Sulawesi Stream Fish Communities Disconnected From the Sea: Absence of Diadromous and Dominance of Exotic Species

ABSTRACT Barriers can profoundly alter the composition of stream communities. In the tropical island of Sulawesi, streams are inhabited by a diversity of secondary freshwater fishes, mostly diadromous species. Many streams, however, are modified by human activities for various reasons, involving the construction of barriers that impede fish migration. At the same time, introduced non‐native freshwater fishes have spread across the island and impact native ecosystems. We tested three hypotheses: (i) Diadromous species that characterise fish assemblages in undisturbed Sulawesi drainages are absent from streams isolated from the sea due to barriers such as dams, water abstraction infrastructure, or major natural obstructions; (ii) habitat niches inhabited by diadromous fishes in natural streams remain vacant in these environments; and (iii) non‐native (exotic) fish species are more diverse and abundant in blocked streams than in unblocked ones, as human‐induced environmental changes create new ecological opportunities and unoccupied habitat niches. Using point abundance electrofishing, we surveyed 17 streams across three regions in Sulawesi (Toraja, Poso, and Kolaka) for fish species diversity, abundances, and associations among species and their habitats. Our assessment revealed 1572 individuals representing 15 fish species from 11 families. Non‐native fishes dominated the communities at most sites in all three regions, comprising 10 of the species and in total more than 70% of fish abundances. Only four endemic species occurred, all at lower frequencies (the goby Mugilogobius sarasinorum , and the freshwater halfbeaks Nomorhamphus rex , N. sagittarius , N. celebensis ). A single amphidromous species (the spinecheek gudgeon Eleotris fusca ) was recorded at a single site in Kolaka, represented by only a few individuals. Outlying mean index (OMI) analyses indicated that pH, temperature, and elevation influenced species distribution. As expected, native diadromous fishes were largely absent from Sulawesi's blocked streams, and non‐native species dominated local communities. High current velocity microhabitats, inhabited by amphidromous gobies and other diadromous fishes in natural streams, remain largely empty. Our findings highlight the substantial impact of barriers on upstream fish communities in Sulawesi. In all cases examined, disconnected streams showed lower fish diversity than natural ecosystems and were dominated by populations of non‐native species. The ecological consequences of dam projects and other habitat modifications blocking streams in Wallacea need further study. Solutions such as efficient fish ladders might mitigate the isolation of native stream ecosystems from the sea, but need to be developed for this specific fauna.