Group of Caribbean reef sharks (Carcharhinus perezi)
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FISH PLAY A VITAL ROLE IN MAINTAINING THE HEALTH AND BALANCE OF OUR OCEANS AND SERVE AS A SOURCE OF SUSTENANCE FOR HUMANS AND COUNTLESS OTHER SPECIES.
Fish are divided into cartilaginous and bony groups.
Cartilaginous fish, called Chondrichthyans, have a skeleton consisting mainly of cartilage, a flexible and light connective tissue. This group includes sharks, rays, sea eagles, and manta rays.
Bony fish, scientifically known as Osteichthyes, are the most diverse and abundant fish, with over 29,000 known species. As their name suggests, they possess a skeleton composed mainly of bone tissue. Examples of bony fish are salmon, trout, tuna and angel fish.
Another notable difference between bony and cartilaginous fish is their reproductive strategy. Bony fish lay eggs externally, while cartilaginous fish give birth to live young.
Blacktip shark (Carcharhinus limbatus)
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Blacktip shark (Carcharhinus limbatus)
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SHARKS PLAY A CRITICAL ROLE AS TOP-LEVEL PREDATORS IN MARINE ECOSYSTEMS, ENSURING THE BALANCE AND HEALTH OF THESE ENVIRONMENTS.
As apex predators, sharks play a crucial role in regulating the populations of other marine species, particularly those lower down the food chain. By feeding on smaller fish and marine mammals, they prevent the overpopulation of certain species, maintaining a balanced ecosystem.
Sharks also play a role in managing the spread of diseases within marine populations. They target weaker or diseased individuals, thereby mitigating the transmission and dissemination of diseases among marine species.Specific species of sharks, such as reef sharks, contribute to the well-being of coral reefs by controlling the populations of herbivorous fish that graze on coral, thereby supporting the health and vitality of these vital marine ecosystems.
Whitetip reef sharks (Triaenodon obesus)
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Whale shark (Rhincodon typus)
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The Whale shark (Rhincodon typus) is the largest fish and non-cetacean vertebrate on Earth, reaching lengths of up to 20 meters and weighing several tens of tonnes. Its large, flat head and frontally positioned mouth distinguish it from other sharks. The body is grey-green, covered with numerous white polka dots, which inspired the people of Madagascar to give it the name "Marokintana", meaning "of a thousand stars". As one of three filter-feeding shark species, it passively filters water for prey rather than actively hunting.
Basking shark (Cetorhinus maximus)
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The Basking shark (Cetorhinus maximus) is the second-largest shark after the whale shark. They reach lengths of 6 to 8 meters and weigh up to 7 tons. These filter feeders primarily consume plankton and are harmless to humans. Basking sharks can be found in temperate and cold waters worldwide, often seen near the surface "basking" in the sun. Unfortunately, basking sharks have suffered population declines in various regions due to overfishing and habitat degradation.
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White shark (Carcharodon carcharias)
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Sharks are ancient creatures that have inhabited the oceans for around 450 million years. With over 500 species, they are considered the ocean's superpredators. Sharks have highly developed senses, including smell, sight, touch, taste, hearing, and the ability to detect electrical signals produced by other animals' muscles through Lorenzini's ampullae.
The white shark, for example, can perceive minute differences in potential (even a millionth of a volt).
Pores of ampullae of Lorenzini in the snout of white shark
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RAYS, SEA EAGLES, AND MANTA RAYS PLAY CRUCIAL ROLES IN MAINTAINING MARINE ECOSYSTEM BALANCE, FROM THE SEA EAGLES' ROLE AS APEX PREDATORS TO THE RAYS AND MANTA RAYS' CONTRIBUTION IN REGULATING PLANKTON POPULATIONS AND NUTRIENT CYCLES.
Rays, sea eagles, and manta rays are fascinating creatures that belong to different groups.
Rays, belonging to the order Rajiformes, are flattened fish with disc-shaped bodies. They have specialized gill slits located on the underside of their bodies. This adaptation allows them to respire while partially buried in the sandy or muddy seafloor, with only their eyes and spiracles (respiratory openings) exposed. Rays inhabit marine environments and possess wide pectoral fins that give them their characteristic appearance. Some species of rays have venomous spines on their tails, which they use for defence.
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Eastern fiddler ray (Trygonorrhina fasciata) eye
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Eastern fiddler ray (Trygonorrhina fasciata)
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Sea eagles, on the other hand, are majestic birds of prey belonging to the family Accipitridae. They are known for their large size, powerful beaks, and sharp talons. Sea eagles can be found in coastal areas and near bodies of water. They rely on their exceptional eyesight and soaring abilities to hunt fish. Sea eagles do not bury themselves in the seafloor-like rays; instead, they use their keen vision to spot prey from above and dive down to catch it.
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Eagle ray (Myliobatidae)
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Lastly, manta rays, belonging to the genus Manta, are gigantic filter feeders that inhabit warm ocean waters. They have triangular bodies, impressive wingspans, and whip-like tails, captivating observers with their acrobatic leaps. Unlike other rays, manta rays lack venomous spines and pose no threat to humans. Instead, they consume plankton and small fish by filtering them through specialized gill rakers, comb-like structures in their mouths. This adaptation allows them to feed on their microscopic prey efficiently.
