Marine Scientists Investigating If Warming At Sea Hurting Fish Populations By Scrambling Feeding, Reproductive Cycles
For decades on the U.S. Mid-Atlantic coast, recreational anglers have braved the cold temperatures of late October and November to chase one of the region’s most iconic fish species, the striped bass. This season, just offshore of New Jersey and New York, the fall run was especially strong. “The amount of fish and [their size] was really, really high,” said Lou Van Bergen, a captain of Miss Barnegat Light, a 90-foot party boat out of Barnegat Light, New Jersey. “Every week, all the way through Thanksgiving, you could go out and catch nicer-sized fish.” From the looks of the boat’s deck this fall, it would have been easy to assume that striped bass, once overfished to dangerously low numbers on the East Coast, had completed a remarkable comeback. Except that in the nearby Chesapeake Bay and in the Hudson River, where the fish return each spring to spawn, the hatching and maturation of juveniles “has been abysmal,” said John Waldman, an aquatic conservation biologist at the City University of New York. Waldman, an avid fisherman himself, called the low levels of striped bass recruitment, or spawning success, in these historically fertile estuaries “a real mystery.”
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.”
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https://e360.yale.edu/features/ocean-migration-mismatch