For decades along the U.S. Mid-Atlantic coast, recreational anglers have braved the cold temperatures of late October and November to pursue one of the region’s most famous species, the striped bass. This season, off the coast of New Jersey and New York, the fall run has been particularly strong. “The quantity of fish and [their size] has been really, really high,” said Lou Van Bergen, captain of Miss Barnegat Light , a 90-foot party boat out of Barnegat Light, New Jersey. “Every week, up until Thanksgiving, you could go out and catch nicer fish.”
From the look of the boat deck in the fall, it would have been easy to guess that striped bass, once overfished to dangerously low numbers along the East Coast, had completed a remarkable comeback. Except that in the nearby Chesapeake Bay and Hudson River, where the fish return each spring to spawn, juvenile hatchability and maturation have been “abysmal,” said John Waldman, an aquatic conservation biologist at the City University of New York. Waldman, an avid fisherman himself, called the low level of striped bass recruitment or spawning success in these historically prolific estuaries “a real conundrum.”
Warning signs are observed in marine ecosystems worldwide from the North Sea to the South Sea Ocean.
One way to better understand this apparent shift in striped bass recruitment and distribution in the Mid-Atlantic Bight—the coastal region stretching from the Outer Banks of North Carolina to Massachusetts—is to consider 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 where the ocean surface was thick with menhaden for about 40 miles. But just like striped bass, menhaden numbers have been low in the Chesapeake and other estuaries where the fish was once reliably plentiful.
“I don’t know if this is a larger cyclical pattern, if it’s due to how they’re being managed, or if it’s due to rising water temperatures,” said Janelle Morano, a doctoral student at Cornell University who has studied how menhaden distribution has changed over time along the U.S. East Coast. “But something is happening, and it’s real.”
Taken together, the behavioral shifts of these two linked species resemble aspects of a phenomenon observed across the planet from land to sea: phenological mismatches.
Phenology is the seasonal timing of life cycle events such as spawning and migration. Think of how honeybees emerge from their hives just as spring flowers bloom, or how the monarch butterfly migrates south to Mexico in the fall when milkweed is dying in the United States. Phenological mismatch, however, occurs when these complicated, interspecies relationships fall out of synchronization due to environmental changes. Terrestrial cases of phenological mismatch have been well documented. For example, one 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 preventing overflight sites from being reached.
However, far less has been studied in the oceans. Every scientist interviewed for this story noted that while there has been good research on individual species phenology in marine environments, there is little understanding of multispecies phenological mismatch. This subject, they said, urgently requires more focus due to the potential impacts it could have on up and down the food chain. They also cautioned that all species, marine and terrestrial, are susceptible to natural fluctuations in abundance and that decreases or increases can be unrelated to a stressor. Overfishing and stock management are just two external factors that can influence phenological mismatch in the world’s oceans. As the authors of a paper published in Nature Climate Change focused on this lack of knowledge, “Given the complexity, accurately predicting phenological mismatch in response to climate change is an essential test of ecological theory and methods.”
Nevertheless, warning signs are observed in Marine Ecosystems Planetwide from herring and zooplankton in the North Sea to sardines and bottlenose dolphins in the Southern Ocean – along with striped bass – baleen whales and menhaden in the Northwest Atlantic.
The decline of lobster in the Mid-Atlantic has forced older striped bass to compete for food with younger, more agile fish.
Of course, striped bass don’t rely on menhaden as critically as monarchs rely on milkweed. But the fish do appear to be responding to shifts in menhaden behavior and abundance, and experts say both species are likely responding to changes in warming water. Collectively, these ecosystem-wide shifts could reshape where and how striped bass and menhaden spawn, move, feed, and ultimately interact. How these impacts extend beyond the food chain—from effects on planktonic organisms to the human communities that rely on fisheries and the marine environment more generally for economic and cultural survival—is largely unknown.
One of the few certainties in the marine ecosystem is that water temperatures are warming, and rapidly 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, warming has virtually eliminated one of the striped bass’s primary food sources, the American lobster. This contraction in prey diversity can negatively impact striped bass, particularly older individuals that lack the fitness to pursue fast-moving prey like menhaden and mackerel. The disappearance of lobsters has forced them to compete with younger, more agile fish for other resources.
