A team of biologists reports a comparison between museum collection of cichlid fishes kept before a dam closed on the Amazon Tocantins River in 1984, and current species from the Tucurui Reservoir - collected 34 years later.
Led by Craig Albert and doctorate student Chaise Gilbert from the University of Massachusetts Amherst, researchers worked with specialists from Brazil. Together, they tested whether cichlid fishes displayed changes in their physiology. This is inferred from changing habitats and foraging behavior caused by the dam rapidly changing the environment from a clearwater river to a murky reservoir.
A Change in Environment Leading to Morphological Changes
The researchers described the Tocantins as "historically characterized by fast-flowing rapids, sand, and rock covered waterbeds and seasonal floodplains." It is also among the largest clearwater rivers in entire South America.
However, increasing human presence and activity in the area have resulted in the construction of dams along the river, with the Tucurui Hydroelectric Dam being the largest and the oldest of them. Tucurui included a large permanent reservoir of about 70 kilometers (43.5 miles) long and 40 kilometers (24.9 miles) wide.
Cichlid fishes are a source of interest within the scientific community for its propensity for morphological change, demonstrating alterations in as short a period as a single season. Aspects of the cichlid's body shape often match feeding conditions and environmental change, according to Albertson. Specifically, the skeleton of these fishes is sensitive to external factors, prompting researchers to use them as an illustration of organisms' response to anthropogenic activities that cause environmental changes.
Using geometric morphometrics, the study of shapes based on Cartesian landmarked coordinates, researchers compared the morphology of six species of fishes collected from the Tucurui reservoir in 2018 to those samples collected in the same area shortly before the closure of the dam in 1984, or the pre-dam period.
Comparing Samples from the Same Species, 34 Years Apart
To begin the morphological comparison, researchers took a picture of both museum (pre-dam) and field (post-dam) samples with a digital camera, with each photographed sample accompanied by a scale bar to guide coordinate locations. Aside from 2D geometric morphometric information, researchers also collected a set of four linear measures across all available specimens - done to supplement their data and account for possible changes in certain traits.
"Our overarching hypothesis is that the damming of the Tocantins and subsequent formation of the Tucuruí reservoir has induced shifts in habitat and foraging behavior and that the anatomy of resident cichlid populations has changed in ways that allow them to adapt to this novel environmental conditions," Albertson explained.
After comparing the changes between pre-dam and post-dam samples, researchers noted the dramatic changes in the environment and the subsequent effects on the local marine life. They pointed out that the construction of the Tucurui Hydroelectric Dam and its Tocantins reservoir may have possibly helped the success of local Cichla species, citing a previous study that noted the preference of this species toward stable water levels and sufficient littoral zones for reproduction.
While researchers noted that "it is impossible to explicitly connect changes in cichlid morphology detected here to the construction of the Tucuruí dam," the dam has played an important role - noting its changes to the hydrology of the local area, and soon affecting the ecology surrounding it.
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