Manta rakers
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Reef Manta Ray (Mobula alfredi)
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Reef Manta Ray (Mobula alfredi)
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ILLEGAL FIN TRADE POSES A SEVERE THREAT TO MARINE ECOSYSTEMS AND ENDANGERS SEVERAL SHARK SPECIES.
The shark fin trade involves the buying and selling of fins primarily for shark fin soup, with devastating consequences that include overfishing, declining shark populations, and disruptions to ecosystems. Fishermen employ methods such as longline fishing or gillnetting to catch sharks, engaging in shark finning on board their vessels. This cruel practice entails slicing off the fins and discarding the rest of the animal, throwing the finned shark back into the sea. At this point, the shark is still alive but unable to move and remain submerged at a suitable depth, leading to immense suffering as it approaches a slow and certain death by suffocation.
STOP FINNING
Shark fin trade
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Fortunately, numerous countries have taken action by implementing regulations and bans, while NGOs actively work to raise awareness and promote sustainable practices. Nevertheless, to date, the global shark population has declined by 70%, and according to the IUCN Red List of Threatened Species (Dulvy et al., 2021), between 32.6% and 45.5% of shark species are considered threatened with extinction.
BONY FISH PLAY A CRUCIAL ROLE IN THE OCEANIC FOOD CHAIN, OCCUPYING VARIOUS TROPHIC LEVELS AS BOTH PREDATOR AND PREY.
Bony fishes, comprising some 30,000 species, constitute the largest class of vertebrates. They occupy various trophic levels within the oceanic food web, with many species serving as both predators and prey, intricately connecting different trophic compartments. Predatory fish, including tuna, marlin and swordfish, depend heavily on smaller bony fish as a food source. By preying on these smaller fish, larger predators help to regulate their population growth and prevent overgrowth.
In addition, the bony fish contribute significantly to the nutrient cycle in the ocean. During their migrations, they extract and transport nutrients, benefiting different regions. In addition, the excretion of waste products by bony fish adds essential nutrients to the water, supporting the growth of phytoplankton and other primary producers in the marine ecosystem.
Barracuda (Sphyraena) shoal
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The sardine run
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The sardine run is a captivating natural event occurring off the southeastern coast of South Africa during winter, involving the massive migration of millions of sardines. This spectacle draws various predators like dolphins, sharks, whales, and seabirds, contributing to a vital marine feeding frenzy.
Shoal of Glass fish (Parambassis ranga)
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Swordfish (Xiphias gladius)
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Swordfish (Xiphias gladius) are known for their remarkable speed, capable of reaching speeds of up to 100 kilometres per hour. Scientists have made an intriguing discovery related to these fish—they possess a distinctive gland that produces oil. This oil serves a crucial purpose by reducing the friction or resistance on their skin by over 20%. This remarkable adaptation gives swordfish a significant advantage in hunting and navigating the open sea.
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Ocean sunfish (Mola Mola)
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Ocean sunfish (Mola Mola) detail
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The sunfish, also known as Mola mola, is a peculiar ocean-dwelling fish. It is the heaviest known bony fish and belongs to the family Molidae. The name "sunfish" derives from its habit of sunbathing near the surface of the water, where it often floats on its side, basking in the warmth of the sun.
Sunfish have a flattened body structure, with a round or oval shape that can reach impressive sizes. On average, they measure about 1.8 meters (6 feet) in length and weigh around 1,000 kilograms (2,200 pounds). However, some individuals have been recorded weighing over 2,000 kilograms (4,400 pounds), making them one of the largest bony fish in the world.
Sunfish have no true caudal (tail) fin, but they possess a clavus, which is a structure that protrudes from the rear of their bodies and resembles a rudder. They also have a long dorsal fin and an anal fin, which give them a unique appearance in the water. The pectoral fins of sunfish are large and triangular, while their mouth is small and lacks teeth.
OVERFISHING IS A COMPLEX AND PRESSING ISSUE WITH FA-REACHING CONSEQUENCES FOR BOTH MARINE ECOSYSTEMS AND HUMAN SOCIETIES.
Overfishing is a critical environmental issue resulting from catching fish at a rate faster than they can reproduce, with profound consequences for marine ecosystems, economies, and food security.
Dried fish
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OVERFISHING CONSEQUENCES
_Depleted fish stocks: Fish populations decline, increasing the risk of species collapse.
_Economic impact: Fishing industries incur higher costs and reduced profits.
_Ecosystem disruption: Fish are ecosystem regulators; overfishing disrupts the marine food web, causing imbalances and potential cascading effects.
_Bycatch: Large nets and fishing methods unintentionally catch non-target species like dolphins, sea turtles, and seabirds, posing threats to their populations.
_Habitat damage: Destructive methods like bottom trawling harm sensitive marine habitats, including coral reefs and seafloor ecosystems, compounding environmental degradation.
_Food security: Overfishing jeopardizes global food supplies, especially in regions heavily reliant on seafood.
_Climate change impact: Rising ocean temperatures and ocean acidification worsen overfishing's challenges, further endangering fish populations.