Casting Wide Nets

McBride, a rare expert on Gulf Stream strays, led two notable studies on them in the late 1980s and early 1990s (1,2). Much of the fieldwork, during his PhD, was part of a long-term survey of larval fish at the Rutgers University Marine Field Station in Tuckerton, New Jersey. Sea air streamed into McBride’s office window, which was nestled in a small cluster of white clapboard outbuildings, steps from a pier and small cove. McBride hung mesh traps, dangling past the pilings, to catch larval fish in the murky water below.

At the time, several papers reported tropical small fry appearing in southern New England (3,4), and a tiny community of biologists had taken an interest. Ichthyologist Ken Able, McBride’s PhD advisor and director of the station for more than 30 years, had been sampling larvae in the inlet since 1989, dropping a mesh plankton net into the water weekly. As part of an ecological survey, he’d unscrew the net’s cod end and dump the sample into a bucket to go back to the station for sorting. The catch was mostly temperate species. But by the late ‘90s, around the time of McBride’s PhD, tropicals were more commonplace, too. So, McBride wasn’t shocked to pull Caribbean spotfin and four-eyed butterflyfish from below the pier. “But we were astonished at how many,” he says. “Day after day after day, we caught them for months in a row.”

McBride hung his fish traps for four years and caught babies of both species, no bigger than a fingernail, as early as July (1). The young fish grew over the summer to about the width of his hand, before disappearing in November. McBride scooped some of the butterflyfish into buckets and splashed them into aquariums the same temperature as the cove. They did fine until one autumn morning, when all the butterflyfish had sunk to the tank bottoms, having died overnight as water temperatures dipped below 50 degrees Fahrenheit. Others, in a heated control tank, largely survived. His conclusion: The ill-fated butterflyfish in the Rutgers boat cove somehow arrived anew every year, then died with the last heat of summer.

They probably washed in from reefs farther south, McBride reasoned. Many oceanic species, including butterflyfish, cast plumes of sperm and eggs into the waves, usually timed by the lunar cycle and tides. Called broadcast spawning, this produces thousands of sperm, eggs, and, eventually, tiny baby fish called larvae that drift. Butterflyfishes’ larval phase lasts about 40 to 60 days (5), long enough to drift hundreds of miles on a current. Only some luck upon a suitable home. Others, McBride says, “just disperse to the wrong habitat.” Cape Hatteras, North Carolina, is about the closest place with a resident butterflyfish population, though it’s still not clear whether the Rutgers larvae originate there. The Gulf Stream runs 15 miles offshore, and eggs and larvae could easily be swept north.

Then, in the year 2000, McBride published a second study based on fieldwork dragging a seine net through chest-deep water in estuaries around New York, netting lots of young crevalle jacks in the process(2). Young jacks grew fast and, by autumn, reached the size of bluefish, a brawny local species that migrates off the continental shelf. Might jacks migrate too? To find out, McBride scanned data from bottom-trawling surveys, tracking the diversity of fish off the continental shelf. Sure enough, trawls in autumn caught jacks and bluefish out there, suggesting the fish were moving south out of estuaries en masse. McBride also thumbed through the data for butterflyfish. He didn’t see a single one. “With the butterflyfish, it was clear they weren’t making it,” he says. “They didn’t even try to leave.”

McBride came to suspect that delicate territorial species, such as angelfish and butterflyfish, are true strays. But beefier tropicals, such as crevalle jacks, could be sending babies up north on purpose. Larvae would ride the Gulf Stream to sheltered northern nurseries, then migrate back south after summer.

To Unlock the Black Box

The Gulf Stream strays are an East Coast phenomenon. But these Atlantic fish aren’t the only ones with mysterious comings and goings. On the West Coast, too, flame-orange garibaldi and other fish species, typically restricted to kelp beds in Southern California or Baja, can pop up as far north as Monterey when currents change during El Niño.

