Sharks of Onslow County

Category: Sandbar shark

  • How Sharks Carry the Future: Life Histories Written in Tide and Time

    How Sharks Carry the Future: Life Histories Written in Tide and Time

    The Season Beneath the Surface

    Along the North Carolina coast, spring does not arrive all at once. It filters in through temperature gradients, longer light, and currents that shift almost imperceptibly until the water itself feels different. Animals respond before people do. Some move north. Some move inshore. Others arrive carrying a process already underway — reproduction unfolding quietly inside bodies designed to measure time in seasons rather than days.

    This post explores shark reproduction in North Carolina, not as spectacle, but as a system of time, geography, and survival.

    Shark reproduction is rarely visible. There are no surface displays, no spectacle to announce the moment. Instead, lineage advances through anatomical engineering and geographic choreography. The coastline becomes a corridor through which inheritance travels. What appears to be migration is often the hidden architecture of the next generation. Across shark species, reproductive strategies are tightly bound to life history pacing — longevity, growth rate, and investment per offspring — forming evolutionary solutions calibrated to risk and time (Cortés, 2000; Musick & Ellis, 2005).

    Sharks do not share a single blueprint for reproduction. Some lay eggs encased in protective capsules that anchor to the seafloor. Others carry embryos internally and give birth to fully formed young. Between those extremes lies a spectrum of strategies — eggs retained inside the mother, embryos nourished in different ways, gestation stretched across seasons rather than weeks. The diversity is not incidental. It is the result of a lineage experimenting with how best to move the future through water: protect it externally, carry it internally, or invest in a few individuals built to survive from the first moment they enter open ocean (Carrier et al., 2012; Cortés, 2000).

    The Long Circuit of the Dogfish

    Each winter, Atlantic spiny dogfish (Squalus acanthias) thin from our nearshore waters. Their absence is not disappearance but redistribution. Along the Northwest Atlantic coast the species occupies a broad range from Canada to the Carolinas, but this range is not a single undifferentiated mass. Seasonal movements reveal two general latitudinal tendencies — a northern contingent centered toward New England and Canadian waters, and a southern contingent extending toward North Carolina. In spring, portions of both groups converge in mid-Atlantic shelf waters, where overlapping migrations create temporary reproductive mixing before adults disperse again toward their habitual ranges (Carlson et al., 2014).

    This convergence is not random drift. It is structured migration. Satellite tracking shows that spiny dogfish follow repeatable north–south circuits tied to temperature and habitat gradients rather than wandering opportunistically (Carlson et al., 2014). During these seasonal overlaps, sex and maturity stage influence where individuals position themselves within the shared corridor. Females and mature animals use space differently from juveniles, reflecting reproductive status and energetic demand (DeVries et al., 2025). The result is a coastline briefly braided by lineage: individuals from distant home waters exchanging genetic material before returning south or north to complete gestation.

    migration patterns atlantic spiny dogfish

    Atlantic spiny dogfish do not disappear when they leave our waters; they redistribute. Each triangle marks where a tagged shark surfaced months after deployment, tracing seasonal circuits that braid northern and southern populations together before they separate again. The shaded regions show the broad envelope of movement and the smaller core areas used most consistently. Migration here is not wandering — it is structure. Reproduction moves along these same corridors, written into geography long before it is visible at the surface. | Graphic credit: Carlson et al., 2014

    After fertilization, females carry embryos for nearly two years — among the longest gestation periods recorded in sharks (Hamlett, 2005). A single pregnancy produces relatively small litters, commonly averaging six to twelve pups, each representing a substantial maternal investment spread across seasons rather than weeks (Hamlett, 2005; Cortés, 2000). Birth does not occur in the same waters where mating took place. Instead, adults retreat toward their familiar temperature zones and feeding grounds, and the next generation enters the ocean already geographically sorted. Migration and reproduction form a loop rather than a point. Each cycle redistributes genes across the coast while preserving the regional rhythms that structure the population.

    This extraordinary investment in time creates vulnerability. Sharks with slow growth, delayed maturity, and extended gestation replace themselves gradually, making populations sensitive to elevated fishing pressure (Cortés, 2000; Musick & Ellis, 2005). Removing a late-term female represents not a single loss, but the collapse of years of biological investment in a species evolved for endurance rather than speed.

    Reading the Body

    Female sharks often carry scars along their flanks and fins — pale arcs and punctures that appear deliberate enough to invite explanation. These marks are frequently attributed to mating, and sometimes that interpretation is correct. During copulation, males grip females with their teeth to maintain position in moving water, producing patterned abrasions consistent with tooth spacing (Pratt & Carrier, 2005). But the body of a coastal predator is an archive of many encounters, not all of them reproductive.

    Mating scars recorded on female blue sharks. The pale arcs and punctures along the flank, gill region, and fins are bite marks left during courtship, when males grip females to maintain position in open water. Some individuals carry a single mark; others bear layered evidence of repeated encounters. These scars are not pathology but record — the body retaining brief moments of reproductive contact long after the act itself has vanished into current. What remains visible is the aftermath: lineage written lightly into skin. | Image credit: Vossgaetter et al., 2025
    Mating scars recorded on female blue sharks. The pale arcs and punctures along the flank, gill region, and fins are bite marks left during courtship, when males grip females to maintain position in open water. Some individuals carry a single mark; others bear layered evidence of repeated encounters. These scars are not pathology but record — the body retaining brief moments of reproductive contact long after the act itself has vanished into current. What remains visible is the aftermath: lineage written lightly into skin. | Image credit: Vossgaetter et al., 2025

    Fishing gear produces different signatures: hooks damage the jaw, entanglement leaves constricting linear abrasions, and vessel strikes create irregular trauma. Healed injuries accumulate across a lifetime, recording survival rather than singular events. Marine biologists interpret these marks through context — season, species behavior, wound geometry — understanding that a scar is evidence, not confession (Pratt & Carrier, 2005). The ocean rarely supplies a single explanation.

