Marine Environment - Fish

The vertebrates are made up of 5 classes of backboned animals; fish, amphibians, reptiles, birds and mammals. The fish are the most successful vertebrates that have ever lived on Earth and they make up 50% of all known vertebrate species. They are found from lakes at 5,000 metres above sea level to trenches in the deep oceans at 10,000metres below sea level. They range from tiny fish, which grow to only 10mm long, to the enormous great white shark reaching 12 metres in length.

Fish are aquatic vertebrates which obtain oxygen from the water by means of gills. They are usually divided into two main groups; the cartilaginous fish such as sharks and rays, and the bony fish such as herring and cod. As the names imply, the cartilaginous fish have skeletons that are made of cartilage, which is a softer, more flexible material than bone and is found in humans at the end of our noses and around joints. Their skin lacks scales and is important in maintaining their body shape so it is tough and flexible. Cartilaginous fish have external gills arranged in 5 or 7 pairs an each side of the head and lack gill covers. Some of the sharks and rays also have openings called “spiracles” which allow them to continue to obtain enough oxygen even whilst they remain stationary.

The other group, the bony fish do have a bony skeleton and skull as well as scales and a cover to their gills, called an operculum. They also have rays of bony material in their fins and tails. Many have swim bladders to allow them to control their position at any depth in the water with little effort and they are able to pump water across their gills to maintain gas exchange even when stationary.

The basking shark Cetorhinus maximus

The seas around the Argyll Islands abound in fish of all types. One of the most spectacular however is the basking shark which is the second largest fish in the world, reaching lengths of over 11 metres. Its name comes from its habit of floating gently across the surface of the water as it feeds. It has a typical pointed dorsal fin and a huge mouth and can seem very sinister but it only feeds on plankton by filtering the water. When it opens its mouth special bristles alongside the gills are made to stand up and they act like a sieve catching the tiny plankton in their mouth. A basking shark can filter up to 1½ million litres of water every hour.

Being a cartilaginous fish the basking shark has no swim bladder and in order to be able to float just under the surface it has a great deal of oil stored in its body and especially its liver. This oil, called squalene, was once used to light the streets of Dublin . The basking shark is still hunted for the squalene but a greater threat comes from the increasing demand for shark fin particularly in Asia . The fin is removed from the still living shark and the animal is thrown back in the sea to die slowly. Basking sharks are protected within UK waters but this only stretches to 12 nautical miles offshore.

Lesser spotted dogfish Scyliorhinus canicula,

Also known as the rough hound or dogfish this harmless species is the commonest European shark. Found from 6ft (1.8m) to 700ft (210m) they are mainly bottom-living, feeding on mollusks, crustaceans, and bottom-dwelling fish. The upper body and sides are a sandy brown colour, dappled with small, darker spots. The undersides are creamy-white. They hatch from egg capsules (a ‘mermaid's purse') aged between 5 – 11 months at about 4in (10cm) long. They can grow to nearly 40in (100cm) in length.

Greater spotted dogfish Scyliorhinus stellaris,

Also known as the nursehound, this species usually lives over rocky sea-beds. It differs from the lesser spotted dogfish by having the dorsal fin positioned further forward, and nasal flaps that are distinctly separate. An average adult is about 48in (120cm).

Porbeagle Lamna nasus,

Belonging to the mackerel sharks, the group that includes the great white shark, the porbeagle normally lives near the surface, hunting among shoals of mackerel or herring. The body is a deep grey-blue above and pale cream below. It is characterized by having a strong keel along each side of the tail and a smaller keel below it on the tail fin. It grows up to 8ft (2.4m) in British waters, has keen eyesight, bears live young and has sharp, three-pointed teeth.

Rays

All rays have flattened cartilaginous bodies with broad pectoral fins that give them a diamond shaped outline. Related to sharks, most of the larger species are known as skates and have long snouts. The smaller, short-snouted ones are called rays. As these fish mature slowly, and produce few eggs, they are very vulnerable to over fishing.

Spotted ray Raja montagui,

Spotted rays live in deep water and are only seen in shallow waters during egg-laying. They take nine years to reach maturity and a size of 30in (76cm). An adult female lays about 70 eggs a year, each ‘mermaid's purse' egg may take 3-15 months to hatch. Its body is brown with many dark brown spots that don't extend to the edge of the wings.

Blonde ray Raja brachyura,

The blonde ray is similar in shape to the spotted ray with dark spots that extend right to the edge of the wings. Blonde rays may reach 43in (110cm) long.

Cuckoo ray Raja naevus,

Smaller, at 28in (70cm) long, full grown, Cuckoo rays are distinguished by black and yellow ‘roundel' markings on the wings.

Thornback ray Raja clavata,

The commonest ray, it can be distinguished by its mottled grey-brown colouring on top, blue-grey underside, and course prickles including large spines with swollen bases in adults. Males are spiny all over the upper surface, but females have spines only in the front part. Females grow to about 9ft (2.7m), males are slightly smaller.

Conger eel Conger conger,

Like the common eel, the conger has a smooth, apparently scales skin and no pelvic fins. It differs in having a slightly protruding lower jaw, a dorsal fin that starts near the head, and larger gill slits. A voracious eater of almost anything that moves on the sea-bed, it grows to a maximum length of 9ft (2.7m). During the day congers hide in crevices, especially submerged wrecks.

Herring Clupea harengus,

Once the most important fish in the Scottish diet herring were over fished but are now increasing in numbers again. ‘Silver darlings' herrings have slender bodies with delicate, silvery scales. The back is grey-green or brownish with one short dorsal fin. There is no lateral line. They grow up to 16in (40cm). Between three and seven years old females become mature and lay about 40,000 eggs, spawning taking place off the Norwegian coast. Plankton feeders herring may move several hundred feet vertically through the water in search of food during the course of a day.

