Species-specific adaptations, individual condition, and life-history-induced dispersal strategies of fish larvae along a transverse gradient in a large river

Early life-history properties reveal a considerable variation among riverine fish species and may play a major role in the downstream drift of larval fish. This role, however, remains largely unexplored. Assemblages of drifting fish larvae and their condition during the peak seasons were analyzed along a cross-section from littoral to midstream zones in a free-flowing section in the middle Yangtze River. Assemblage structure and larval body injuries revealed pronounced transverse patterns. These patterns were mainly explained by hydraulic gradients. Highest species occurrence was recorded in the littoral zone. Larval abundance of most species and larval body injury rates were higher in littoral and sublittoral zones. Relative inshore and nearshore larval abundance revealed a significant negative relationship with species-specific initial body size, and varied among species. Larval abundance, body injury rate and feeding intensity were all higher at night. Our findings revealed considerable habitat preferences of drifting larval fish and suggest important adaptations for reducing starvation- and predation risks. These factors should be considered in species conservation and river management.