    The skin of a white shark carries a record of encounters. Different wounds trace different histories: restrained bite marks associated with courtship (A & B), deeper bites from conflict (C & D), punctures and scratches left by struggling prey (E & F), abrasions from contact with reef or hard bottom (G), and the unmistakable geometry of propeller strikes (H). Each mark is a fragment of interaction preserved after the moment has passed. To read a shark’s body is to read a map of relationships — mating, hunting, collision, survival — written not as drama, but as accumulation. | Photo credit: Anderson et al., 2025
    The skin of a white shark carries a record of encounters. Different wounds trace different histories: restrained bite marks associated with courtship (A & B), deeper bites from conflict (C & D), punctures and scratches left by struggling prey (E & F), abrasions from contact with reef or hard bottom (G), and the unmistakable geometry of propeller strikes (H). Each mark is a fragment of interaction preserved after the moment has passed. To read a shark’s body is to read a map of relationships — mating, hunting, collision, survival — written not as drama, but as accumulation. | Photo credit: Anderson et al., 2025

    Scars are only one layer of interpretation. Sharks also carry quieter markers of sex and maturity written into their form. Males develop elongated claspers — modified fins that trail beneath the body — visible even at a distance once the animal reaches reproductive age. In immature males these structures are short and flexible, almost decorative. With maturity they lengthen and calcify, projecting clearly behind the pelvic fins like paired shadows. A school viewed from a pier often reveals this difference in motion: some bodies carry that trailing geometry, others do not. Even without knowing species, an observer is watching a mixed population divided by sex and age.

    Females, lacking claspers, present a cleaner silhouette. During pregnancy their bodies shift subtly. The abdomen rounds, not dramatically but enough to change how light moves across the flank. Experienced observers recognize gravid females less by size than by proportion — a redistribution of mass that suggests internal cargo rather than surface injury.

    The clasper itself is an evolutionary innovation — a modification of pelvic fins that allows internal fertilization in a fluid environment where external fertilization would disperse gametes too widely to ensure success (Hamlett, 2005). It is a structural solution to a problem posed by water: how to keep lineage from dissolving into current.

    Sex in sharks is written into the silhouette. Males carry paired claspers — elongated extensions of the pelvic fins that lengthen and stiffen with maturity — while females lack them entirely. Even at a distance, the trailing geometry changes how the body reads in motion. What looks like a uniform school from the surface is already divided by anatomy: juveniles, adults, males, females, each stage visible to anyone patient enough to watch. | Photo credit: National Oceanic and Atmospheric Administration
    Sex in sharks is written into the silhouette. Males carry paired claspers — elongated extensions of the pelvic fins that lengthen and stiffen with maturity — while females lack them entirely. Even at a distance, the trailing geometry changes how the body reads in motion. What looks like a uniform school from the surface is already divided by anatomy: juveniles, adults, males, females, each stage visible to anyone patient enough to watch. |
    Photo credit: National Oceanic and Atmospheric Administration

    These signals are quiet. They require patience. To read a shark in the water is to read a body moving through stages — juvenile, mature, gravid — each phase revealing that reproduction is not a single event but a condition carried across seasons. The distinction is anatomical literacy learned slowly, the way birdwatchers learn silhouettes or botanists learn leaf shape. Bodies announce their histories to those patient enough to look.

    Timing Written Into the Body

    Maturity does not arrive uniformly across a population. In many coastal sharks, size is a better predictor of reproductive readiness than age. Warmer water accelerates metabolism and growth, allowing juveniles in southern nurseries to reach maturity sooner than their northern counterparts (Cortés, 2000; Musick & Ellis, 2005). Temperature becomes a developmental force. A difference of a few degrees can compress or extend the timeline by years, shaping when an individual enters the reproductive pool.

    Juveniles and adults often sort themselves accordingly. Young sharks cluster in shallower, warmer margins where rapid growth offsets vulnerability. Larger, mature individuals occupy deeper or more exposed water, their size granting a margin of safety (Heupel et al., 2007). When mixed schools appear near piers, the variation in body shape reflects overlapping life stages sharing temporary habitat. What looks like a single group is often a layered demographic — future breeders moving alongside current ones.

    During mating seasons, additional cues surface. Mature males display fully calcified claspers held stiff against the body, while gravid females carry the rounded proportions of pregnancy. These changes are not theatrical. They are subtle adjustments in geometry, visible only to observers willing to compare silhouettes over time.

    Nurseries and Geographic Memory

    Many coastal shark species rely on estuaries as nursery grounds, where shallow, structured habitat increases juvenile survival by buffering predators and concentrating prey (Heupel et al., 2007). Young sharks enter a world scaled to their size. Warmer water accelerates growth, and complex shoreline geometry provides refuge during early vulnerability.

    Some females exhibit fidelity to nursery regions, returning to the same coastal systems that once sheltered them (Heupel et al., 2007). Habitat becomes inheritance. When nursery grounds degrade, the disruption extends beyond a single generation — it interrupts geographic memory embedded in the population itself.

    Multiple Ways to Continue

    Sandbar Sharks — Durability Over Speed

    A sandbar shark range from New England to Brazil. | Photo Credit: G.P. Schmahl/NOAA

    Sandbar shark | Photo Credit: G.P. Schmahl/NOAA

    Sandbar sharks (Carcharias plumbeus) invest heavily in durability. They mature late, produce relatively small litters, and rely on long development to generate robust juveniles capable of extended survival (Musick & Ellis, 2005). This strategy favors stability over speed. When mortality rises, recovery unfolds slowly because the species was never designed for rapid turnover.

    Sandbar shark reproduction unfolds slowly even by shark standards. Gestation lasts roughly 9–12 months, with litters typically ranging from 6 to 13 pups, though regional variation is common (Musick & Ellis, 2005). Along the mid-Atlantic coast mating generally occurs in spring and early summer, while birthing follows the next year in warmer estuarine margins. The delay is part of the design. Juveniles arrive when prey is abundant and water temperature accelerates growth, aligning birth with a narrow ecological window where survival odds briefly tilt in their favor.

