Declined fitness in larvae born from long-distance migrants of anadromous Coilia nasus in the lower reaches of the Yangtze River, China

Anadromous Coilia nasus is a socioeconomically important species from the middle and lower reaches of the Yangtze River. Here, we compared growth and feeding of C. nasus larvae in four reaches along the full migration corridor of the river (640 km) to determine how fitness varies between progenies of short- and long-distance migrants. Results demonstrated that larvae collected in downstream sections grew faster, exhibited higher feeding intensities, and consumed larger zooplankton (a favored food resource). Our results did not support the parent-offspring trade-off theory, which predicts that the costs and benefits of migration increase with migration distance, and higher parental costs with long migration should be offset by increased offspring fitness. We suggest pervasive human impacts along the river are likely driving the observed ecological patterns. Overfishing has resulted in a truncated body size in migrants, which shortens their migration distance; isolation of floodplain lakes from the river restricts fish spawning and nursing to suboptimal lotic river habitats; and higher discharge experienced by larvae born from long-distance migrants in the upstream river reaches during the later flooding season results in declined feeding intensity and slower growth compared to those produced from short-distance migrants in the earlier season. We predict that a fishing ban in the Yangtze River will allow fish to grow larger and older so they can access floodplain lakes further upstream, which will further enhance recruitment of the C. nasus population.