Long-term data reveal impact of dams and drought on migratory shrimp in Puerto Rico

Angela Hsiung, an.hsiung25@uga.edu

Contact: Jessica Chappell, jchapp@uga.edu

Athens, Ga. – From the ocean surrounding Puerto Rico, masses of shrimp begin their journey inland, climbing toward their freshwater birthplace. As larvae, these shrimps migrate downstream through rivers and streams into the ocean where they mature before beginning their upstream migration to the headwaters to mate and produce offspring.

A growing number of dams, combined with seasonal changes in water availability, have made it increasingly difficult for the shrimp to reach their destinations, according to a recent study from the University of Georgia. The research, based on 37 years of streamflow data from El Yunque National Forest, found that connectivity declined by 27% as low-head dams were installed to accommodate drinking water intakes in the lower reaches of area watersheds.

“Migratory shrimp provide important ecosystem services for the El Yunque streams,” said lead author Jessica Chappell, who received her doctorate in integrative conservation and ecology from UGA in 2019. She explained shrimp break down leaf litter, facilitating nutrient cycling; keep water clear by reducing sediment and algae buildup; and serve as a food source for other aquatic species. “So it’s important to have these shrimp populations maintained in the system,” she said.

Low-head dams are built to create pools of water deep enough to submerge drinking water intakes for local municipalities. Previous research has shown that Puerto Rico’s freshwater shrimp are able to overcome barriers like dams and waterfalls up to 100 feet high, but only when water is flowing over them. When water levels are too low to allow that, even the much smaller low-head dams are insurmountable.

Not being able to reach the ocean could potentially reduce survival of larval shrimp, while failure to reach spawning ground could reduce adult shrimps’ ability to reproduce; both could cause population decline.

To help water resources managers make decisions that have less impact on the shrimp populations in Puerto Rico, Chappell and colleagues evaluated how human-built structures affect habitat connectivity for shrimp, both over the long term and month-by-month. In particular, they were interested in the effects of drought and dry seasons, as well as the locations of dams and water intakes.

The researchers analyzed monthly water discharge, collected by U.S. Geological Survey water gages, and withdrawal data in seven watersheds within El Yunque from 1980 through 2016.

Their findings revealed that total habitat connectivity decreased by 27% and connectivity of refugia habitat—areas the shrimps’ predators are unable to reach—decreased by 16% over the 37-year span.

Furthermore, the data showed that the proportion of water withdrawn from the watersheds did not change significantly even as more intakes were added, indicating that the decrease in habitat connectivity is not necessarily due to having less water in the system.

“This really highlighted the importance of thinking about intake location within the watershed”, said Chappell. “The lower down they are in the watershed, there’s the possibility of disconnecting the whole watershed if that one barrier completely blocks water movement.”

Chappell and her colleagues also modeled changes in connectivity during drought years and dry seasons. They found that habitat connectivity for migratory shrimp decreased during drought years by 17% compared to non-drought years. Additionally, connectivity decreased by 7% in the study area during the dry season compared to the wet season.

Taken together, the addition of dams in the lower reaches of El Yunque’s watersheds combined with seasonal changes in water availability have made it more difficult for migratory shrimp in Puerto Rico to move between habitats. And with climate change projected to lead to prolonged dry seasons throughout the Caribbean, connectivity may be reduced even further.

Chappell said she hopes that the study’s findings will help water resources managers make decisions to meet the needs of people while taking into account the needs of Puerto Rico’s freshwater shrimp.

“We think potential management actions could be more informed if you really know what’s happening in these systems,” she said.

The paper, “Long-term (37 years) impacts of low-head dams on freshwater shrimp habitat connectivity in northeastern Puerto Rico,” was published in River Research and Applications. It is available online at https://onlinelibrary.wiley.com/doi/full/10.1002/rra.3499. Coauthors are S. Kyle McKay of the U.S. Army Corps of Engineers, Mary C. Freeman of the U.S. Geological Survey, and Catherine Pringle of the University of Georgia Odum School of Ecology. Support from the research came from the National Science Foundation, the International Institute of Tropical Forestry, the USDA Forest Service, the U.S. Army Corps of Engineers Ecosystem Management and Restoration Research Program, and the University of Georgia.