    In Onslow County waters, juvenile sandbar sharks use shallow estuary margins as thermal accelerators. Warm, protected water shortens the time required to reach a size less vulnerable to predation. Growth in these early months is not cosmetic; it is survival measured in centimeters. A difference of a few inches can determine whether a young shark passes unnoticed beneath larger predators or becomes part of their diet (Heupel et al., 2007). The nursery functions as a buffer against probability. By compressing early growth into a brief window of ecological generosity, sandbars convert geography into longevity.

    Blacktip Sharks — Timing as Opportunity

    Atlantic blacktip sharks | Photo credit: Shutterstock
    Atlantic blacktip sharks | Photo credit: Shutterstock

    Blacktip sharks (Carcharhinus limbatus) align reproduction with seasonal pulses. Birth coincides with warm water and prey abundance, creating a temporary ecological advantage for juveniles. This strategy accepts higher early mortality but compensates through timing — survival synchronized with opportunity (Heupel & Simpfendorfer, 2008).

    Blacktip sharks compress their timeline. Gestation averages 10–12 months and litters often contain 1 to 10 pups, with smaller litters more common in northern portions of their range (Heupel & Simpfendorfer, 2008). Mating occurs in late spring and summer; pups are born the following late spring when baitfish concentrations peak in shallow coastal waters. Their strategy hinges on synchronization. Birth is timed not to safety, but to opportunity — a calculated arrival into abundance.

    Along our piers in late spring and summer, blacktip juveniles appear in pulses that mirror the prey fields they depend on. Schools of baitfish create moving refuges — density as defense — and young blacktips learn to survive inside motion itself. Survival belongs to individuals able to exploit brief windows, grow fast, and disperse before scarcity returns (Heupel & Simpfendorfer, 2008).

    Bonnethead Sharks — Redundancy and Retention

    Bonnethead shark | Photo credit: NC Aquariums
    Bonnethead shark | Photo credit: NC Aquariums

    Bonnethead sharks (Sphyrna tiburo) operate on one of the shortest reproductive cycles among coastal sharks. Gestation lasts approximately 4–5 months, and litters commonly range from 4 to 16 pups depending on female size (Hamlett, 2005). Mating generally occurs in late summer, but sperm storage allows fertilization to be delayed until environmental conditions favor gestation. Pups are born in late spring and early summer, entering warm shallow waters that function as immediate nurseries. The speed of the cycle reflects a species built for resilience through repetition — rapid turnover as insurance against instability.

    Bonnetheads add evolutionary contingency. Rare cases of parthenogenesis — reproduction without fertilization — demonstrate biological redundancy when mates are scarce (Chapman et al., 2007). Such flexibility underscores a principle of lineage persistence: survival tolerates complexity if complexity improves continuity.

    Bonnetheads, often glimpsed in shallow surf or near pilings, compress life history into shorter cycles, allowing populations to respond quickly to environmental change. Unlike many coastal sharks, females are capable of storing viable sperm for extended periods, delaying fertilization until conditions favor successful gestation (Hamlett, 2005). This ability decouples mating from pregnancy, allowing reproduction to align with environmental timing rather than immediate opportunity. Redundancy becomes insurance in a fragmented coastal landscape. Their persistence is not brute strength but flexibility — an evolutionary acknowledgment that coastlines are rarely stable for long (Cortés, 2000).

    Sand Tiger Sharks — Survival Before Birth

    Sand tiger shark | Photo credit: Mitchell, 2024
    Sand tiger sharks | Photo credit: Mitchell, 2024

    Sand tiger sharks (Carcharias taurus) represent an uncompromising alternative. Embryos compete within the uterus, and only the strongest survive to birth through intrauterine cannibalism — a process that produces a small number of highly developed juveniles (Hamlett, 2005). From a human perspective the mechanism appears brutal. In evolutionary terms it is a concentrated investment in pre-birth survival.

    Sand tiger gestation stretches close to 9–12 months, but the internal competition that defines their development reduces litters to one or two surviving pups per uterus despite a much larger initial embryo count (Hamlett, 2005; Branstetter & Musick, 1994). Mating occurs offshore in cooler months, and births typically follow in spring or early summer. The resulting juveniles are large at birth — already capable hunters — trading quantity for immediate competence. Survival is front-loaded. The species invests in a few individuals built to endure rather than many built to gamble.

    For sand tigers occasionally seen near South Topsail Island, this pre-birth selection produces juveniles that enter the water already comparable in size to many adult coastal fish. They arrive as functioning predators. Instead of a long vulnerable childhood, sand tigers begin life past the most dangerous bottleneck. Their subsequent behavior reflects this early security: slow movement, energy conservation, and longevity built on having cleared the lethal threshold before birth (Branstetter & Musick, 1994).

    It is tempting to read personality into origin. Yet adult sand tigers move with calm efficiency, rarely engaging in unnecessary conflict. A harsh developmental filter does not predict a harsh adulthood. It simply ensures survival past the most intense threshold.

    Together, these strategies map the same coastline through different biological clocks. Some sharks survive by accelerating early growth. Others invest in a few individuals built to last. Still others hedge their future with redundancy. Diversity is not excess — it is resilience expressed through bodies.

    The Coast as a Clock

    Longevity is the silent partner in every reproductive strategy. Long-lived sharks can afford to reproduce slowly, distributing investment across decades. Shorter-lived species compress reproduction into tighter intervals. Neither strategy is superior in isolation. Each is calibrated to environmental tempo (Cortés, 2000).

    The coastline holds many clocks at once — tides measured in hours, migrations in seasons, lineage in centuries. Sharks survive by aligning their bodies to the clock that fits their niche. Gestation becomes a wager on stability. Migration becomes inheritance in motion. A nursery becomes infrastructure for continuity.

