Pelagic fish live in the pelagic zone, which is the open water column of oceans and lakes. They are free-swimming fish that do not live near the bottom or shorelines. Pelagic fish can be divided into two main types based on their behavior and habitat – epipelagic fish and mesopelagic fish.
Epipelagic fish live in the upper sunlit layer of the open ocean called the epipelagic zone. This zone extends from the surface down to around 200 meters in depth. Epipelagic fish include fast-swimming predatory fish like tuna, billfish, and jacks that hunt in the open water. Other epipelagic fish feed on plankton and small organisms. Some examples are anchovies, herring, and sardines. Many epipelagic fish form large schools and undertake extensive migrations across oceans following food sources or for spawning.
In contrast, mesopelagic fish inhabit the mesopelagic zone, which spans from around 200 meters down to 1000 meters deep. This zone receives little sunlight, so mesopelagic fish have adapted with bioluminescent organs, large eyes, and dark camouflage coloration. Mesopelagics migrate vertically, coming up at night to feed in nutrient-rich surface waters under the cover of darkness. They avoid surface waters during the day to escape predators. Lanternfish, bristlemouths, fangtooth fish, and hatchetfish are examples of mesopelagic fish.
Major Types of Pelagic Fish
There are many different families and species of pelagic fish found around the world. Some of the major groups include:
Herrings
Herrings are small, schooling forage fish belonging to the family Clupeidae. Some important species are the Atlantic herring, Pacific herring, Araucanian herring, and American shad. Herrings feed on plankton and small crustaceans and form a vital prey species for larger predators. They have an elongated body shape with a single dorsal fin. Herrings undertake seasonal migrations for feeding and spawning. They are abundant in cool temperate waters and are caught commercially for food.
Anchovies
Anchovies are small, schooling forage fish in the Engraulidae family. There are over 140 species found globally in tropical and temperate seas. Some major species are the European anchovy, Peruvian anchovy, Japanese anchovy, and northern anchovy. Anchovies have a slender, fusiform shape with a protruding snout and large mouth. They feed on plankton and are an important prey species for larger fish, seabirds, and marine mammals. Several anchovy species support large commercial fisheries, valued for fishmeal, animal feed, and human consumption.
Sardines
Sardines comprise over 20 species of the genus Sardinella and other related genera. These small, oily fish form large schools in temperate and tropical oceans around the world. Major species include the European pilchard, Japanese pilchard, South American pilchard, and Indian oil sardine. Sardines feed on plankton near the surface and undertake seasonal migrations. As a nutrient-rich forage fish, sardines are prey for larger predators and support commercial fisheries for food, fishmeal, and bait.
Mackerels
Mackerels are fast-swimming predators belonging to the family Scombridae. This diverse group includes species like Atlantic mackerel, Pacific mackerel, Indian mackerel, and Spanish mackerel. Mackerels have an elongated, streamlined body and a deeply forked tail for speed and maneuverability. They hunt smaller fish as well as crustaceans and cephalopods. Many mackerel species migrate long distances and school in large numbers. They are caught commercially and used for food due to their rich flavor.
Tuna and Billfish
Tuna, marlins, swordfish, and sailfish comprise the open ocean hunters in the suborder Scombroidei. Tuna species like yellowfin, bluefin, bigeye, albacore, and skipjack have a robust, torpedo-shaped build with fins and physiology adapted for high-speed swimming to hunt prey. Their meat is prized by sushi and sashimi consumers. Billfish like marlin and sailfish have elongated spear-like snouts to slash and stun prey. These large pelagic fish are apex predators that hunt fish, squid, and crustaceans. They support extensive commercial and recreational fisheries around the world.
Jacks and Trevally
Jacks and trevally belong to the family Carangidae and are found in tropical and temperate waters worldwide. They have a deep, compressed body shape with a blunt snout. Species range from small fish like blue runner to large fish like giant trevally, which can exceed 100 pounds. Jacks are aggressive predators that hunt in packs, feeding on fish, squid, and crustaceans. Many carangids have a silvery coloration and dark spotting or bars on their sides for camouflage when hunting. They are caught by commercial fisheries and also prized as gamefish.
Lanternfish
Lanternfish comprise over 250 species in the family Myctophidae and are one of the most abundant mesopelagic fish. They inhabit dimly lit ocean waters between 200-1000m depth during the day and migrate closer to the surface at night. Lanternfish have light-producing photophores on their body to camouflage their silhouette from predators when viewed from below. Their large goggling eyes allow them to see in low light. Lanternfish feed on plankton like copepods and serve as an important prey species for whales, squid and larger fish.
