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The seas around the UK contain a fabulous array of fish life, adapted to life in shallow tidepools or to the deep sea. Humans have long depended on a bountiful supply of edible fish for food - it is only in recent times that stocks of many fish species have been drastically reduced.
Agnatha : Primitive jawless fish e.g. hagfish and lampreys comprising 0.3% of all species.
Gnathostoma : Fish with jaws comprising 97.7% of all species.
Chondrichthyes : Cartilaginous fish comprising 3.7% of all species.
Teleostomi : Jawed fish with bony skeletons comprising 96.0% of all species.
Elasmobranchii : Sharks and Rays
Dipneusti : Lungfishes.
Crossopterygii : Coelacanth.
Holicephali : Chimaeras e.g. rabbit fish.
Actinopterygii : Ray-finned fishes.
There are approximately 22-25,000 species of marine and freshwater fish, representing over 50% of all known vertebrates. These are classified into approximately 36 Orders and more than 400 Families. Fish may be divided into two groups - (1) primitive jawless fish, the Agnathans, represented by the eel like hagfish, the most primitive fish surviving today, and lampreys - and (2) fish with jaws, the Gnathostomata.
Gnathostomata are further divided into the cartilaginous fish - Chondrichthyes, represented by 2 groups-- Elasmobranchil (sharks and rays) and Holicephali (chimaeras) - and the bony fish Teleostomi, comprising Dipneusti (lungfishes), the Crossopterygii (coelacanth) and the Actinopterygii (ray-finned fishes), dominated by the more advanced bony fishes, the Teleostei. Most living bony fish are teleosts. 20,000 species comprise 48.2% of all known vertebrates and it is to this group that the majority of fish of commercial importance belong. The cartilaginous sharks, skates and rays are described in a separate page.
Fish are extremely diverse and have adapted to a range of habitats, from tropical reefs to the almost barren waters of high mountain lakes. The properties of water, determined by factors such as temperature and salinity, have important consequences for the fish that live in it. Water is much more dense than air, therefore aquatic animals do not need to develop strong skeletons to overcome the effect of gravity. Movement is easy for well-muscled organisms such as fish. Compared with air, water is a good transmitter of sound. Many fish have devices for producing sound or use sound to detect prey. In contrast to sound, light is poorly transmitted in water. Even in clear oceanic water, sunlight can penetrate only to 1 km, while in turbid waters; all light is absorbed within a few metres of the surface. Despite this, many fishes have well developed eyes and in some cases there is evidence that they have colour vision.
Typical features of teleost fish include bony vertebrae, two pairs of ribs, a swim bladder, a tail and fins (which have fin rays made of bone). In many teleosts the fin rays are individually mobile and are sometimes modified to form hard spines. The anglerfish Lophius piscatorius is a particularly good example of this type of modification, where the fin rays have been adapted into ‘lures’ to attract prey.
Adult fishes feed in a wide variety of ways ranging from sieving phytoplankton to devouring other fishes whole. The range of food taken by a species depends on its ability to detect, capture and process the food.
One of the most remarkable internal features of most teleosts is the swim bladder. This is an air-filled àrgan that is used to regulate the buoyancy of the fish. The amount of gas in the organ is controlled by a gas gland called a rete miriable which secretes gas from haemoglobin in the blood into the swim bladder, sometimes against enormous pressures. Some fish such as the dab and anglerfish have, through evolution, lost the swim bladder since it acts as a good reflector and is easily detected by animals (including man) that use sonar systems to locate prey. It may also function as a resonance chamber for sound production and in some fishes it has a respiratory function.
Most fish swim by oscillating their bodies. Fish have three ranges of speed - sustained or cruise, prolonged, and burst swimming. The capacity for short bursts of high-speed swimming can be crucial for chasing prey or escaping from predators or fishing nets. Fish have two types of muscle fibre - red and white.
Red muscle is used for sustained swimming and white muscle for burst swimming. Fast swimming fish like tuna and mackerel have more red muscle than slower species such as cod or plaice.
Most reproduction in fish is oviparous, this means eggs - usually thousands at a time - are laid by the female and fertilized externally by milt (sperm) from the male. The fertilized eggs are then left to develop, hatch and grow without parental care. There are, however, a number of live-bearers, as well as some egg-layers that practice internal fertilization with subsequent parental care. These species are referred to as viviparous’. Of the 20,000+ existing species of teleosts, around 500 are viviparous.
Sensory organs amongst fish show tremendous variation. Vision, perhaps the major sense of teleost fish, varies from acute perception to complete blindness in cave-dwelling species. Sound is important to fish as it enables them to seek out prey, predators, and their own kind, sometimes at great distances under conditions where other senses may be less effective. Fish hearing abilities are confined to low frequencies.