    To observe a pregnant shark offshore is to witness a process already years in motion. The animal carries not only embryos but evolutionary decisions accumulated across millennia: how many to produce, when to move, where to shelter, how long to live. Reproduction is less an event than a continuity. Its future depends not on spectacle, but on whether the slow mathematics of these lives can continue unfolding inside waters still capable of carrying them forward.

    References

    Branstetter, S., & Musick, J. A. (1994). Age and growth estimates for the sand tiger in the northwestern Atlantic Ocean. Transactions of the American Fisheries Society, 123(2), 242-254. https://doi.org/10.1577/1548-8659(1994)123<0242:aageft>2.3.co;2

    Carlson, A. E., Hoffmayer, E. R., Tribuzio, C. A., & Sulikowski, J. A. (2014). The use of satellite tags to redefine movement patterns of spiny dogfish (Squalus acanthias) along the U.S. East Coast: Implications for fisheries management. PLoS ONE, 9(7), e103384. https://doi.org/10.1371/journal.pone.0103384

    Carrier, J. C., Musick, J. A., & Heithaus, M. R. (2012). Biology of sharks and their relatives (2nd ed.). CRC Press.

    Chapman, D. D., Shivji, M. S., Louis, E., Sommer, J., Fletcher, H., & Prodöhl, P. A. (2007). Virgin birth in a hammerhead shark. Biology Letters, 3(4), 425-427. https://doi.org/10.1098/rsbl.2007.0189

    Cortés, E. (2000). Life history patterns and correlations in Sharks. Reviews in Fisheries Science, 8(4), 299-344. https://doi.org/10.1080/10408340308951115

    DeVries, C., Gartland, J., & Latour, R. J. (2025). Patterns in spiny dogfish consumption by sex and maturity stage relate to prey availability and environmental forcing in the Northwest Atlantic. Frontiers in Marine Science, 12. https://doi.org/10.3389/fmars.2025.1621343

    Heupel, M., Carlson, J., & Simpfendorfer, C. (2007). Shark nursery areas: Concepts, definition, characterization and assumptions. Marine Ecology Progress Series, 337, 287-297. https://doi.org/10.3354/meps337287

    Heupel, M., & Simpfendorfer, C. (2008). Movement and distribution of young bull sharks Carcharhinus leucas in a variable estuarine environment. Aquatic Biology, 1, 277-289. https://doi.org/10.3354/ab00030

    Musick, J. A., & Ellis, J. K. (2005). Reproductive evolution of chondrichthyans. In Reproductive Biology and Phylogeny of Chondrichthyes (1st ed., pp. 45-79). Science Publishers.

    Pratt, H. L., & Carrier, J. C. (2005). Elasmobranch courtship and mating behavior. In Reproductive Biology and Phylogeny of Chondrichthyes (1st ed., pp. 129-169). Science Publishers.

  • The Winter Guild: Nearshore Sharks and the Ecology of Cold Water

    The Winter Guild: Nearshore Sharks and the Ecology of Cold Water

    On a winter morning in Surf City, the beach feels emptied of its usual cast. Pelicans still cruise the shoreline in loose, patient lines, rising and settling with the wind. Gulls hover over seams in the water where green folds into brown. The tide sounds heavier now, denser, carrying cold through the shallows.

    Along Topsail Island, the nearshore zone becomes a narrow corridor of motion and restraint. Waves collapse without urgency. The water clears between fronts. What summer spreads wide, winter compresses.

    To most people, this looks like absence. The season reads as retreat.

    But the coastal system has not gone dormant. It has been edited.

    Cold water does not simply slow life along the coast; it reorganizes it. As the air cools, shallow waters lose heat first. Deeper layers follow. The sharp thermal steps of summer—warm surface, cool bottom—soften into sameness. What had been stacked becomes blended. Oceanographers describe this seasonal collapse as thermal and density homogenization (Cai et al., 2021), but on the shore it feels like weight: the water darker, heavier, less willing to give anything up.

    This change reshapes everything that lives within it. Temperature governs metabolism. Light governs production. Density governs movement. Winter redraws those rules.

    Predation does not vanish. It narrows.

    Reading the Cold and Salt

    Winter along the Carolina coast is defined less by dates than by gradients. Surface waters along the inner shelf commonly cool into the range of about 8–12 °C (46–54 °F), while deeper waters offshore remain slightly warmer under the influence of slope waters and the Gulf Stream (Atkinson et al., 1983; Cai et al., 2021; Rasmussen et al., 2005). The warm-bottom refuges of summer collapse. The mixed layer deepens. A single, colder column replaces the layered world of warm months.

    Sea surface temperature (SST) range for Topsail Island, NC from 1981-2005. The thick yellow line shows average SST compared to 1984, while thin, black lines show extreme temperatures. | Photo credit: Surf-forecast.com, 2005
    Sea surface temperature (SST) range for Topsail Island, NC from 1981-2005. The thick yellow line shows average SST compared to 1984, while thin, black lines show extreme temperatures. | Photo credit: Surf-forecast.com, 2005

    Salinity follows a similar simplification. In winter, open shelf waters settle into a narrow band—typically around 32–35 parts per thousand (ppt), the same saltiness as the open Atlantic. Freshwater input diminishes, and stronger winds and tides smooth what summer once layered.

    Average salinity utilizing historical ship and buoy data. Notice practical salinity is higher (red) along the North Carolina coastline. | Photo credit: World Ocean Atlas, 2009
    Average salinity utilizing historical ship and buoy data. Notice practical salinity is higher (red) along the North Carolina coastline. | Photo credit: World Ocean Atlas, 2009

    In summer, that chemistry fractures. Heavy rains, river discharge, and weak vertical mixing dilute nearshore and estuarine waters into the low 20s ppt or even the teens, sending plumes of brackish water outward from creek mouths and sounds (Singer et al., 1980). Onslow Bay becomes chemically patchworked: stratified sounds, plume-fed inlets, and salinity fronts that drift and reform with each tide.

