Multiple factors determine the effect of anthropogenic barriers to connectivity on riverine fish

Habitat fragmentation is a key anthropogenic factor in biodiversity decline, particularly in aquatic ecosystems. We predicted that differences in fish assemblage composition due to the impact of fragmentation would most strongly affect migratory species, and these effects would be dependent on the interaction between the characteristics of each barrier and the antecedent flow conditions that determine temporal variation in connectivity. These hypotheses were applied to a coastal river network in eastern Australia that is fragmented by multiple weirs and dams, including some with passage facilities. How these facilities interact with flow to mediate hydrological connectivity and hence patterns of community structure is unknown. Five distinct assemblages were identified that were associated with different combinations of environmental factors and barrier characteristics (spatial arrangement, passability), and key differences were due to variation in migration traits. Two spatially distinct assemblages were associated with fragmentation by two impassable barriers. However, the migration traits that accompanied these community changes were inconsistent between these groups, and likely reflected effects of barriers near the estuary and in the middle of the stream network on diadromous and freshwater-migratory species, respectively. Two assemblage groups in the vicinity of passable weirs varied temporally as a function of hydrology and the seasonal upstream movement of juvenile diadromous species. The effect of habitat loss in conjunction with fragmentation was evident, with a further assemblage group occurring in reaches where riparian vegetation and instream habitat have been altered by poor management of agriculture. This study indicates that the impact of habitat fragmentation in rivers depends on the interaction of the migration characteristics of biota, temporal variation in hydrology which mediates connectivity, and the location of anthropogenic barriers. Conservation policies aimed at minimizing human impacts on aquatic biodiversity need to jointly account for the separate impacts of habitat fragmentation and habitat loss.