Along the sides of the body many fishes have a lateral line or a system of lines, usually in the form of a tube with a series of pores that open externally. These pores lead to sensory cells that sense vibrations in the water.
Many benthic fish are perfectly camouflaged against the seabed. Flatfish such as plaice, turbot and halibut can change their body colouration to match the background. Halibut kept in net-pens under laboratory conditions have been observed to change their colour completely within a minute.
In fish that swim near the surface ‘counter shading’ is used, where the back is dark and the belly lighter. Camouflage may also be achieved by reflection - an example of this is in the herring where scales are used to reflect light thus reducing the contrast of the fish relative to its background.
Although fresh water covers only 1% of the earth’s surface and accounts for less than 0.01% of its water, about 40% of fish species live in fresh water. The rest
live in the sea.
There are so many species of fish in UK seas that it would be impossible to describe even half of them here.
You may come across fish on the shore, often in rockpools but also, in the case of the shanny Lipophrys pholis, exposed in the open air when the tide has receded. The large male lumpsucker Cyclopterus lumpus occupies shallow rockpools in late spring, faithfully guarding its eggs. The slippery butterfish Pholis gunnel/us, Cornish sucker Lepadogaster lepadogaster, long spined seascorpion Taurulus bubalis and various blennies and gobies can be found, depending on geographical location, around the UK.
A little deeper, larger fish such as bass Dicentrarchus labrax and thick-lipped grey mullet Chelon labrosus inhabit the water column of inshore shallows, along with the juveniles of many open-water fish. Wrasses are also found close inshore. These are often very colourful, particularly the males of each species.
Mackerel Scomber scombrus and herring Clupea harengus tend to be found at the surface of the sea. Fish in the cod family (Gadus species) include whiting, ling, pollack and saithe as well as cod. Many of these species tend to live near the bottom of the sea.
Plaice Pleuronectes platessa, flounder Platichthys flesus and dab Limanda limanda are common ‘flatfish’. Flatlfish are almost always found on the seafloor, adopting a stationary position to avoid predation and await small prey. Topknots Zeugopterus punctatus are found over rocky surfaces while turbot Psetta maxima prefer sand.
Other fish come in many shapes and sizes - pipefishes and eels are long and tubular, the John Dory Zeus faber is narrow and tall. Gurnards and the pogge Agonus cataphractus have armour- plating, while dragonets are long and lizard-like. Records of ‘exotic’ species appear to be becoming more frequent. Atlantic species such as grey trigger fish Balistes carolinensis and sunfish Mo/a mo/a are regularly spotted in the south west.
Deep-water species are recognised as those commonly found in water between 400 and 1700m depth and are very different from the fish most people are familiar with. Unlike their shallow water relatives, deep-water fish live in an environment which is permanently cold, dark and at high pressure, and they have become highly evolved and adapted to these particular conditions. Many, although not all, grow very slowly and live to a very great age. The most extreme example is the orange roughy Hoplostethus atlanticus which
• has been estimated, using radiometric techniques, to - live for a staggering 149 years. Other fish
. such as roundnose grenadier, Argentine Argentina silus and bluemouth Helicolenus
dactylopterus are also long-lived.
The scarcity of food means that many deep-water fish become sexually mature only once growth has slowed or stopped, and this can be at a late age. Again the most extreme example is the orange roughy which does not reach maturity in the North-east Atlantic until its mid-thirties. In contrast the traditionally exploited shallow- water species are capable of reproduction within the first few years of life.
Deep-water fish also characteristically have low reproductive rates. Many species produce large eggs in numbers which are typically an order of magnitude lower than the shelf species. Fecundity is also low because these fish do not spawn every year, and there can be long periods, in the order of decades, when recruitment is poor.
Two major groups of deep-water fish are recognised, distinguished by their habitat. Bathypelagic fish swim free in the water column while benthopelagic fish are associated with the sea bottom. The latter are found in greater abundance and grow to a larger size than the bathypelagic fish and it is these deep-water fish that are the target of the deep-water fisheries.
Overfishing has been recognised for over 100 years and is now acknowledged as the greatest threat to marine biodiversity. The Food and Agricultural Organisation (FAO) report that 11 of the world’s 15 most important fishing areas are in decline and 60% of major fish species are now either fully or overexploited. In the North Sea, between 30 and 40% of the biomass of exploited fish species is removed each year.
Other threats to fish include those associated with the exploitation of marine and land-based resources that can impact upon fish stocks.
The main categories of pollution that affect the marine environment are nutrients, oil, chemical and radioactive substances, heavy metals, litter, sewage, thermal discharges, noise and acoustic disturbance. All these types of pollutants, from a wide range of sources, have the potential to adversely affect marine fish. Whilst nutrients are essential for life in the sea and their supply is the principal factor controlling primary production in the marine environment, excessive inputs as a result of human activities associated with agriculture and industry cause nutrient enrichment or eutrophication in coastal waters.