    Winter erases that mosaic. Only near inlets and estuary mouths do sharp gradients persist, briefly stacking fresher creek water over denser seawater before winds and tides flatten them again. Where summer offered a quilt of chemical habitats, winter replaces it with continuity.

    Light changes too. Shorter days and deeper mixing reduce phytoplankton growth, pushing the productive layer below the surface and dimming the water’s green cast. Satellite and in situ records from the Mid-Atlantic shelf show winter as the seasonal low point for surface chlorophyll and primary productivity (Xu et al., 2011). The surface darkens. The food web thins from its base upward.

    Winter does not remove life. It rearranges it.

    Benthic invertebrates burrow or slow. Many fishes retreat or become lethargic. Bait compresses into fewer corridors—thermal seams, nearshore troughs, inlet mouths, shelf breaks—where temperature and oxygen remain tolerable. What summer scattered across marsh, creek, sound, and surf now funnels into lines.

    This is what the filter does. It sheds surplus. It strips away species that require warmth, shallow stability, or dense prey. What remains are animals built to endure cold, exploit structure, or move precisely between systems.

    Winter does not simplify the coast. It sharpens it.

    3D perspective of coastline of Topsail Beach inner shoreface visualizes depth. | Photo credit: Greenhorn & O'Mara Consulting Engineers & Geodynamics, 2007, p. 221
    3D perspective of coastline of Topsail Beach inner shoreface visualizes depth. | Photo credit: Greenhorn & O’Mara Consulting Engineers & Geodynamics, 2007, p. 221

    The Inshore Winterer: Atlantic Spiny Dogfish

    By midwinter, nearshore waters along this coast settle into a narrow thermal band—often between about 8 and 12 °C (46–54 °F) from surface to bottom (Atkinson et al., 1983; Cai et al., 2021). For many coastal fishes, that range marks the edge of activity. For Atlantic spiny dogfish (Squalus acanthias), it is home.

    Across the North Atlantic, dogfish remain active in waters as cold as 4–6 °C (39–43 °F), with their highest densities often occurring well below the temperatures that drive other sharks away (Bangley & Rulifson, 2014; Sulikowski et al., 2010). Their physiology, growth patterns, and life history are tuned to persistence rather than speed, favoring endurance in lean systems over burst performance in rich ones (Tribuzio et al., 2010).

    When winter flattens the water column into uniform cold, dogfish are not pushed to the margins of survival. They are moving within their preferred envelope.

    Their bodies are built for this season. Unlike sharks bound to a narrow depth band, dogfish move freely through the water column—from the surface to depths approaching 200 meters—tracking temperature and prey through vertical space (Campana et al., 2009; Carlson et al., 2014; Sulikowski et al., 2010). In winter, when baitfish and invertebrates compress into fewer layers, that vertical freedom becomes a hunting advantage. A predator locked to one plane must wait. Dogfish can follow.

    Nearshore waters and inlets become conveyor belts in winter. Tides concentrate prey flushed from estuaries. Cold fronts reorganize the column. What looks empty from the beach is often a thin band of movement just beyond the breakers. Dogfish occupy that band—persistent, economical, often in loose groups—feeding on schooling fishes and benthic prey even as energy margins tighten (Bangley & Rulifson, 2014).

    Winter does not exclude them. It clears room for them.

    The Atlantic spiny dogfish moves freely through the water column from the surface up to 200 meters. | Photo credit: ©Malcolm Francis
    The Atlantic spiny dogfish moves freely through the water column from the surface up to 200 meters. | Photo credit: ©Malcolm Francis

    The Threshold Species: Sandbar Sharks

    Sandbar sharks (Carcharhinus plumbeus) read that same winter map very differently. In warm months, juveniles rely on shallow estuaries where bottom temperatures routinely exceed 15–18 °C (59–64 °F), conditions that accelerate growth, digestion, and survival (Bangley et al., 2018; Collatos, Abel & Martin, 2020). These flats become engines of development—wide, shallow spaces where warmth turns food into body.

    By winter, those same bottoms cool into the low teens Celsius—often around 10–13 °C (50–55 °F) and sometimes lower. The estuary does not become lethal. It becomes unprofitable. Feeding no longer offsets the energetic cost of movement and digestion in cold water, particularly for juveniles still building mass.

    Unlike dogfish, sandbar sharks cannot remain inside a cold system and adapt to its structure. Their bodies and life histories are tuned to warm, shallow stability. Their performance drops rapidly as temperature declines; muscle efficiency, digestion, and growth all slow (Crear et al., 2019). When that stability collapses, they respond laterally rather than vertically—sliding down the coast or into slightly deeper nearshore waters where bottom temperatures remain marginally warmer (Bangley et al., 2018).

    Where dogfish remain and work winter’s compression, sandbars leave it.

    They are still part of the region, but they are absent from the nearshore corridor. They become threshold species—present in the seasonal arc, absent from the winter system itself. The estuary no longer belongs to them.

    Sandbar sharks will move to warmer water in winter and will move to deeper offshore waters or move southward down the coast. | Photo credit: (c) The Wet Lens, 2023
    Sandbar sharks will move to warmer water in winter and will move to deeper offshore waters or move southward down the coast. | Photo credit: (c) The Wet Lens, 2023

    The Offshore Presence: Dusky Sharks

    Dusky sharks (Carcharhinus obscurus) follow a third geometry. They tolerate cool water, but within a narrower band than many coastal sharks—most often occupying waters between roughly 10 and 20 °C (50–68 °F) (Bangley et al., 2020; Manz et al., 2025). When nearshore temperatures along this coast fall into that range, duskies can be present. When they rise beyond it in spring and summer, they are gone.