Local phenomena offer case studies that inform a larger, global mystery as to where fish disperse and why, says fisheries oceanographer Robert Cowen, director of the Hatfield Marine Science Center, the hub of coastal research and education for Oregon State University in Newport. Knowing the journeys of baby fish is crucial to siting a marine reserve or setting fishing rules for neighboring islands, he says.

On a video call from his office in Oregon, Cowen holds up both of his hands. Imagine, he says, that’s he’s holding Jamaica in his left and the Virgin Islands in his right. If you assume that all the fish larvae are dispersing broadly, across his hands, then the Virgin Islands should replenish overfished stocks in Jamaica. But that doesn’t seem to be the case—the fish don’t tend to get that far, for reasons that aren’t entirely clear.

To test the scale of dispersal, Cowen simulated the larval journeys of three Caribbean species (a damselfish, a wrasse, and a grouper) in a 2006 study (6). He overlaid a physical model of currents rippling around the region with a biological model simulating each fish’s behavior during its 13- to 45-day larval phase. All three species float for their first week, then sink as they age to avoid predators. When Cowen simulated releasing billions of larvae into the model, he found they dispersed “a much shorter distance” than expected. Many settled on the same islands as their parents. Sinking behavior is probably why, Cowen says. Strong wind-driven currents at the surface push larvae offshore, but deeper currents tend to suck them back inshore. The larvae didn’t wash out far enough before sinking to escape the tug of their home island.

“What this paper suggested,” Cowen says, is that “most of the larvae aren’t going long distance.” They’re not dispersing across both of his hands. In other words, “if you overfish your own island,” he explains, “you won’t replenish it.”

Marine biologists today know the general whereabouts of most fish species around the globe. And thanks to pioneering work like Cowen’s, they’re beginning to draw arrows on that globe, connecting populations through the exchange of eggs and larvae. Blind spots still abound, though.

“That’s what we call the ‘black box,’” says ichthyologist Richard Coleman at the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science, on Virginia Key in Florida. He led one of the newer studies trying to unlock it—to explain fish dispersal around O’ahu, Hawaii.

Published in 2023, the work focused on the convict tang, a silvery-yellow surgeonfish named for its vertical stripes (7). Convict tang are ubiquitous in Hawaii, swirling everywhere from remote northwest reefs to the surf lapping at Waikiki Beach. Hawaiians fish for convict tang, and to inform long-term food security, communities around Kaneohe Bay on the windward coast of O’ahu wanted to know where their fish come from—whether breeding locally or washing in as larvae from other islands. Coleman worked with local fishermen to collect about 1,500 tissue samples from adults and young in the bay and at 23 sites around O’ahu. Sometimes, he went spearfishing. Other days, he’d wade into tide pools. Coleman analyzed the tissue at the Hawaii Institute of Marine Biology on Coconut Island. He compared markers of genetic variation, called single-nucleotide polymorphisms (SNPs), to make parent–offspring matches. Ultimately, 68 parent–child pairs shook out of the 1,200 samples.

Most of the pairs turned up around southern and eastern O’ahu. Parents and offspring could live within a few hundred meters. Many of the convict tang that hatched in Kaneohe Bay stayed there. The work revealed a surprising pattern. “Even though this thing is distributed across the entire Indo-Pacific, a lot of larvae stay close to home,” says senior author Brian Bowen, Coleman’s PhD advisor at the University of Hawaii at the time. Tang, like butterflyfish, are larvae for 40-plus days. Coleman and Bowen expected babies to wash much farther from their parents in that time, even to other islands.

All of which is to say, larvae must have some control over how they ride the tide. If they were passive drifters, the length of their larval phase would predict how far they get.

Tiny Wayfinders

Indeed, larvae may look like hapless specks in the swell, but they’re actually doing a lot, says marine ecologist Kim Selkoe, who founded Get Hooked Seafood in Santa Barbara and maintains a research position at the University of California campus there. Studies in the last decade find that larvae orient to light and dark, change their buoyancy, and even use chemoreceptors to sniff out coral reefs on which to settle (8–10). They plunge from the sunlit surface into deeper water, where the washing machine of currents can push them inshore or offshore. By their vertical movement, larvae exert some control over their destinies, though that’s “likely a bit of an accident,” Cowen says. The larvae dive because they develop better eyesight and can hide in deeper, darker water, he explains. The baby fish may scoot into an inviting pocket of water, a pocket that also just happens to be a current ripping inshore.