Hatchetfish
Hatchetfish are named for their distinct hatchet-shaped, compressed body. There are around 9 genera and 50 species in the family Sternoptychidae found in mesopelagic waters worldwide. They have large upward-facing eyes to scan for silhouetted predators and bioluminescent photophores along their belly for counter-illumination camouflage. Hatchetfish undertake daily vertical migrations hundreds of meters up at night to feed on plankton near the surface before retreating to deeper waters during the daytime.
Bristlemouths
Bristlemouths comprise over 260 species in 11 genera of the family Gonostomatidae. They are small, silver-colored fish with a terminal mouth bristling with sharp fang-like teeth. Their oversized pectoral fins near the throat act like paddles for maneuverability to capture prey. Bristlemouths inhabit mesopelagic depths between 500-4000m in tropical and temperate oceans worldwide. They feed on small crustaceans like copepods and serve as prey for larger predators. Bristlemouths are potentially the most abundant vertebrates in the ocean.
Fangtooths
The family Anoplogastridae contains fangtooth fish, which inhabit mesopelagic to bathypelagic zone depths of 300-5000m in tropical and temperate waters. Their most striking feature is large, depressible fangs sprouting from their lower and upper jaws. The needle-like teeth allow them to impale and consume prey like small fish and squid. Fangtooths have expansive mouths and stomachs that can distend to swallow prey larger than themselves. Their intimidating appearance matches their role as fierce predators in deep ocean food webs.
Geographic Distribution
Pelagic fish are distributed throughout the world’s oceans, although different species have distinct habitat ranges:
Tropical pelagic fish
– Skipjack tuna – tropical and subtropical oceans
– Yellowfin tuna – tropical and subtropical oceans
– Kawakawa – Indo-Pacific oceans
– Mahi-mahi – tropical and subtropical oceans
– Wahoo – tropical and subtropical oceans
Temperate pelagic fish
– Atlantic herring – North Atlantic Ocean
– Pacific sardine – North Pacific Ocean
– European anchovy – Mediterranean and Black Sea
– Bluefish – Atlantic, Indian, Pacific oceans
– Atlantic mackerel – North Atlantic Ocean
Polar pelagic fish
– Antarctic silverfish – Southern Ocean around Antarctica
– Glacier lanternfish – Arctic and Antarctic oceans
– Antarctic toothfish – Southern Ocean
– Blackfin icefish – Southern Ocean
– Polar cod – Arctic oceans
Deep sea (mesopelagic) pelagic fish
– Lanternfish – worldwide deep sea habitat, 200-1000m
– Hatchetfish – worldwide oceans, below 200m
– Bristlemouth – Atlantic, Pacific, Indian oceans 500-4000m
– Fangtooth – tropical and temperate oceans, 300-5000m
– Viperfish – Atlantic, Pacific, Indian oceans 250-5000m
Unique Adaptations
Pelagic fish have evolved special anatomical and physiological adaptations that enable them to thrive in the open ocean habitat:
Streamlined bodies
Most pelagic fish have an elongated, fusiform shape that tapers at both ends to reduce drag in open water. This streamlining allows fast, energy-efficient swimming to catch prey or migrate long distances.
Thunniform swimmers
Highly migratory pelagic fish like tuna use a thunniform mode of swimming. Their muscular, crescent-shaped tails provide thrust in a side-to-side motion that minimizes drag but allows sustained high speeds and long-distance treks across oceans.
Schooling behavior
Schooling offers protection from predators and improves hunting success through coordinated efforts. Pelagic fish like herring and anchovies form huge shoals that can evade predators through synchronized maneuvers.
Counter-shading camouflage
Many pelagic fish are dark above and light below, blending into the ocean’s photic environment. From above, their dark dorsal surface matches deep water depths. From below, their pale underbelly matches the sunlit water surface. This counters their silhouette and makes them invisible to predators.
Bioluminescence
Mesopelagic fish use bioluminescent light organs and photophores to glow, disguise their silhouette, lure prey, or communicate in the deep sea twilight zone where sunlight cannot penetrate.
Large goggling eyes
In dim ocean depths, large tubular eyes maximize light capture. Some mesopelagics have eyes directed upwards to spot the silhouettes of predators approaching from above.
Lifecycles and Behaviors
Pelagic fish exhibit interesting behaviors and lifecycles adapted to the open ocean:
Spawning aggregations
Many pelagic fish migrate to specific areas annually to form spawning aggregations for reproduction. This synchronizes mating and increases reproductive success in open waters where finding a mate can be difficult.
Daily vertical migration
Mesopelagic fish like lanternfish avoid surface predators during the day by retreating hundreds of meters into dark depths. At night, they rise to epipelagic depths to feed under the cover of darkness.
