Migratory shrimp require connectivity to live out their life cycle in which they travel downstream from the headwaters of the watershed to the ocean as larva, and then they travel from the ocean back upstream as juveniles. Migratory shrimp populations are decreasing from habitat loss and fragmentation caused by dams in watersheds in Puerto Rico. Shrimp are important in this ecosystem for nutrient cycling and keeping algae under control, as well as being an important component to the food web for predators.
According to a study published in River Research and Applications, researchers found shrimp habitats are decreasing due to the decreasing of connectivity in streams.
Dr. Jessica Chappell, a PhD researcher for the Odum School of Ecology at the University of Georgia and coauthors, used data from the U.S. Geological Survey (USGS) gages and gathered data from water intake stations to observe the daily total water withdraw from each watershed in the El Yunque National Forest (EYNF). Water intake stations withdraw water from the streams for use in the cities.
The watersheds were monitored for 37 years from 1980 to 2016, and over that time period connectivity of the streams decreased by 16% for refugia habitat and 27 % for total habitat. Stream connectivity refers to the continuous flow of a stream from the headwaters to the ocean. Any obstacles, such as dams and water intakes disturb the connectivity of a stream, as it decreases the ability of some organisms to migrate through the area. The total habitat for shrimp is where both life cycles occur, while refugia habitat is a section of habitat where larva live 5 meters above a waterfall. Refugia habitat has no predators, and these are hotspots for the shrimp population to grow, as no predators are impacting the population.
Chappell and her collaborators observed both total and refugia habitat, and it was found that over time the total habitat decreased by 27 percent and the refugia habitat decreased by 16 percent. These decreases are the result of new water intakes placed downstream. Refugia habitat is more sensitive to the location of water intakes, because these habitats are usually in the headwaters where the waterfalls are located. Adding one new water intake near the headwaters can highly impact the refugia habitat, as it decreases the connectivity early in the stream. This can be vital for the shrimp populations, since the larva need to be able to travel downstream to become juveniles. However, stream connectivity is not completely cut off, because when the stream flow is high enough, then shrimp larva can pass over the top of obstacles like low head dams and water intake areas.
The research showed that total habitat connectivity and refugia habitat connectivity was lowest during dry and drought months. During dry and drought months, the water flow is already low, so shrimp are unlikely to be able to pass over the barriers naturally. If an abundance of water is withdrawn during this time, then the connectivity decreases further, as there is even less water flowing in the streams. Habitat decreases in the total connectivity and refugia habitats of these shrimp can negatively impact the shrimp populations. This problem may be further amplified by climate change impacts, as it is predicted that there will be more drought periods.
The effects of climate change on stream flow and continued water withdraws from humans to satiate growing populations in cities will impact the habitat of freshwater shrimp. With the impacts climate change, it is important to monitor these changes on stream flow, especially during the dry and drought periods.
Chappell said it’s important to keep the flow of the rivers at above a certain threshold, so that it does not get too low and further decrease the connectivity of the streams. A decrease in the connectivity of rivers can impact the life cycles and habitats of organisms leading to population declines. Stream connectivity needs to be supported to protect the organisms and ecosystem of the area.
Works Cited
Chappell, J., McKay, S. K., Freeman, M. C., & Pringle, C. M. (n.d.). Long-term (37 years) impacts of low-head dams on freshwater shrimp habitat connectivity in northeastern Puerto Rico. RIVER RESEARCH AND APPLICATIONS. https://doi-org.proxy-remote.galib.uga.edu/10.1002/rra.3499