    Their winter preference therefore lies not in the compressed nearshore column, but along offshore corridors where temperature is steadier and remains within that narrow envelope. Shelf and slope waters influenced by the Gulf Stream often stay buffered within those bounds even as inshore waters swing widely (Atkinson et al., 1983; Rasmussen et al., 2005).

    Dusky sharks do not attempt to work winter’s compression. They choose stability instead. Rather than hunting within a narrowed corridor, they reposition along thermally buffered routes where cold arrives slowly and predictably.

    From land, this reads as vacancy. The surf appears emptied. Yet beyond the bar, duskies remain active in a parallel winter economy—tracking prey along shelf edges and slope corridors invisible from shore.

    They have not disappeared. They have changed address.

    Dusky sharks prefer cooler temperature waters and may be present in our nearshore waters. However, when waters warm beyond 20 °C (68 °F), they will move to cooler waters. | Photo credit: jmartincrossley, iNaturalist, 2026
    Dusky sharks prefer cooler temperature waters and may be present in our nearshore waters. However, when waters warm beyond 20 °C (68 °F), they will move to cooler waters. | Photo credit: jmartincrossley, iNaturalist, 2026

    Winter as an Ecological Filter

    By February, the coast has shed its surplus.

    The fish that needed warmth are gone. The invertebrates that depended on light have slowed or buried. What remains are species built to move through cold, to wait, or to follow structure instead of abundance.

    Winter filters by physics first.

    A deep mixed layer removes warm-bottom refuges (Cai et al., 2021). Bottom-water intrusions in Onslow Bay and adjacent shelf waters restructure the vertical habitat, replacing summer’s layered gradients with cold continuity (Hofmann et al., 1981). Uniform salinity simplifies the chemical landscape. Reduced light and productivity shrink the food web’s base (Xu et al., 2011). Currents sharpen boundaries between inshore and offshore waters, and periodic Gulf Stream intrusions create moving seams of heat and prey (Atkinson et al., 1983; Rasmussen et al., 2005).

    These constraints become biological.

    Prey compress. Movement costs rise. Energy budgets tighten. Species that require constant warmth, shallow stability, or dense forage are excluded. Those that remain are specialists: animals that tolerate cold, exploit vertical structure, or reposition with precision.

    Dogfish persist because they can work the layered winter column, remaining inside nearshore waters and inlets even as surface and bottom temperatures converge. Sandbar sharks withdraw from estuaries and shallow flats, shifting down the coast or into slightly deeper nearshore habitats where bottom temperatures remain metabolically tolerable. Dusky sharks relocate more completely, leaving the coastal corridor for offshore shelf and slope waters where Gulf Stream influence preserves thermal stability and prey remains distributed.

    Each species reads the same season and answers it differently—not by disappearance, but by repositioning within a changing map.

    Winter does not erase complexity. It concentrates it.

    Meteorological seasons are based upon the annual temperature cycle and differ from the astronomical seasons based upon the position of the sun relative to the Earth. The meteorological seasons help scientists track climate and weather changes. | Photo credit: NOAA, 2024
    Meteorological seasons are based upon the annual temperature cycle and differ from the astronomical seasons based upon the position of the sun relative to the Earth. The meteorological seasons help scientists track climate and weather changes. | Photo credit: NOAA, 2024

    A Season That Is Beginning to Shift

    The cues that drive migration—photoperiod, temperature thresholds, energetic margins—are no longer fixed. Recent models suggest that warming oceans are already altering how and when coastal sharks move, stretching winter windows and delaying departures (Manz et al., 2025).

    This does not announce collapse. It signals reorganization.

    From the beach, the change may remain imperceptible. The surf will still look calm. Pelicans will still cruise. The marsh will still pale. But beneath that surface, the system will continue to read its invisible clocks.

    Winter along Surf City and Topsail Island is not an end. It is a narrowing—a season where cold, salt, light, and current decide who remains.

    They are still here. They have simply become harder to see.

    References

    Atkinson, L. P., Lee, T. N., Blanton, J. O., & Chandler, W. S. (1983). Climatology of the southeastern United States continental shelf waters. Journal of Geophysical Research: Oceans, 88(C8), 4705-4718. https://doi.org/10.1029/jc088ic08p04705

    Bangley, C. W., Curtis, T. H., Secor, D. H., Latour, R. J., & Ogburn, M. B. (2020). Identifying important juvenile dusky shark habitat in the Northwest Atlantic ocean using acoustic telemetry and spatial modeling. Marine and Coastal Fisheries, 12(5), 348-363. https://doi.org/10.1002/mcf2.10120

    Bangley, C. W., Paramore, L., Dedman, S., & Rulifson, R. A. (2018). Delineation and mapping of coastal shark habitat within a shallow lagoonal Estuary. PLOS ONE, 13(4), e0195221. https://doi.org/10.1371/journal.pone.0195221

    Bangley, C. W., & Rulifson, R. A. (2014). Feeding habits, daily ration, and potential predatory impact of mature female spiny dogfish in North Carolina coastal waters. North American Journal of Fisheries Management, 34(3), 668-677. https://doi.org/10.1080/02755947.2014.902410

    Cai, C., Kwon, Y., Chen, Z., & Fratantoni, P. (2021). Mixed layer depth climatology over the Northeast U.S. continental shelf (1993–2018). Continental Shelf Research, 231, 104611. https://doi.org/10.1016/j.csr.2021.104611

    Campana, S. E., Jamie, A., & Gibson, F. (2008). Stock structure, life history, fishery and abundance indices for spiny dogfish (Squalus Acanthias) in Atlantic Canada (2007/089). Canadian Science Advisory Secretariat. https://www.researchgate.net/publication/268389393_Stock_Structure_Life_History_Fishery_and_Abundance_Indices_for_Spiny_Dogfish_Squalus_acanthias_in_Atlantic_Canada

