Effects of climate‐driven hydrological changes in the reproduction of Amazonian floodplain fishes

Abnormal hydroclimatic years in the Amazon have been increasingly frequent in the last two decades, creating more prolonged droughts and severe floods. These events are expected to impact organismal phenology, including seasonal reproduction of fish. Droughts are also expected to increase fish mortality and vulnerability to fishing. However, empirical evidence on the impact of these novel conditions on fish reproduction and demography is still limited. Here, we evaluate how changes in hydrological conditions and fishing affected reproduction and demographic parameters for the 16 most common floodplain fish species in the central Amazon. We used water level data collected for 113 years together with a 19‐year dataset of fish biology, sampled monthly from a floodplain lake at the confluence of Amazon and Negro rivers. We observed a lower proportion of ripe females after long, drier low‐water and abrupt rising‐water seasons. For many species, we detected a progressive reduction in the female’s size at sexual maturity, the average size of the ripe females and in the abundance of larger adult females. These effects were observed for both fished and non‐fished species, suggesting that the effect of the recent hydroclimatic events might be affecting most fish species. However, fished species showed a steeper decline in the average body size of ripe females, suggesting that size reduction is a combined effect of drought severity and fishing pressure. Policy implications. Our results for the proportion of females in reproduction mediated by hydrological conditions and temporal changes of demographic and life‐history parameters suggest that drought events can reduce the resilience of fish populations in the central Amazon. The increasing frequency of droughts and rapid changes in fish reproductive parameters highlight the need for conservation policies to consider the impact of droughts in addition to fisheries and habitat degradation. Our results should be taken as an early warning regarding the conservation and sustainable use of Amazon aquatic biodiversity. Conserving fish diversity and fish stocks will require substantial ecosystem management in a large portion of the Amazon floodplains and a greater enforcement of fisheries management regulations in years of predicted drought.