Eutrophication can cause both temporary and long term effects in marine ecosystems. Enhanced plant growth reduces dissolved oxygen when the plants decay, which can lead to anoxia (oxygen depletion) in bottom layers of the water column, resulting in kills of invertebrates and fish.
Flat fish species - plaice and dab - are particularly vulnerable to the affects of oil pollution associated with polyaromatic hydrocarbons (PAH5), which are responsible for causing skin lesions and suppressing the immune systems of fish living in contaminated waters. Commercially harvested fish and shellfish that are contaminated with oil have a tainted flavour and show increased incidence of fin rot and skin lesions.
Chemical pollution can affect marine life at every trophic level. Fish (and other fat-containing foods) are known to accumulate dioxins and polychlorinated biphenyls (PCB5). These are very persistent chemicals and are ubiquitous in the environment. A recent survey of these chemicals in samples of various marine fish species, carried out by the Ministry of Agriculture Fisheries and Food (MAFF) (now Department of Environment, Fisheries and Rural Affairs (DEFRA)), has found that people who eat more than the recommended intake of oily fish (larger doses are likely to be found in oily fish because of their high fat content) may exceed dioxin and PCB intake levels considered safe by the World Health Organisation (WHO).
Metals are conservative pollutants i.e. unlike organic wastes (oil and sewage) they are not subject to bacterial decay. They are permanent additions to the marine environment and can accumulate in animal tissue. Mercury, cadmium, copper and zinc are of particular concern for marine organisms.
Aggregate dredging can lead to loss of habitats and species and the suspension of sediments, causing increased turbidity and remobilisation of pollutants in the water column. Studies on the impacts of dredging have shown that the removal of the substrate has a considerable effect on bottom topography and benthic communities. Removal of prey sources at the base of the food web can cause localised food shortages for fish populations whilst the re-deposition of disturbed sediment can smother eggs on spawning grounds. Plumes of suspended sediment created in the extraction process can also clog fish gills. Fish and their eggs may also be extracted’ in the process.
Fish, particularly those of commercial interest have been traditionally conserved or managed for economic reasons rather than for reasons associated with biodiversity.
The regulatory framework for managing fisheries resources to which UK fishermen have access may be considered at a national, European and International level.
In England and Wales, Sea Fisheries Committees (SFC5) have a statutory role in the regulation of inshore or coastal waters i.e. those waters within the 6 nautical mile (nm) limit. SFCs are empowered to make byelaws and can impose regulations on all vessels within the 6nm limit for the management and conservation of their district fisheries.
In Scotland, inshore fisheries are regulated under the Inshore Fishing (Scotland) Act 1984 and administered by the Scottish Executive. The Scottish Fisheries Protection Agency, an agency of the Executive, has responsibility for enforcing fisheries legislation and regulations in the 140,000 square miles of sea area (including inshore waters) around Scotland and in Scottish ports. In Northern Ireland the Department of Agriculture and Rural Development (DARD) has responsibility for administering and managing fisheries.
The UK Government’s response to the Biodiversity Convention, Biodiversity: the UK Action Plan (UK BAP), published in 1994, sets out the broad strategy for conserving and enhancing wild species and wildlife habitats in the UK. A number of marine species, including a plan for commercial fish, common skate and deep-water fish, have been included in the UK BAR A UK Biodiversity Steering Group, co-ordinated by the Department of the Environment, Food and Rural Affairs (DEFRA) has responsibility for implementing the plans. DEFRA and the Scottish Executive, nominated as lead partners, will have specific responsibility for implementing the plans for commercial fish and deep- water fish respectively.
In the waters of the European Union (EU) the framework for management of fisheries resources is the Common Fisheries Policy (CFP). Probably the most well known and most criticised of all fisheries management policies, it currently seeks to harmonise biological, economic and social objectives in a single management policy. Technical Conservation Measures (TCMs) is the term applied to legislative methods within the framework of the CFP which regulate fishing activity. Examples of TCMs are:
the use of minimum mesh sizes
the use of Minimum Landing Sizes (MLS)
limitations on fishing within certain areas with specific gear types
limitations on fishing during certain periods with specific gear types
These measures attempt to improve the selectivity of fishing gears and minimize catches of juvenile fish and thus conserve fish stocks.
Following the review of the CFP after 2002, the future of all fisheries in Europe will become apparent.
WHAT YOU CAN DO
If you eat fish, only eat fish that is caught using selective and environmentally sensitive
methods from well managed stocks or farms.
Make enquiries about the method and area of capture.
If you shop at a supermarket enquire about their environmental and fishing policies.
Avoid eating deep-water fish species. Send a SAE to MCS for a copy of our information
brochure ‘A Dead-end for Deep-water Fish?’
Join the Marine Conservation Society and support our campaigns to protect the marine environment for wildlife and future generations.
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