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Studying blue crabs and their surrounding aquatic environments can tell us about species higher in the food chain.

Blue crabs are 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 where species live. Salinity and water temperature, for example, influence the presence of predatory fish, but so does the availability of prey.

If conservation measures result in the quick recovery of a dominant fish predator, this can lead to large decreases in its prey populations. 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 areas where both predator and prey overlap.

What We Investigated

We examined 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. We tested whether the number of predators depend only on environmental factors — like water temperature — or also on how much prey is around.

What We Found

As we expected, we found that both the environment and the abundance of prey drive where predators live.

The abundance of blue crab as prey influenced the number of their 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 consume 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 as a food source may explain why they occupy the same areas.

What It Means

Knowing where animals live is essential for managing wildlife, and good models can show us this. Our work helps 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 predator numbers solely from prey numbers. Estuaries are complex systems with large swings in temperature and salinity, as well as animals that move around a lot, and therefore we need more data to capture these changes to predict the numbers of predators more accurately.

the full study
“Predicting Predator Abundance from Prey in Estuaries: Insights from Single and Joint Species Distribution Modeling” in Estuarine, Coastal, and Shelf Science

more

“Why Is North Carolina’s Blue Crab Population Declining?”

“Will North Carolina’s Blue Crabs Go Extinct?”

Sally Dowd 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. Visit: sallycdowd.com.

This story originally appeared in North Carolina Sea Grant’s award-winning Hook, Line & Science series. More at HookLineScience.com

lead photo credit: Sally Dowd.