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

Abstract

1. 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.2. 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.3. 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.4. Policy implications. Our results for the proportion of females in reproduction mediated by hydrological conditions and temporal changes of demographic and lifehistory 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.

Seasonal patterns in the structure of the fish larvae assemblages in the lower Amazon River

“…In the rising water, medium-distance migrators were predominant as indicated by other studies conducted in the Amazon Basin (Mariac et al, 2021;Ponte et al, 2017;. Most species in this group also are periodic strategists with maturation at small size (Röpke et al, 2019(Röpke et al, , 2022. Many species of herbivorous TA B L E 2 List of species and density (no.…”

“…In the rising water, medium‐distance migrators were predominant as indicated by other studies conducted in the Amazon Basin (Mariac et al, 2021; Ponte et al, 2017; Zacardi et al, 2017). Most species in this group also are periodic strategists with maturation at small size (Röpke et al, 2019, 2022). Many species of herbivorous (e.g., M. albiscopum ) and detritivorous ( P. amazonica ) fishes tend to reproduce in the rising water as well because the marginal areas of forests flood and connectivity increases, facilitating larval access to floodplains (Zacardi, Bittencourt, et al, 2020).…”

Effects of anomalous climatic events on the structure of fish larvae assemblages in the eastern Amazon

“…In addition, periodic species, as well as medium-distance migrators, should be more abundant during La Niña, due to favorable conditions for the reproduction of these fish, such as greater amounts of rainfall and river flow (Zacardi et al, 2017b;Humphries et al, 2020). Finally, we believe that commercially exploited species will have their reproduction more affected, given that previous studies have observed the effect of climatic events on the reproductive parameters of these fish (Röpke et al, 2022).…”

“…Zacardi et al (2018) verified the relationship between the production of landed fish and the abundance of larvae in the lower stretch of the Amazon River and observed that the fishing effort undertaken on adults compromises the spawning stock and, consequently, the annual recruitment of resources. The reproductive parameters of commercially exploited species are more susceptible to the effects of climatic events (Röpke et al, 2022), suggesting that environmental factors and fishing intensity can act simultaneously for the failure or success of fish recruitment in each period. Therefore, relatively minor changes in the reproductive success of large periodic can lead to poor recruitment for at least 2 years (Bayley et al, 2018;Castello et al, 2019;Cataldo et al, 2022).…”

“…The climatic anomalies associated with ENSO impact several population aspects of fish, especially reproductive ones. For example, prolonged droughts and mild floods can cause high mortality of larvae and juveniles, lower proportions of females with mature gonads, and decreased size at first maturity (Castello et al, 2019;Röpke et al, 2022). On the other hand, some species of periodic and migratory fish benefit from larger nursery areas, feeding areas, and the connectivity between the main channel and the floodplains during La Niña events when the flood is intense (Oliveira et al, 2014;Camacho Guerreiro et al, 2021;Cataldo et al, 2022).…”