Swordfish
Fish · Last reviewed 2026-08-10
A large oceanic predator with a flat sword-like bill and specialised tissue that warms its eyes and brain, allowing it to hunt in cold deep water.
Safety
Species that bite — Can cause serious injury or death
The flat sword is sharp-edged and long, and a green fish brought alongside or aboard can cause severe injury with a sweep of the head.
What reduces it: Control the bill with a gloved hand or purpose-made device before attempting anything else, keep all crew clear during landing, and never approach the head of an unsubdued fish.
Swordfish is the only member of its family and one of the most physiologically remarkable fish in the ocean. It carries a long, flat, sharp-edged bill used to slash through prey, and it makes daily vertical migrations from the surface down into cold water hundreds of metres deep.
What makes that possible is a specialised heat-producing tissue behind the eyes that warms the eyes and brain well above the surrounding water temperature. Warm eyes see faster — the temporal resolution of vision improves markedly — and that is what lets a swordfish hunt effectively in cold, dark, deep water where its prey cannot see it coming.
Adults have no scales and no teeth. The bill does the work.
Identification
FishingHQ holds no photograph of this species whose identity and licensing it can vouch for, so it is not showing one. Misidentified fish photographs teach identification wrongly, and a picture with an unchecked caption would be worse than none. The identification points below are what to look at instead.
- Scientific name
- Xiphias gladius
- Family
- Xiphiidae
- Water
- Saltwater
- Conservation status
- Near Threatened
- Regions
- Global, Mediterranean, North America, Caribbean, Indo-Pacific
- A long, flat, sharp-edged bill — broad in cross-section, unlike the rounded spear of a marlin.
- No pelvic fins at all, and adults have neither scales nor teeth.
- A single tall crescent first dorsal fin that does not fold into a groove.
- A single strong keel on each side of the caudal peduncle, and very large eyes.
Size
- Typical
- 30–90 kg (66–200 lb)
- Large
- 150–300 kg (330–660 lb)
Exceptional: FishBase records a maximum of 455 cm and 650 kg
Lifecycle and reproduction
Fast-growing and long-lived, with females growing considerably larger than males. Highly migratory across ocean basins, and subject to international rather than national management.
Spawns in warm oceanic waters, with females releasing very large numbers of eggs in batches over an extended season. There are no aggregation sites of the reef kind.
Feeding and senses
Takes squid and pelagic fish, slashing through prey with the bill before consuming it. Adults are toothless, so prey is cut or stunned rather than gripped.
Vision is the dominant sense and is unusually specialised: heat-producing tissue warms the eyes and brain, improving visual performance in cold deep water where the fish hunts by day and rises to feed near the surface at night.
Through the year
Undertakes extensive seasonal migrations following warm water and prey, and makes daily vertical migrations between deep water by day and near the surface at night.
Distribution
Atlantic, Indian and Pacific: tropical and temperate and sometimes cold waters, including the Mediterranean Sea, the Sea of Marmara, the Black Sea, and the Sea of Azov. Highly migratory species. Mt DNA restriction analysis reveal that genetic differentiation occurs between populations inhabiting the Mediterranean Sea and the tropical Atlantic ocean, indicating little genetic exchange occurring between the two
This is a biological range — where the species occurs, not a prediction that you will catch one. To browse the fishing geography FishingHQ holds, start at fishing places.
Why anglers care
A major big-game target, fished by deep-drop daytime techniques and by night fishing near the surface with squid baits and lights. Its significance is as one of the premier oceanic gamefish and as a commercially important species subject to international management. Public health authorities in several countries advise limiting consumption because of mercury accumulation in large individuals, which is documented advice rather than a precaution invented here.
Handling and returning
A very large, powerful fish with a bill capable of causing serious injury; boated swordfish have injured crew. Control the bill before anything else, keep bystanders clear, and use appropriate tackle and gaffs. Fish intended for release should be handled in the water alongside the boat.
