Studying blue crabs and their surrounding aquatic environments can tell us about species higher in the food chain.
Research need
Blue crab is a major commercial fishery in North Carolina, but the population has been declining for over a decade, prompting regulators to consider new restrictions. The reasons for the decline are unclear. People catch a lot of blue crabs, but other factors are also in play — from water quality to predation to habitat availability, and even hurricanes.
Both living and non-living parts of an ecosystem impact which waters species frequent. Salinity and water temperature influence predatory fish, for example, but so does the availability of prey.
If conservation measures result in quick recovery of a dominant fish predator, this can lead to large decreases in the populations of its prey species. Yet, population assessments rarely account for the effects of predators on their prey.
To see if predation plays a key role in the decline of the blue crab population, we must first study the areas that both predator and prey frequent.
What did we study?
We studied the distribution of red drum, southern kingfish, and black drum in North Carolina’s Pamlico Sound and its surrounding waters. These predatory fish eat blue crabs, commonly frequent the estuaries, and share similar habitats with one another.
We built models for these three predators using 18 years of data that field biologists with the N.C. Division of Marine Fisheries collected annually in May and June. These models use field observations to show where species live and what affects this. We tested whether the number of predators depend only on environmental factors (like water temperature) or also on how much prey is around.
What did we find?
As we expected, we found that both the environment and the abundance of prey drive where predators live.
The abundance of blue crab prey influenced the number of predators in an area — particularly red drum and southern kingfish, but not black drum. Previous surveys in Pamlico Sound found that red drum and southern kingfish eat more blue crabs (about 30% of their diet) than black drum (3%). The stronger reliance of both red drum and southern kingfish on blue crabs for prey may explain why they occupy the same areas.
So what?
Knowing where an animal lives is important for managing wildlife. Good models can show us this. Our work helps us improve these models in dynamic places by looking at prey and other environmental factors.
These models showed how predator, prey, and the environment are connected, but it is hard to predict predators’ numbers solely from prey numbers. Estuaries are complex systems with large changes in temperature and salinity and animals that move around a lot. We need more data to capture these changes to predict the numbers of predators more accurately.
Reading
Dowd, S., Roberts, S. M., Bacheler, N. M., & Nye, J. A. (2025). Predicting predator abundance from prey in estuaries: Insights from single and joint species distribution modeling. Estuarine, Coastal and Shelf Science, 313, 109083.
Sally Dowd (above) is a graduate student at the University of North Carolina at Chapel Hill. She is based at the coast at UNC’s Institute of Marine Sciences in Morehead City. See sallycdowd.com for more information on her research and artwork.
lead photo credit: Sally Dowd.
The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant.
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