Cod Gadus morhua,

Cod have three dorsal and two ventral fins, with a conspicuous single barbel under the lower jaw. Darkly mottled on the dorsal surface colour varies from brownish to pale grey according to habitat. Cod often make lengthy migrations to reach their spawing grounds where they congregate in large shoals and where females normally lay between 2 and 3 million eggs. They grow to 47in (119cm).

Haddock Melanogrammus aeglefinus,

Haddock have a lower jaw shorter than the upper with a very short chin barbel. On the side between the base of the pectoral fin and lateral line is a conspicuous black blotch. The upper body is purplish or greenish-grey, whilst the sides are silver and the belly white. They grow up to 30im (76cm) long.

Poor Cod Trisopterus minutus,

Only growing up to 9in (23cm) the Poor Cod has a long chin barbel under an overlapping upper jaw. The back is yellowish, the underside silvery-grey. On the upper part of the pectoral fin base is a small black spot.

Saithe / Coley / Coalfish Pollachius virens,

A member of the Cod family these fish have jaws of equal length, a straight, light coloured lateral line and dark green fins. They grow up to 24in (60cm).

Whiting Merlangius merlangus,

The whiting is more slender and smaller than the cod, normally 18in (45cm) long but sometimes growing up to 28in (70cm). They are sandy-green on the back with silver sides. A distinctive black spot is situated at the base of the pectoral fin. They have a shorter lower jaw and no barbel.

Ling Molva molva,

An elongate, slim fish, ling have a single long barbel on the lower jaw and a greenish-brown marbled body. Both the second dorsal fin and the anal fin are elongated. The dorsal and anal fins have white tips and a dark spot marks the hind edge of the first dorsal fin. They grow to 80in (200cm).

Angler Fish Lophius piscatorius,

This bottom-dwelling fish varies from reddish-brown to grey-green above, with white below. The large head is surmounted by a lure formed from spines of the first dorsal ray. They may grow up to 6ft (1.8m) long.

Red and Grey Gurnards Trigla spp.

With an upper body distinctly red or pink and the underside orange or white, an armoured head and large pectoral fins, the gurnard is also unusual in other ways. It ‘walks' on the sea-bed by means of pectoral rays and it ‘grunts' – perhaps to stay in contact with others of its kind. They may grow up to 30in (77cm). Grey gurnards are more common than red gurnards and is also a bottom feeder, living off crustaceans, small fish and molluscs.

Mackerel Scomber scombrus,

Designed for fast swimming with slender crescent-shaped fins, these can be pulled into depressions and flattened against the smooth scaled sides to reduce drag. Characteristic iridescent blue-green stripes on the upper surface and white undersurface help camouflage it in the open sea. It has two well-spaced dorsal fins with four to six finlets behind the second dorsal. A similar number of finlets lie between the ventral fin and tail. Mackerel grow up to 16in (40cm) but can reach 26in (66cm).

Red band-fish Cepola rubescens,

This large eyed, slender bodied fish is sometimes found stranded on beaches. It has a red or orange body with a long dorsal fin. They may grow up to 28in (70cm) long.

Cuckoo wrasse Labrus mixtus,

Handsome looking fish the female is reddish-orange with three dark blotches along the back, whilst the male has a brilliant blue head and blue streaks along its sides contrasting with a yellow or orange background. When spawning the male has a large white patch on its head. They grow to 13in (33cm).

Ballan wrasse Labrus bergylta,

Enigmatically all ballan wrasse are born female, taking six years to become sexually mature. After several spawning seasons some females change sex, becoming males. They are deep bodied with small mouths containing pharyngeal teeth for crushing molluscs and crustaceans. Most are greenish-brown, a few are reddish-brown, older males may be a deep chestnut colour. The scales characteristically have a dark edge and a pale centre giving the fish a spotted appearance. They reach a maximum length of 24in(60cm).

Lesser sand eel Ammodytes toblianus,

The sand eel is not a true eel but has an eel-like shape for swimming, and burrowing in the sand. They have a shiny silver body tinged with yellow and a silver belly. There are no pelvic fins but the dorsal is long and composed of soft rays. There are two pectoral fins and an extended anal fin. The lower jaw protrudes on the pointed snout. They grow to a maximum of 6in (15cm).

Flatfish.

Many bottom dwelling fish have developed flattened, camouflaged bodies that conceal them on the sea-bed. Whilst skates and rays are flattened from top to bottom so that they lie belly-down, other flat-fishes are compressed from side to side. On the sea-bed they lie on either their right or left sides and have become adapted by having both eyes on one side – the upper side – of the head.

Turbot Scophthalmus maximus,

These fish vary in colour, closely matching their background. Usually they are sandy-brown with small darker brown speckles. The body is almost circular in outline, the eyed side being covered in scattered bony tubercles, not scales. They grow up to 40in (100cm).

Plaice Pleuronectes platessa,

The skin is brown in colour, with bright orange or red spots on the eyed side and white beneath. The skin feels smooth, with four to seven bony knobs running in a curved line from just behind the eyes to the beginning of the slightly arched lateral line. Usually about 20in (50cm), plaice can live up to 20 years and grow up to 36in (90cm) and weigh as much as 8lb (3.6kg).

Dab Limanda limanda,

On their eyed side these sandy-brown fish feel rough to touch and are usually covered in a mass of darker speckles. They have a strongly curved lateral line. They reach a maximum length of about 15in (38cm).

Halibut Hippoglossus hippoglossus,

Mottled sandy-brown in colour, halibut have a large mouth and pointed strong teeth. They are voracious predators on other fish and large crustaceans. The lateral line is strongly arched. The largest of the flatfish halibut grow up to 100in (250cm).

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