A bill, a deck, and a fish that arrives from very deep water
Swordfish combine the billfish hazard with a set of circumstances that make it worse than the marlin case in some respects and better in others.
THE BILL IS FLATTENED AND EDGED rather than round like a marlin's, and it is used in a lateral slashing motion. A swordfish alongside a boat is dangerous in the same way and for the same reason, and the same rules apply: one person on the leader, gloves, no fixed wraps, everybody else clear, and the fish never brought aboard alive.
THE DEPTH IS THE COMPLICATION. Much swordfish angling is deep-drop fishing, and a fish brought up from several hundred metres has undergone an enormous pressure change. Deep-caught swordfish frequently arrive in poor condition and the practical reality is that this is largely a retention fishery — a fish from that depth is often not a viable release.
AND THE FISHING HAPPENS AT NIGHT OR IN THE DARK EARLY HOURS in many fisheries, which multiplies every hazard on a deck: lines under tension, gaffs, an animal with a weapon, and poor visibility.
WHERE A FISH IS RELEASED, it is done alongside with a long dehooker or by cutting the leader, without lifting.
AND THE EYES ARE WORTH KNOWING ABOUT because they explain the fishery: swordfish have very large eyes and a specialised tissue that warms the brain and eye, which is why they hunt effectively in cold deep water and why the fishing is what it is. It is also why a fish brought into warm surface water and bright light is under a load that goes beyond the pressure change.
Practitioner consensus
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Dentition, fin and spine morphology, body form, air-breathing capability and distribution for the species, which are the anatomical facts the handling practice below follows from.States this directlyPassage read and verified
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice for handling this species, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
A vertical habitat: warm at night, near-freezing by day, and eyes that are heated
Swordfish occupy a habitat defined by depth rather than by geography, and the daily range they cover through the water column is more extreme than almost any other large fish manages.
THE PATTERN IS DIEL VERTICAL MIGRATION AT SCALE. Swordfish spend daylight hours at depth — commonly several hundred metres and sometimes far deeper, in water close to freezing and effectively without light — and rise toward the surface at night to feed in the upper layers. They are following the deep scattering layer: the vast assemblage of squid, small fish and crustaceans that rises at dusk and descends at dawn, which is the largest daily movement of biomass on the planet.
SO THE HABITAT IS A COLUMN, AND THE FISHERY FOLLOWS IT. This is why swordfish are historically a night-time surface fishery and why deep-drop daytime fishing works: the fish is in two different places on a schedule.
THE PHYSIOLOGY REQUIRED IS REMARKABLE AND IT IS THE REASON THEY CAN DO IT. Swordfish possess a specialised heater organ — modified eye muscle tissue that generates heat and warms the brain and the eyes substantially above ambient water temperature. That matters because visual processing speed collapses in cold water: a cold eye is a slow eye, and a predator hunting agile prey in near-freezing darkness needs its vision to work. Warming the eye maintains temporal resolution where it would otherwise be lost, and it is what makes the deep daytime habitat usable at all.
THE EYES THEMSELVES ARE ENORMOUS, as they must be to gather light at depth, and the species has among the largest eyes relative to body size of any billfish.
THE BILL IS THE FEEDING ADAPTATION and it is used to slash rather than to spear — a flattened blade swept through concentrations of squid and fish to injure them for easier capture, which suits a predator working on dense aggregations in the dark.
GEOGRAPHICALLY they are found worldwide in temperate and tropical seas, more tolerant of cold water than most billfish precisely because of the heater organ, and they associate with productive frontal systems, seamounts and shelf edges where the scattering layer is richest.
Scientific evidence
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Depth range, oceanic habitat association, temperature preference and vertical distribution recorded for the species.States this directlyPassage read and verified
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: Recognition of the species as an oceanic sport fish and the water types its recognised fisheries operate in.Background and mechanism onlyPassage read and verified
Where to go next
How it is caught
- Big-game fishingDeep-drop daytime baits and night fishing near the surface with squid.
- Deep-drop fishingDeep-drop daytime baits and night fishing near the surface with squid.
