By Roberta Attanasio, IEAM Blog Editor
With their sapphire-tinted claws, their olive green shell, and their paddle-shaped rear swimming legs, blue crabs are easily recognizable. They’re famous not only for their looks, but also for their telltale scientific name (Callinectes sapidus), which translates roughly to “savory beautiful swimmer.” Indeed, they’re prized for their tender meat and sweet, delicate flavor and are, not surprisingly, the most heavily harvested crustaceans in the geographical areas in which they live. They’re found in brackish coastal lagoons and estuaries from Nova Scotia, through the Gulf of Mexico, and as far south as Uruguay. Notably, they’re considered the Chesapeake Bay’s signature crustaceans.
Over the past 60 years, blue crabs have dominated Chesapeake Bay commercial and recreational fisheries, with an estimated one-third of the nation’s blue crab catch coming from the Bay, which is the nation’s largest estuary. Moreover, as both predator and prey, blue crabs are a keystone species in the Chesapeake Bay food web and are therefore key players in the Bay’s ecosystem. However, they’re vulnerable to pollution, habitat loss, and harvest pressure, and their abundance fluctuates over time.
For most of the last two decades, the total number of blue crabs in the Chesapeake Bay has lingered below the long-term average. In 2019, though, there has been a nearly 60 percent crab population increase compared with 2018. The Chesapeake Bay Program, through the Chesapeake Bay Watershed Agreement signed in 2014, oversees the restoration of the Bay, its tributaries and the lands that surround them, recognizing that water quality improvements, underwater grass restoration, and proper harvest management are critical to maintaining, among others, blue crab populations at healthy levels. Now, it’s becoming clear that climate change may also become a major contributing factor in the fluctuation of blue crab populations—and possibly in a good way.
Investigators at the University of Maryland Center for Environmental Science carried out a study, recently published in PLOS One, to explore how changes in water temperature may impact the overwintering behavior and winter survival of blue crab in the Chesapeake Bay in the next 100 years. They used computer-modeled projections of future temperature from the World Climate Research Programme’s Coupled Model Intercomparison Project and predicted that winters will be up to 50% shorter by 2100. The shortening of winter will be accompanied by increases in average wintertime temperature—thus, overwinter survival of the blue crab will increase by at least 20% compared with current conditions.
Similar to all other crustaceans, blue crabs grow by molting—there are rapid increases in size associated with the shedding of the external carapace (growth per molt), followed by periods of stasis (intermolt period). The intermolt period is temperature dependent. Therefore, the external temperature determines growth rate and key life history events such as maturation. In simple words, warmer waters make the blue crabs grow faster. Study co-author Tom Miller said: “Blue crabs are a climate change winner in the bay. As the bay gets warmer, they will do better because they are a more tropical species. We always hear about those species that are going to struggle or move. Blue crabs are going to do better.”
Not all is as good as it seems, though. For example, climate change not only leads to warming temperatures, but also to increased variability in temperatures, and could therefore complicate wintertime management of the crab populations. Miller added: “If crabs start moving and feeding year-round, they represent an added predation pressures on the bay’s ecosystem, and we don’t know how the ecosystem will respond.”
Anson Hines, director of the Smithsonian Environmental Research Center in Edgewater, Maryland, told The Washington Post that a warming climate could lure more southern predators that enjoy snacking on blue crabs to the Chesapeake. In addition, warmer temperatures may have negative impacts on resources that are good for crabs, including underwater grasses in which they hide from predators. Their own major food source—the Baltic clams—might not survive the warmth.
Interestingly, naturally warmer temperatures are helping blue crabs to establish themselves in the Mediterranean as invasive species. They were first sighted in the Ebro Delta on Spain’s Mediterranean coast in 2012, where they probably arrived via ships’ ballast tanks. Since then, the population has expanded exponentially, wiping out native species. While in their natural habitat blue crabs become inactive during the winter, in the Mediterranean they keep growing thanks to the higher temperature. Intensive fishing seems the only way to control their expansion, therefore they’re now somewhat viewed as a resource, and there is a growing profitable market for it—they’re making their way into local signature dishes, such as paella.
Invasive blue crabs are also present in Tunisia, and now they’re expanding even in Italy’s southern Apulia region, a holiday hot-spot, where experts have warned parents, and particularly tourists, to watch their children and keep them away from the aggressive species that, with large, sharp claws and powerful jaws, destroys fishermen nets and may harm people that mistake them for native crustaceans.
As blue crab populations keep widening their habitats, let’s expect to hear more about them.