon, a famous example of marine-derived subsidy, migrate upstream in pulses at high latitudes, diverse amphidromous fish species in temperate rivers of low-to-middle latitudes exhibit species-specific phenologies during their upstream migration. As a result, marine-derived subsidies may be prolonged. This study aimed to determine whether this prolonged subsidy is incorporated into the diet of catfish (Silurus asotus), a top predator in river ecosystems. We conducted weekly sampling surveys through a year combined with sulfur stable isotope (δ³⁴S) analysis, which exhibits higher values in the ocean than in rivers and indicates the proportion of marine-derived material in body tissues. In the Tahara River, southern Wakayama Prefecture, nine amphidromous fish species migrated upstream at different seasonal patterns, facilitating the transport of marine-derived resources for approximately one year (47/51 weeks). Additionally, δ³⁴S analysis of lens layers, which form incrementally with growth, was used to reconstruct the feeding history of catfish. The results suggested long-term utilization of marine-derived resources across seasons throughout the year. Furthermore, based on the seasonal data for nine species, we extracted species groups consisting of three to nine species and calculated the duration of marine-derived recourse transport for each group to simulate the relationship between species richness and the duration. Species group extraction was performed in two ways: one reflecting the species richness and composition of actual rivers based on the National River Survey, and the other with randomly selected species. The results showed a tendency for the duration to increase with species richness. However, compared to randomly assembled species groups, those based on actual river conditions showed a rapid increase in transport duration at 3–5 species, as phylogenetically distant species with different transport durations were added. In contrast, at 6–9 species, the addition of closely related species with similar transport durations led to a plateau in transport duration. This study suggests that the year-long ocean-river connectivity driven by diverse species may contribute to the long-term dynamics of river communities through a top-down pathway by supplying resources to top predators. Furthermore, in this system, phylogenetic diversity may be linked to seasonal diversity, suggesting that maintaining phylogenetic diversity may be crucial for sustaining long-term ocean-river connectivity.