What these connections rest on (2)
Practitioner consensus — 2 connections
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Big-game fishing
A practitioner claim about how Xiphias gladius is commonly fished, following from where and how it feeds.
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated practitioner accounts of fishing for swordfish.Citation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Practitioner treatment of the methods used for swordfish and comparable species.Citation checked — the work exists and is described correctly
Deep-drop fishing
A practitioner claim about how Xiphias gladius is commonly fished, following from where and how it feeds.
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated practitioner accounts of fishing for swordfish.Citation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Practitioner treatment of the methods used for swordfish and comparable species.Citation checked — the work exists and is described correctly
Gear for this fish
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Squid and cuttlefish
A practitioner claim about tackle commonly used for Xiphias gladius.
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated practitioner accounts of this presentation for swordfish.Citation checked — the work exists and is described correctly
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Practitioner treatment of comparable presentations for the species group.Citation checked — the work exists and is described correctly
Where it lives
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Open ocean
A habitat association for Xiphias gladius taken from the environment and distribution data on its FishBase summary.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: The environment, depth range and habitat description recorded for Xiphias gladius.States this directlyPassage read and verified
Where you will find it
- ItalyTaken in the Strait of Messina by a visually hunted harpoon fishery worked from high-masted boats, which is among the most distinctive traditional fisheries surviving in Europe.
- The Strait of GibraltarOccurs in the deeper water either side of the sill and is taken by the longline fisheries operating from both coasts.
- The Strait of MessinaComes to the surface in the strait, basking with dorsal and caudal fins showing, which is the behaviour that made a harpoon fishery possible here and nowhere much else.
What these connections rest on (3)
Scientific evidence — 3 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Italy
That a species occurs in these waters, drawn from distribution science and stock assessment. It is a statement about where the animal lives and is not a statement that anybody will catch one, still less where.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Recorded distribution, depth range and habitat association of the species named, including its occurrence in the waters of the jurisdiction named.States this directlyPassage read and verified
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Stock distribution and assessment across the European ecoregions, which is the basis for stating that a species occurs in a given national sea area.States this directlyCitation checked — the work exists and is described correctly
The Strait of Gibraltar
A distribution association between an animal and a described water body, which is what the `inhabits` predicate is for — the kind of water rather than a named mark.
- FAO Species Identification Guides for Fishery Purposes and the FAO Species Catalogues — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Distribution, habitat, depth and fishery accounts for the species of the eastern central Atlantic and Mediterranean in the FAO regional identification guides.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Distribution, habitat association and depth range recorded for the species.States this directlyPassage read and verified
The Strait of Messina
A distribution association between an animal and a described water body, which is what the `inhabits` predicate is for — the kind of water rather than a named mark.
- FAO Species Identification Guides for Fishery Purposes and the FAO Species Catalogues — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Distribution, habitat, depth and fishery accounts for the species in the FAO regional identification guides for the ocean area concerned.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Distribution, habitat association and depth range recorded for the species.States this directlyPassage read and verified
Understanding this fish
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Contaminants, and what accumulates in a fish
A link from a species or a place to the accumulation mechanism that decides whether what is caught there should be eaten, resting on published seafood-hazard guidance and on the species' recorded diet and trophic position.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Contaminant monitoring in marine biota and the accumulation of contaminants through marine food webs.States this directlyCitation checked — the work exists and is described correctly
- Fish and Fishery Products Hazards and Controls Guidance — US Food and Drug Administration, Center for Food Safety and Applied Nutrition, 2022 (Standards body)Relied on for: Methylmercury in seafood, the species in which it accumulates, and the advisory approach taken to it.States this directlyPassage read and verified
Sources for this page
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Family, maximum size, depth and environment, distribution and the IUCN assessment for Xiphias gladius.States this directlyPassage read and verified
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: The Near Threatened assessment for Xiphias gladius, assessed 01 May 2021.States this directlyCitation checked — the work exists and is described correctly
How FishingHQ labels evidence is explained in the evidence policy.