“Fluctuations in the abundance of prey populations can…drive predators to consume less energy-dense but more abundant prey, leading to a decline in predator condition.” Waldman’s study of striped bass feeding behavior has indeed revealed a narrowing in their diet. “It used to be that striped bass would come along the shore in small groups throughout the fall, eating cockles, eels, crabs, and lobsters,” he said. “But now it’s shifted to this almost complete focus on large aggregations of baitfish.”
A similar dietary shift is observed in the Southern Ocean off South Africa, where the annual KwaZulu-Natal sardine run is one of the planet’s most spectacular examples of phenology. As Southern Hemisphere winter approaches in May, large schools of sardines emerge from deeper water and congregate along the coast of South Africa, moving north on a current of cold water. Over millennia, countless species, from bottleneck dolphins to sharks, penguins, and gannets, have tuned their life cycles—their survival—for the event.
Krill haven’t simply moved north. Instead, they’re concentrating in cold water pockets wherever they can be found.
Over the past 60 years, however, sardines have arrived increasingly later, as their instinct to follow cold water has been confounded by the southward creep of warmer water. As a result, the arrivals of many of the sardine’s predators are out of sync with the feast. Scientists studying the KwaZulu-Natal sardine run have hypothesized that this mismatch has reduced the abundance and distribution of Cape gannets and African penguins. According to one study, bottleneck dolphins have shifted their dietary focus from sardines to mackerel. “When such events are disrupted, it can have an impact,” Stephanie Plön, a marine biologist at South Africa’s Stellenbosch University and co-author of the study, told the BBC in June.
Phenological mismatches like these aren’t isolated to the upper levels of the food chain. There are likely reverberations that extend all the way down to the base.
In the Northeast Atlantic and the North Sea, zooplankton and phytoplankton have declined over the past half century. Herring plankton is critical to the success of a given season’s spawning class. In a study conducted in the North Sea, researchers found that the success of herring larvae is closely linked to the abundance of zooplankton and phytoplankton, both of which are highly sensitive to temperature. Like the rest of the world’s marine regions, the North Sea is experiencing significant warming. “Although the causal mechanisms are unclear, the functioning of the major planktonic life forms in the Northeast Atlantic is a major concern for the future of food webs,” concluded the authors of another study of North Atlantic zooplankton and phytoplankton.
One of the most critical zooplankton species in the marine food web is krill, a shrimp-like crustacean that everything from whales to penguins to squid and seabirds depends on for survival. In 2021, a team of French and British scientists found that krill has declined sharply throughout the North Atlantic. Krill has also been moving northward in response to the steady creep of warm water toward the Arctic. Instead, they are experiencing a “habitat squeeze”—essentially condensing into pockets of cold water wherever they can be found. “We would expect krill populations to simply shift northward to avoid the warming environment,” said Martin Edwards, one of the study’s authors. “However, this study shows… in the North Atlantic, marine populations are not just shifting their distributions northward.”
Dave Secor, a professor of fisheries science at the University of Maryland’s Chesapeake Biological Laboratory for Environmental Sciences, noted that in recent years, the behavior of North Atlantic right whales in the mid-Atlantic, whose diets depend heavily on zooplankton, does not fit neatly with the so-called “poleward march” theory. “There is evidence that there has indeed been a southward shift in their concentrations,” Secor said. “Oceanography is not linear. Things happen in fits and starts.” Regarding striped bass in the region, Secor said there has clearly been a shift in the timing of spawning and migration. “The question is, can this be sufficiently adaptive to the rapid changes we’ve seen in recent years?”
Just as the KwaZulu-Natal sardine run is crucial for 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 crucial to local economies driven by recreational fishing in the Mid-Atlantic and New England. Ultimately, this means that the impacts of phenological shifts and mismatches between marine ecosystems and less dynamic human ecosystems extend beyond marine ecosystems. As Waldman said, the species least able to adapt to changes in the oceans could be the ones that lose out. “Some people are going to lose the fisheries where they grew up and made their living,” he said. “And there’s nothing we can do about it.”