All these tricks complicate where larvae will go. Some of the most sophisticated simulations of their dispersal come from the National Oceanic and Atmospheric Administration (NOAA), where ecologists map the larval journeys of the fish we eat.

Fish biologist Ana Vaz pulls up one such map on her laptop at the Southeast Fisheries Science Center in Miami, Florida. Swirls of turquoise illuminate ocean currents in the Gulf of Mexico. Red dots clustered along the coast represent red snapper eggs at inshore breeding grounds. Thin red lines stretch into the Gulf, predicting where the snapper will drift over the course of a year. Many of the dots swirl south, threading through the Florida Keys, and up the Atlantic coast as far as North Carolina.

Vaz’s map combines oceanographic modeling with field data that NOAA collects every spring and fall. Ecologists drive their research boats into the Gulf, towing huge nets to collect eggs and larvae. Vaz gathers the resulting data, including where adults spawn, the buoyancy and density of their eggs, how larvae change depth, and many other factors. She plugs that data, combined with sea surface temperatures and other physical variables, into an oceanographic model to simulate releasing an egg into the Gulf.

The resulting map traces flows of larval fish across state lines, linking southeast snapper populations (11–13). “Our model suggests some states might receive most of their supply from outside their boundaries,” Vaz says. That means overfishing breeding grounds in one state could hurt snapper fisheries in another. Since fisheries and recreational licenses are managed at the state level, Vaz hopes this kind of study will encourage cooperation between jurisdictions.

Lines in the Sand

Maps like Vaz’s also set a baseline, establishing where larval fish travel today. Their journeys are already changing course as climate change heats up the seas.

Discoveries in the normally frigid waters of Maine offer a case in point. Coastal ecologist Jeremy Miller remembers the moment in 2017 when he found a spotfin butterflyfish in a net full of larvae from the Gulf of Maine. Miller’s discovery remains the only record of a larval butterflyfish in the state, which is normally too cold, even in summer, for tropicals to survive (14).

But the Gulf of Maine is warming fast. Coupled with more frequent storms and runoff that alters water chemistry, the conditions are inviting new species (15,16).

Miller leads a 17-year survey at the Wells Reserve, a NOAA-protected estuary in Wells, Maine, netting and identifying fish larvae off the local pier. Maine had a heat wave in 2012 that shattered records for the Gulf by 3 degrees Fahrenheit, and, since then, “all these southern species come into our dataset,” Miller says. New arrivals include black sea bass, northern sennet (a type of barracuda), and summer flounder. Miller is even netting few-day-old larvae, too young to have drifted far. The parents must be spawning farther north than they used to, the researchers reason. In response, Miller has tracked how the array of larval species has changed since surveys began in 2008. “We need to keep an eye on it,” he says, to see if newcomers start displacing native fish.

New tropicals have also washed into the Rutgers inlet, where ecologists still net butterflyfish in summer. The long-term survey that Able began 36 years ago marches on. Water temps have warmed there, too, and “we are seeing fewer northern species,” Able says, but more southern ones year-round. Little brown gobies, for example, normally darting south of Cape Hatteras, now appear resident (17).

Climate trends mean more change is coming. As corals bleach and die, the sounds and smells of shallow seas will shift, and baby fish may not have the auditory and scent cues to find reefs and settle, Coleman says. He’d like to replicate his convict tang study from O’ahu at another time of year and in combination with oceanographic modeling—ocean circulation patterns change seasonally, meaning larval pathways may change as well.

“A lot of things are happening out there that could affect currents, and currents affect fish,” McBride says. “It would be important to have an understanding of that to know where fishes will be abundant, not only today but in the future.”

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