Long-distance migrations
Pelagic predators like tuna traverse entire ocean basins during annual migrations between rich foraging grounds and spawning sites. Some migrations can exceed 10,000 kilometers.
Schooling and shoaling
Anchovies, herrings, and sardines form coordinated schools and shoals of thousands to tens of thousands of individuals for protection from predators. Their silvery flashing reflects light that may confuse predators.
Fast growth
Many pelagic fish grow rapidly, reaching sexual maturity within 1-2 years. Their short lifespans of 5-10 years favor early reproduction to replenish populations. High mortality from predation keeps generation times fast.
High fecundity
Pelagic fish produce enormous numbers of eggs to flood the ocean with offspring, ensuring some survive the high mortality rates. A single female tuna or herring may spawn millions of eggs in a season.
Ecological Importance
As abundant open ocean fish, pelagics play vital ecological roles:
Energy transfer to predators
Small pelagic fish like sardines and anchovies convert plankton productivity into biomass that sustains marine food webs. They transfer energy up the chain by providing food for larger predators.
Scavenging
Opportunistic pelagic fish scavenge on dead animals and debris that sink down through the water column from surface waters. They recycle nutrients and break down organic matter throughout the depths.
Prey for marine mammals and seabirds
Many whales, dolphins, sharks, and seabirds rely on energy-rich pelagic fish like herring, mackerel, and lanternfish as their primary food source. Abundant pelagics help sustain top ocean predators.
Nutrient cycling
Through vertical migrations, pelagic fish transport nutrients from deep, nutrient-rich water to nutrient-poor surface water, fertilizing production. Their excretion at the surface releases nutrients like nitrogen and iron for phytoplankton growth.
Population control
As voracious predators of zooplankton, fish larvae and other pelagics, large pelagic fish like tuna help regulate and limit prey populations, preventing ecosystem imbalance.
Threats and Conservation
Some pelagic fish face major threats:
Overfishing
Commercial overfishing has depleted populations and damaged ecosystems. For instance, overfishing of Atlantic bluefin tuna and Pacific sardines has led to their decline.
Bycatch
Pelagic species are often caught as accidental bycatch in fisheries targeting other species. Bycatch contributes to population declines.
Habitat degradation
Pollution and climate change are altering marine ecosystems inhabited by pelagics. Acidification also threatens populations.
Prey depletion
Overfishing of small planktivorous pelagics like herring and anchovies depletes the prey base for larger predator pelagics like tuna, billfish and sharks.
Conservation efforts like catch limits, gear modifications, and marine protected areas are helping protect and rebuild some depleted pelagic fish stocks. More ecosystem-based management is needed for sustainable pelagic fisheries.
Fishing Industry Importance
Many pelagic fish species support major commercial fisheries due to their abundance, high nutritional value, and desirable taste:
Canning
Small oily pelagics like sardines, anchovies, herrings, and mackerels are commonly canned, smoked, dried or pickled for preservation. Canned sardine and anchovy fillets are popular items.
Live bait
Small live pelagics are used as bait by recreational anglers and commercial longline fisheries targeting larger pelagics, like tuna. Popular live bait species include Atlantic menhaden, Pacific saury, and Atlantic mackerel.
Meal and oil
After processing and canning, leftovers from small pelagics are ground up into fishmeal and fish oil. These are commonly used as high-protein ingredients in animal feeds for poultry, pigs, and aquaculture.
Luxury seafood
Large pelagics like tuna, swordfish, and mahi-mahi are prized for sushi and as gourmet seafood dishes in restaurants, commanding high market values. A single bluefin tuna can sell for over $1 million at auction in Japan.
Recreational fishing
Big game pelagic predators like marlin, sailfish, tuna, and mahi-mahi are iconic targets for recreational anglers around the world. Charter boat industries cater to sports fishing tourists seeking trophy pelagic catches.
Conclusion
Pelagic fish inhabit the vast open ocean waters of the epipelagic and mesopelagic realms. Major groups range from small schooling fish like herrings and anchovies to large apex predators such as tunas, billfish, and jacks. Pelagics exhibit unique adaptations for life in the water column such as streamlining, schooling behavior, bioluminescence, and daily vertical migrations. As key ecosystem components, they transfer energy up food chains and support other ocean life. Overfishing and habitat degradation threaten some pelagic stocks, requiring sustainable management for the future. At the same time, many pelagics support thriving commercial fisheries and recreational fishing due to their abundance, appeal, and nutritional value. Pelagic fish will continue to mesmerize and inspire with their beauty and diversity in the open ocean.