    Carlson, A. E., Hoffmayer, E. R., Tribuzio, C. A., & Sulikowski, J. A. (2014). The use of satellite tags to redefine movement patterns of spiny dogfish (Squalus acanthias) along the U.S. East Coast: Implications for fisheries management. PLoS ONE, 9(7), e103384. https://doi.org/10.1371/journal.pone.0103384

    Collatos, C., Abel, D. C., & Martin, K. L. (2020). Seasonal occurrence, relative abundance, and migratory movements of juvenile sandbar sharks, Carcharhinus plumbeus, in Winyah Bay, South Carolina. Environmental Biology of Fishes, 103(7), 859-873. https://doi.org/10.1007/s10641-020-00989-2

    Crear, D. P., Brill, R. W., Bushnell, P. G., Latour, R. J., Schweiterman, G. D., Steffen, R. M., & Weng, K. C. (2019). The impacts of warming and hypoxia on the performance of an obligate ram ventilator. Conservation Physiology, 7(1). https://doi.org/10.1093/conphys/coz026

    Greenhorn & O’Mara Consulting Engineers, & Geodynamics. (2007). High-Resolution 3D Bathymetric Assessment of Potential Hard Bottom Habitats: Topsail Island, Surf City and North Topsail Island, NC (Project No. DACW54-02-D-0006, Delivery Order 0035 Modification 01 Nearshore Hardbottom Sidescan Survey for Multibeam Data Collections Topsail Island, NC G&O Project Number 146046.T35.6481.GEO). https://www.saw.usace.army.mil/Portals/59/docs/coastal_storm_damage_reduction/SCNTB/R%20-%20Hard%20Bottom%20Survey%20Reports.pdf

    Hofmann, E. E., Pietrafesa, L. J., & Atkinson, L. P. (1981). A bottom water intrusion in Onslow Bay, North Carolina. Deep Sea Research Part A. Oceanographic Research Papers, 28(4), 329-345. https://doi.org/10.1016/0198-0149(81)90003-0

    Manz, M. H., Shipley, O. N., Cerrato, R. M., Hueter, R. E., Newton, A. L., Tyminski, J. P., Franks, B. R., Curtis, T. H., Fischer, C., Zacharias, J. P., Scott, C., Dunton, K. J., Kneebone, J., Peterson, B. J., Scannell, B. J., Dodd, J. F., & Frisk, M. G. (2025). Predictions of southern migration timing in coastal sharks under future ocean warming. Conservation Biology, 39(6). https://doi.org/10.1111/cobi.70080

    NASA Salinity. (2011, October 20). Average Salinity From Historical Ship and Buoy Data. https://salinity.oceansciences.org/gallery-images-more.htm?id=10

    Rasmussen, L. L., Gawarkiewicz, G., Owens, W. B., & Lozier, M. S. (2005). Slope water, Gulf Stream, and seasonal influences on southern Mid‐Atlantic bight circulation during the fall‐winter transition. Journal of Geophysical Research: Oceans, 110(C2). https://doi.org/10.1029/2004jc002311

    Singer, J. J., Atkinson, L. P., & Pietrafesa, L. J. (1980). Summertime advection of low salinity surface waters into Onslow Bay. Estuarine and Coastal Marine Science, 11(1), 73-82. https://doi.org/10.1016/s0302-3524(80)80030-2

    Sulikowski, J., Galuardi, B., Bubley, W., Furey, N., Driggers, W., Ingram, G., & Tsang, P. (2010). Use of satellite tags to reveal the movements of spiny dogfish Squalus acanthias in the western North Atlantic Ocean. Marine Ecology Progress Series, 418, 249-254. https://doi.org/10.3354/meps08821

    Surf-forecast.com. (2005). Topsail island water temperature (Sea) and wetsuit guide (Carolina north, USA). https://www.surf-forecast.com/breaks/Topsail-Island/seatemp

    Tribuzio, C. A., Kruse, G. H., & Fujioka, J. T. (2010). Age and growth of spiny dogfish (Squalus acanthias) in the Gulf of Alaska: analysis of alternative growth models ( Fishery Bulletin(Vol. 108, Issue 2)). National Marine Fisheries Service. https://go.gale.com/ps/i.do?id=GALE%7CA227944663&sid=googleScholar&v=2.1&it=r&linkaccess=abs&issn=00900656&p=AONE&sw=w&userGroupName=anon%7E4fe11d55&aty=open-web-entry

    Xiu, Y., Chant, R., Gong, D., Castelao, R., Glenn, S., & Schofield, O. (2011). Seasonal variability of chlorophyll a in the Mid-Atlantic Bight. Continental Shelf Research, 31(16), 1640-1650. https://doi.org/10.1016/j.csr.2011.05.019

  • The 12 Days of Estuary Christmas | New River Estuary

    The 12 Days of Estuary Christmas | New River Estuary

    In the season of chilly tides and twinkling pier lights, the New River estuary doesn’t quiet down — it parties in its own salty way. So grab your cocoa, bundle up, and join us for a winter countdown of festive fins, feathers, and the ecological magic beneath the misty surface.

    (Sing along if you dare — apologies in advance.)

    Day 12: Twelve Dolphins Dancing

    12 dolphins dancing

    Bottlenose dolphins along the mid-Atlantic coast shift into cooperative foraging teams in the cooler months — synchronized movements that feel almost choreographed (Torres & Read, 2009). Their leaping, circling, and flipper-flicking tactics help herd fish just like dancers driving the story across a winter stage.

    Cue underwater Nutcracker ballet.

    Day 11: Eleven Stripers Schooling

    11 stripers schooling

    Atlantic striped bass move into estuarine channels when the water cools, fueling popular winter fisheries (Boyd, 2011).

    Cold water? Hot bite.

    Day 10: Ten Blue Crabs Burrowing

    Ten Blue Crabs Burrowing

    Blue crabs overwinter right here — burrowed into sediment, metabolism slowed, waiting for spring, or when water temperatures rise above 9℃ (Glandon, Kilborn & Miller, 2019).

