Every spring, something quiet and largely invisible happens along the eastern coast of the United States. Sharks begin to move. Not the dramatic, circling predators of popular imagination, but common coastal species that most beachgoers would never notice, gliding just offshore as water temperatures begin to climb. They have been making this journey for far longer than anyone has been watching. What science is only now beginning to understand is exactly how they do it, when they do it, and what rising ocean temperatures may eventually do to those patterns.

A new study offers the most detailed portrait yet of these annual migrations. Researchers tracked 201 sharks belonging to four species along nearly 1,800 kilometers of Atlantic coastline from Virginia to the Florida Keys, following individual animals for up to seven years using networks of underwater acoustic receivers. What emerged is a rich picture of seasonal movement that will serve as a critical reference point as warming oceans push these animals into an uncertain future.

Four Species, Four Personalities

The study focused on four shark species that share the southeastern US coastline but approach life quite differently: the blacknose shark, the finetooth shark, the lemon shark, and the Atlantic sharpnose shark. Despite belonging to the same broad family and occupying much of the same geographic range, each species turned out to have a distinct migration personality.

Blacknose, finetooth, and lemon sharks all follow a broadly similar strategy. In winter, their core populations cluster together in a relatively compact zone off east-central Florida, huddled near the point where the warm northward flowing Florida Current diverges from shore, creating a pocket of water mild enough to ride out the cold months. When spring arrives and coastal water temperatures begin to rise, these populations migrate northward through Georgia and into the Carolinas, spreading out along the coast from roughly March through April. They spend the warmer months widely distributed along the South Atlantic coast before returning south to Florida by December.

The Atlantic sharpnose shark operates by a strikingly different logic. Rather than concentrating in Florida during winter, sharpnose sharks disperse widely and can be found off the Carolinas even in the coldest months. Their center of abundance then contracts south into east-central Florida each summer, essentially the reverse of the pattern seen in the other three species. The reasons for this unusual behavior remain genuinely unclear to the researchers.

The Scale of the Operation

The numbers behind this study are impressive. Over 11.5 years of data collection, the research team recorded more than 1.4 million valid detections of tagged sharks across 573 acoustic receiver stations. The receivers, part of two collaborative tracking networks spanning the eastern seaboard, were deployed offshore at nine strategically chosen locations from Chesapeake Bay to the Florida Keys. Every time a tagged shark swam close enough to one of these underwater listening stations, its unique coded signal was recorded, building a detailed log of its movements over time.

The 201 sharks in the study were tracked for an average of about three years each. Finetooth sharks proved the most reliable migrants, with all 50 individuals tracked for more than a month completing annual round trips. One male finetooth made the journey from Cape Canaveral, Florida, to Port Royal Sound, South Carolina, in just five days in April 2018, covering roughly 80 kilometers per day. Blacknose sharks were similarly committed to migration, with over 80 percent undertaking it regularly, though a small number of individuals appeared content to remain in Florida waters year round, sometimes for up to seven consecutive years, confirming that long distance migration is not obligatory for the species.

What Is Actually Driving the Journey?

Temperature is clearly central to these migrations, but the study found that the story is more nuanced than a simple response to warm and cold water.

Sea surface temperature turned out to be one of the two most important predictors of when sharks arrived at key locations along the coast, accounting for roughly 21 to 36 percent of the statistical explanation for migration timing in the models the researchers ran. The other major factor was the rate of change in day length over the preceding month, which accounted for a similar proportion of migration timing. This makes biological sense: day length acts as a more reliable long range calendar than temperature, which can fluctuate unpredictably from week to week. A shark that departs for its summer feeding grounds based purely on ambient water temperature risks being caught out by a late cold snap. One that also responds to the lengthening days of late winter gets an advance signal that the season is genuinely turning.

Interestingly, the study found that sharks generally migrated northward at slightly cooler temperatures than they migrated southward, suggesting that the spring urge to move north can get ahead of the water warming. This may actually be advantageous. The coastal waters off Georgia and South Carolina are exceptionally productive in spring, with satellite measurements showing chlorophyll levels in March and April running roughly double those found off Cape Canaveral. These productive waters support dense populations of prey including menhaden, Atlantic croaker, and shrimp. Arriving early means more time in prime feeding territory.

Reproduction also plays a role. For blacknose and finetooth sharks, May and June are pupping months, and the data showed that pregnant females tagged in Florida were consistently detected off Georgia and South Carolina in April or May of the same year, having traveled hundreds of kilometers specifically for the purpose of giving birth. Newborn finetooth and lemon sharks in this region associate with river mouths, estuaries, and beachfronts, making the warm productive shallows of the Georgia and Carolina coastlines essential nursery habitat for the next generation.

What the Satellite Cannot See

One of the reasons this kind of study matters is that traditional methods of monitoring coastal sharks have consistently missed what acoustic telemetry reveals. Fisheries surveys and landing records are snapshots. They tell you how many sharks of a given species were present in a particular place during a sampling trip. They miss the early arrivals and late departures, the transient visitors passing through, and the individuals who use habitats infrequently enough to avoid gear.

