As Oceans Warm, Predators Are Falling Out of Sync with Their Prey

One way to better understand this apparent shift in striped bass recruitment and distribution in the Mid-Atlantic Bight— the coastal region that stretches from North Carolina’s Outer Banks to Massachusetts — is to look at similar shifts in the behavior of one of its key food sources, the Atlantic menhaden, a forage fish in the herring family. In recent years, menhaden have also been seen in high numbers off the New Jersey and New York coasts — Van Bergen described an early November trip in which the ocean surface was thick with menhaden for some 25 miles. But just like striped bass, menhaden numbers in the Chesapeake and other estuaries, where the fish was once reliably abundant, have been low.

“I don’t know if this is a larger cyclical pattern, if it’s driven by how they’re managed, or if it’s because the water temperature is increasing,” said Janelle Morano, a doctoral student at Cornell University who has been studying how menhaden distribution has changed along the U.S. East Coast over time. “But something is going on, and it is real.”

Taken together, the shifts in behavior of these two interconnected species resemble aspects of a phenomenon that is being observed across the planet, from land to sea: phenological mismatch.

Phenology is the seasonal timing of lifecycle events, like spawning and migration. Think of how honeybees emerge from their hives just as spring flowers bloom, or how in autumn, the monarch butterfly migrates south to Mexico as milkweed begins to die off in the United States. Phenological mismatch, however, occurs when these intricate, interspecies relationships fall out of sync due to changes in the environment. Terrestrial cases of phenological mismatch have been well documented. For example, detailed analysis has shown that, over the past 29 years, monarch migration has been delayed by six days due to warming temperatures, triggering mismatches with food availability during the journey and failures to reach overwintering sites.

But in the oceans, phenological mismatch has been far less studied. Every scientist interviewed for this story noted that while there has been good research on single-species phenology in marine environments, there remains precious little understanding of multispecies phenological mismatch. The subject, they said, urgently requires more focus because of the potential knock-on effects that mismatches could cause up and down the food chain. They also cautioned that all species, marine and terrestrial, are prone to natural swings in abundance, and that declines or increases can’t be pinned to any one stressor. Overfishing and stock management are just two external factors that may be influencing phenological mismatch in the world’s oceans. As the authors of a paper published in Nature Climate Change that focused on this lack of knowledge put it, “Given the complexity involved, accurately forecasting phenological mismatch in response to climate change is a major test of ecological theory and methods.”

Nevertheless, warning signs are beginning to be observed in marine ecosystems planetwide, from herring and zooplankton in the North Sea, to sardines and bottlenosed dolphins in the Southern Ocean, to — along with striped bass — baleen whales and menhaden in the northwest Atlantic.

To be sure, striped bass don’t rely on menhaden as critically as monarchs rely on milkweed. But the fish does seem to be responding to shifts in menhaden behavior and abundance and, experts say, both species are likely responding to changes that have occurred in the Mid-Atlantic Bight and the Gulf of Maine over the past quarter-century — in particular, to warming water. Collectively, these ecosystem-wide shifts could be reshaping where and how striped bass and menhaden spawn, move, feed, and, ultimately, interact. How these effects ricochet across the food chain — from impacts on planktonic organisms all the way up to the human communities that rely on fisheries and the marine environment in general for economic and cultural survival — remains largely unknown.

One of the few certainties in the marine ecosystem is that water temperature is on the rise, and rapidly so in the Northwest Atlantic. For example, between 2004 and 2019, the Gulf of Maine warmed more than seven times the global average, or “faster than 99 percent of the global ocean,” as the Gulf of Maine Research Institute puts it. In the southern Gulf of Maine and the Mid-Atlantic Bight, the heating has virtually eliminated one of the striped bass’s key food sources, the American lobster. This contraction in prey variety may be negatively impacting striped bass, especially older individuals, which can lack the fitness necessary to chase fast-moving prey, like menhaden and mackerel. The disappearance of lobster has forced them to compete for other resources with younger, more agile fish.