    The ultimate cozy blanket fort.

    Day 9: Nine Oysters Filtering

    Nine Oysters Filtering

    Oysters continue filtering water through the winter, though more slowly — still improving water quality and boosting biodiversity (Grabowski & Peterson, 2007).

    Nature’s tiny elves never clock out.

    Day 8: Eight Croakers Drumming

    Eight Croakers Drumming

    Atlantic croaker remain common in NC coastal waters during cooler months, shifting to deeper estuarine areas (Miller et al., 2003).

    Rumble, rumble — underwater holiday percussion.

    Day 7: Seven Specks Still Striking

    Seven Specks Still Striking

    Speckled seatrout stay active in winter, especially in deeper holes and marsh channels where prey concentrates and water temperatures remain above 7℃ (Ellis, Buckle & Hightower, 2017).

    Even cold-blooded fish love a good holiday snack.

    Day 6: Six Sharks Snow-Birding

    Six Sharks Snow-Birding

    Juvenile coastal sharks like sandbars and sharpnose depart estuaries in late fall, migrating offshore and southward (Bangley et al., 2018).

    “See you after the thaw!”

    Day 5: FIVE… OYS-TER REEFS!

    Five oyster reefs

    Oyster reefs provide the essential winter housing market — structured refuge for juvenile fish, crustaceans, and invertebrates (Coen et al., 2007).

    Deck the reefs with beds and breakfasts..

    Day 4: Four Buffleheads Diving

    Four Buffleheads Diving

    These small sea ducks, buffleheads, arrive from the Arctic and forage in our coastal waters all winter long (Gauthier, 2014).

    Feathered travelers escaping the Arctic freeze.

    Day 3: Three Terrapins Burrowed

    Three Terrapins Burrowed

    Diamondback terrapins overwinter in marsh sediments, lowering heart rate and waiting out the cold (Harden, Midway & Willard, 2015).

    A brumation vacation.

    Day 2: Two Menhaden Shoals

    Two Menhaden Shoals

    Atlantic menhaden form huge winter schools offshore and near inlet mouths, fueling predator energy budgets (Orth, 2023).

    The estuary’s holiday punch bowl.

    Day 1: And a Red Drum in the Mar-sh-Tree

    And a Red Drum in the Mar-sh-Tree

    Red drum remain year-round, feeding in creeks and marsh edges even in winter low-temp slow-motion (Bacheler et al., 2009).

    Our coastal Christmas (and state) mascot.

    The Estuary Never Sleeps

    Even as we wrap gifts and check lists twice, life beneath the cold surface hustles on — feeding, moving, filtering, and keeping the New River ecosystem healthy through the darkest season.

    So here’s to the citizens of our winter waters —
    May your tides be merry and bright!

    References

    Bacheler, N., Paramore, L., Buckel, J., & Hightower, J. (2009). Abiotic and biotic factors influence the habitat use of an estuarine fish. Marine Ecology Progress Series, 377, 263-277. https://doi.org/10.3354/meps07805

    Bangley, C. W., Paramore, L., Dedman, S., & Rulifson, R. A. (2018). Delineation and mapping of coastal shark habitat within a shallow lagoonal Estuary. PLOS ONE, 13(4), e0195221. https://doi.org/10.1371/journal.pone.0195221

    Boyd, J. B. (2011). Maturation, fecundity, and spawning frequency of the Albemarle/Roanoke striped bass stock (2011. 1510474) [Doctoral dissertation]. ProQuest Dissertations and Theses Global.

    Coen, L., Brumbaugh, R., Bushek, D., Grizzle, R., Luckenbach, M., Posey, M., Powers, S., & Tolley, S. (2007). Ecosystem services related to oyster restoration. Marine Ecology Progress Series, 341, 303-307. https://doi.org/10.3354/meps341303

    Ellis, T., Buckel, J., & Hightower, J. (2017). Winter severity influences spotted seatrout mortality in a southeast US estuarine system. Marine Ecology Progress Series, 564, 145-161. https://doi.org/10.3354/meps11985

    Gauthier, G. (2014, July 14). Bufflehead – Bucephala albeola. Birds of the World – Cornell Lab of Ornithology. Retrieved November 29, 2025, from https://birdsoftheworld.org/bow/historic/bna/buffle/2.0/introduction

    Glandon, H. L., Kilbourne, K. H., & Miller, T. J. (2019). Winter is (not) coming: Warming temperatures will affect the overwinter behavior and survival of blue crab. PLOS ONE, 14(7), e0219555. https://doi.org/10.1371/journal.pone.0219555

    Grabowski, J. H., & Peterson, C. H. (2007). Restoring oyster reefs to recover ecosystem services. Theoretical Ecology Series, 281-298. https://doi.org/10.1016/s1875-306x(07)80017-7

    Harden, L. A., Midway, S. R., & Williard, A. S. (2015). The blood biochemistry of overwintering diamondback terrapins (Malaclemys terrapin). Journal of Experimental Marine Biology and Ecology, 466, 34-41. https://doi.org/10.1016/j.jembe.2015.01.017

    Mead, J. G., & Potter, C. W. (1995). Recognizing two populations off the bottlenose dolphin (Tursiops Truncatus) of the Atlantic coast of North America-Morphologic and Ecologic Considerations. https://repository.si.edu/server/api/core/bitstreams/9c563919-2b27-4ac4-bba1-92e7d090fd72/content

    Orth, D. J. (2023). Fish, fishing and conservation. Blacksburg: Virginia Tech Department of Fish and Wildlife Conservation.Torres, L. G., & Read, A. J. (2009). Where to catch a fish? The influence of foraging tactics on the ecology of bottlenose dolphins (Tursiops truncatus) in Florida Bay, Florida. Marine Mammal Science, 25(4), 797-815. https://doi.org/10.1111/j.1748-7692.2009.00297.x