An extreme drought in 2005 decreased many freshwater fish species abundance in areas like Lago Catalão, and many haven’t recovered yet.
Drought overturned the ecology of the lake over time – big fish populations declined while little fish boomed.
The shift has direct impacts on diets in the region since many local people depend on fish for protein, meaning that climate change is already influencing food reserves here.
Life beneath the waters of the Amazon is probably not the first thing that comes to mind when you think of the world’s largest rainforest. All of the bright, primary colors direct your eyes aboveground. But beneath the murky waters of the Amazon swims the highest diversity of freshwater fish on Earth.
This stunning diversity may be “out of sight, out of mind” for most, but not for Dr. Kirk Winemiller from Texas A&M AgriLife Research and his Brazilian colleagues.
Investigating long-term data from a floodplain lake near the confluence of the Amazon and Negro rivers, Winemiller and his team found that fish populations drastically changed after a severe drought in 2005. And numbers of many species haven’t recovered since.
Such changes are not only ecologically significant: fish are an important source of protein for people residing in these regions, and these life-sustaining fisheries are now imperiled by overfishing and climate change. Climate change is increasing drought intensity and frequency – and this is likely to impact freshwater fish populations in the Amazon and beyond.
Changes in biodiversity directly affect “the lives of most organisms living in and around freshwater environments, including humans” said Dr. Cristhiana Röpke, the lead author of the paper from the Instituto Nacional de Pesquisas da Amazônia in Manaus, Brazil.
Floods and droughts
River life in the Amazon is extreme. When precipitation levels are high during the rainy season, Amazonian floodplains fill. But when there is not enough water in the dry season, floodplains can suffer droughts. For large rivers like the Amazon, much of the cycling is dictated by how much rain falls on the headwaters.
Cycling between floods and droughts affects water connectivity and quality, species interactions, and primary production – an important ecological step where primary producer like plants make energy available through photosynthesis.
Now, climate change is putting its stamp on this naturally occurring cycle, by shifting rainfall patterns and making these extreme events more frequent and intense.
But climatic changes are also affecting headwaters differently. For instance, extreme floods are occurring more often in the headwaters of the Negro river, which is the second largest tributary of the Amazon river. The Madeira river – the largest tributary – has seen more extreme droughts.
Many studies have focused on how changes in rainfall may affect forest dynamics in the tropics, but few studies look deeper, into the murky freshwater.
Fish out of water
Winemiller and his Brazilian colleagues analyzed data from monthly surveys of freshwater fish conducted in Lago Catalão, a floodplain lake near the confluence of the Amazon and Negro rivers. Between 1999 and 2014, the surveys gathered detailed information about the fish, like species identity, primary food source, and size.
With roughly 400 species sampled to date, Lago Catalão represents a large portion of the freshwater fish found in the Amazon floodplains. It is also important for tourism. However, there are minimal conservation efforts in the area, the largest of which are local initiatives for fisheries management.
The study shows that rainfall patterns have important direct and indirect effects on lake ecology. Indirectly, droughts may cause shifts in what fish are consuming, causing trickle-down changes throughout the ecosystem. Seasonal water changes determine when the floodplain lake is connected to both rivers. Sometimes it can be completely cut off.
In October 2005, an intense drought dried up 70 percent of the floodplain habitats in this region. Lago Catalão was disconnected from the Negro river for about three months.
After the drought, fish populations dramatically changed. But not every change occurred immediately. This suggests that intrinsic biological factors – like reproduction, range, and diet – changed as well.
Currently, many fish species are less abundant in the floodplain lake than before the drought, including many large fish species that are important for human consumption.
Declines in large fish may be a result of an inability to migrate from river to lake. On the other hand, the study found that small fish that reproduce quicker are now higher in abundance than before the drought.
One example is the tambaqui (Colossoma macropomum),a seed and fruit eating giant that weighs up to 88 pounds. Tambaqui is a migratory species that declined after the drought. The year before the drought, the catch per unit efficiency (CPUE) – a measure of abundance for fish – of tambaqui was approximately 0.035. After the drought, this declined to approximately 0.0025 – more than a 90 percent drop from the pre-drought CPUE. Unfortunately, this species is also important to local fish markets and has a high-market value.
In contrast, the study found only small changes in the abundance of primary consumers, i.e. fish that rely on only plant material as their food source. However, omnivores and secondary consumers – fish that rely on animals like insects or other fish – markedly declined.
Fishing in troubled waters
Despite the importance of fish for local consumption, many species with consumer value are overexploited, and are not successfully farmed.
“Our study found that some of the most affected fish species were also the ones that were valuable in local fish markets. Therefore, climate change in the Amazon region likely will influence fisheries,” explained Winemiller.
As climate change increases the intensity and frequency of droughts, these effects could worsen over time. During droughts, fishermen catch more fish. More droughts may lead to more overexploitation. And this would be on top of the declines already seen in this study.
As the research shows, this can greatly affect the lake’s ecology and the resilience of the fish communities as a whole. Each fish species fulfills a different role in the community. They may eat different food, reproduce at different times, and live in different parts of the lake. If a specific role is taken away it may not be filled as quickly – or at all – after extreme events.
“Reduced supply of fish would possibly result in starvation and migration of these people to other areas,” said Röpke.
She added that the study highlighted the “need for protection areas in large rivers and floodplains because these areas could work as refuges for fish population preventing collapses and biodiversity loss under a scenario of increased frequency of drought.”
Between the number of hooks in the water and prevalent drought conditions, the situation looks murky for Amazonian fish. The need for change is clear, even if the water is not.
Röpke, C.P., Amadio, S., Zuanon, J., Ferreira, E.J.G., de Deus, C.P., Pires, T.H.S., Winemiller, K.O. (2017) Simultaneous abrupt shifts in hydrology and fish assemblage structure in a floodplain lake in the central Amazon. Scientific Reports, 7, 40170.