“Fluctuations in the abundance of prey populations may… drive predators to consume less energy-dense but more abundant prey, leading to declines in predator condition,” Robert Murphy, a social scientist at the Northeast Fisheries Science Center, and colleagues wrote in a 2022 study of striped bass feeding behavior. In his observations of striped bass, Waldman has indeed noted a constriction in diet. “It used to be that striped bass would come in small groups along the shore over the whole autumn and eat cockles and eels and crabs and lobster,” he said. “But now, it has shifted to this almost complete focus on big aggregations of bait fish.”

A similar change in diet is being observed in the Southern Ocean off South Africa, where the annual KwaZulu-Natal sardine run is one of the most spectacular examples of phenology on the planet. As the Southern Hemisphere winter approaches in May, great schools of sardine emerge from deeper water and congregate along the coast of South Africa, moving northward with a current of cold water. Over millennia, myriad species, from bottlenosed dolphins to sharks, penguins, and gannets, have timed their lifecycles — their survival — to the event.

But in the past 60 years, the sardines have been arriving progressively later, as their instinct to follow cold water has become confused by the southerly creep of warmer water. As a result, many of the sardine’s predators arrive too late to take advantage of the feast. Scientists who have studied the KwaZulu-Natal sardine run have hypothesized that this mismatch has diminished the abundance and distribution of Cape gannets and African penguins. According to one study, bottlenosed dolphins have shifted their dietary focus from sardines to mackerel. “When events like this are disrupted, it can have a knock-on effect,” Stephanie Plön, a marine biologist at South Africa’s Stellenbosch University and coauthor of the study told the BBC in June.

Phenological mismatches like these are also not isolated to the upper levels of the food chain. There are likely reverberations reaching all the way to the base.

In the Northeast Atlantic and in the North Sea, zooplankton and phytoplankton have been declining over the last half-century. For herring, plankton is critical to the success of a given season’s spawning class. In one study conducted in the North Sea, researchers found that the success of herring larvae is closely related to the abundance of zooplankton and phytoplankton, both of which are highly sensitive to temperature. Like the rest of the world’s oceanic regions, the North Sea is experiencing significant warming. “Although the causal mechanisms remain unclear, declining abundance of key planktonic lifeforms in the North-East Atlantic… are a cause of major concern for the future of food webs,” the authors of another study of North Atlantic zoo- and phytoplankton concluded.

One of the most critical types of zooplankton to the marine food web are krill, a shrimp-like crustacean that everything from whales to penguins to squid and seabirds relies on for survival. In 2021, a team of French and British scientists found that krill have been in steep decline throughout the North Atlantic. Krill have also not simply moved north in response to the steady creep of warm water toward the Arctic. Instead, they are experiencing a “habitat squeeze” — essentially, they are condensing in cold pockets of water, wherever they may occur. “We would expect the krill populations to simply shift northward to avoid the warming environment,” Martin Edwards, one of the study authors, said. “However, this study shows… in the North Atlantic, marine populations do not simply just shift their distributions northward.”

Dave Secor, a professor of fisheries science at the University of Maryland Center for Environmental Science’s Chesapeake Biological Laboratory, noted that in recent years in the Mid-Atlantic Bight, the behavior of North Atlantic right whales — whose diet relies heavily on krill — does not cleanly track with what has been termed the “poleward march” theory. “There is evidence that there has actually been a southerly shift in their concentrations,” Secor said. “Oceanography is not linear. Things are happening in fits and starts.” Regarding striped bass in the region, Secor said there clearly has been a shift in the timing of spawning and migration. “The question is whether that can be sufficiently adaptive to the more rapid changes we’ve experienced in recent years.”

Just as the KwaZulu-Natal sardine run is critical to commercial fisheries in South Africa, and the availability of herring in the North Sea sustains cultural culinary traditions in European countries, striped bass and menhaden are critical to local economies driven by recreational fishing in the U.S. Mid-Atlantic and in New England. Ultimately, that means the knock-on effects of phenological shifts and interspecies mismatches will reverberate beyond marine ecosystems and into more entrenched and less dynamic human ecosystems. As Waldman said, the species that might be the least capable of adapting to the changes underway in the oceans might be us. “Some people will lose the fisheries they grew up on and made their livings from,” he said. “And there may be nothing we can do about that.”