The acoustic data painted a more complete picture. Blacknose sharks, for instance, were detected in South Carolina waters from April through January, and finetooth sharks from April through November, in both cases two months longer than previously documented by a six year statewide field survey. The tracking network also revealed that juvenile lemon sharks make regular annual visits to Georgia and South Carolina waters, despite being very rarely captured by traditional sampling gear because of their preference for shallow surf zones that are difficult to sample.

The study also confirmed something important for fisheries management. Not a single one of the 201 tagged sharks was detected crossing into the Gulf of Mexico. This reinforces that the separate Atlantic and Gulf of Mexico stock classifications for blacknose, finetooth, and sharpnose sharks are scientifically appropriate, since the populations do not appear to mix through the Florida Straits. For lemon sharks, tracked individuals showed minimal connectivity between the east coast juvenile population and the robust lemon shark community in the Florida Keys, suggesting that exchange around the Florida peninsula may be largely limited to adults.

Winners, Losers, and the Warming Question

The central long term purpose of this baseline data is to prepare for a future that looks different from the present. Sea surface temperatures have already risen nearly one degree Celsius above pre-industrial levels globally, and projections point to further increases of one to three degrees by 2100. For marine species whose migrations are calibrated to temperature thresholds and seasonal cues, even modest warming can shift the timing and geography of annual movements in ways that cascade through ecosystems and fisheries.

The researchers offer a cautiously optimistic note about the four species they studied. All four are generalists with flexible diets, broad thermal tolerances, and the ability to move quickly across large distances. Unlike species tied to coral reefs, sea ice, or other habitats at acute risk, these sharks can in principle shift their ranges northward if warmer water allows or requires it. Similar habitats do extend further north along the Atlantic coast, and the data suggest these species are already comfortable across a wide range of temperatures during their annual cycles.

However, the Gulf of Mexico stocks of the same species face a more constrained future. A northward range shift is not possible for sharks living in an enclosed sea. Their options are more limited, and the consequences of continued warming may be correspondingly more severe.

There is also the question of timing mismatch. If warming temperatures cause sharks to arrive in northern nursery and feeding areas earlier in the season, but the prey species those sharks depend on respond to warming differently or more slowly, the animals may arrive before their food does. These kinds of ecological mismatches are a well documented consequence of climate change in other taxa and could undermine the benefits of migration even as the physical ability to migrate is preserved.

A Listening Network for the Future

Perhaps the most practical contribution of this research is the methodological groundwork it lays for future monitoring. By establishing precise, year by year records of when specific individuals arrived at specific locations under specific temperature conditions, the study creates a benchmark that researchers decades from now can compare against.

The researchers are candid about the limitations of the current dataset. Almost all the tagging occurred at a single location, Cape Canaveral, which risks creating a biased picture of sharks that have a strong affinity for that particular overwintering area. A fully representative survey would tag sharks distributed along the entire coastline. They also note that the acoustic receiver networks, while extensive, have limited coverage in deeper offshore water, which likely explains why sharpnose sharks, with their preference for deeper habitats, proved harder to track comprehensively.

Looking ahead, the researchers argue for making migration data collection more systematic and permanent. Acoustic telemetry lines anchored at a relatively small number of critical chokepoints along the coast could provide standardized, ongoing records of migration timing that would be comparable across decades. These stations could potentially be integrated into existing coastal infrastructure such as piers, offshore energy platforms, and oceanographic monitoring buoys, reducing the cost of long term maintenance. All tracking data, they emphasize, should be archived in publicly accessible regional databases so that future researchers can revisit and reanalyze the raw material as new questions emerge.

Reading the Thermometer Through a Shark

There is something fitting about using sharks as instruments to measure an ocean in transition. These animals have been responding to seasonal temperature cues for millions of years, tuning their internal calendars to the rhythms of warming and cooling water along coastlines that looked very different from the ones they navigate today. Their migrations encode information about ocean conditions that no satellite can fully capture.

What this research has done is begin to decode that information in a rigorous, quantitative way. The blacknose shark that departs Cape Canaveral in late March when the water hits a certain temperature, the finetooth shark that accelerates northward as the days begin to lengthen, the pregnant lemon shark making her way to Georgia to give birth in shallow estuarine waters: each of these movements is a data point about how ocean conditions shape animal behavior. Together, they form a baseline against which the effects of a warming world can one day be measured.

That measurement is coming. The question is whether the monitoring infrastructure will be in place to make it.

Publication Details:

Year of Publication: 2025; Journal: Marine Ecology Progress Series ; Publisher: Inter-Research: DOI: https://doi.org/10.3354/meps14823

Credit & Disclaimer: This article is based on the peer-reviewed research paper. All scientific facts, findings, and conclusions are drawn directly from the original study and remain faithful to its content. Readers are encouraged to consult the full research article for complete data, methodology, and supplementary materials.