Fish & Shellfish

Fish & Shellfish is readily available relatively cheap source of food which is high in proteins, vitamins and minerals. Its extremely  perishable, so  it must be cooked as quickly as possible after being caught and must be kept cold until it is cooked .There are hundred of species of fish & shellfish,  sea fish from the ocean and fresh water fish from lakes, rivers and streams.  

While much of the fish & shellfish consumed is still wild caught, there is distress in fish populations around the globe. It was once thought that the oceans were an inexhaustible source of fish and seafood but today we know that many fish species once in abundance like cod, herring, swordfish, sole, and tuna are in danger of being depleted to extinction. Increasing consumer demand and sophisticated fishing techniques have taken their toll on fish populations and the environment. Fish farming has helped alleviate some over-fishing, but has created environmental pollution problems from intensive farming techniques, and additional negative effects to wild fish because the meal fed to farm-raised fish is often derived wild sources. It will take a concerted global effort to devise ways to harvest fish in ecological and sustainable ways to preserve fish stocks for future generations.


Aquaculture, mariculture, and aquaponics are names used for a variety of farming techniques for fish, seafood, and marine produce. The techniques are used for freshwater and salt water fish and are employed in above ground tanks, ocean pens, and in open waters. Fish farming produces a steady supply of fish that are raised in a carefully controlled environment   to uniform size and formation, much like breeding poultry or cattle. Farmed fish can be quickly processed and delivered fresh to market in a short time. They can also relieve the pressure on endangered fish species caused by over fishing.


Classification Of Fish
fish classification
click to enlarge

Whitefish is generally a mild-flavored, often slightly sweet fish, which can be interchangeable in recipes. These include wild Alaska pollock, bass, cod, grouper, haddock, and halibut. These are great for pan-frying, pan-searing, using soups and chowders, and baking. There are also some thinner fillets of white fish, which can be used in these recipes but keep in mind they cook much faster, including flounder, perch, plaice, and sole. 

john dory white fish
John Dory White Fish

Round fish:

These are the fish that have eyes on both sides of their body. They are found swimming near the surface of the water. E.g. cod, hake, haddock, whiting, eel, mackerel, salmon, trout. 

red mullet round fish
Red Mullet Round Fish

Flat fish:

These are the fish, which have eyes on the same side of their body. They are found at great depths at the bottom of the sea. E.g. Halibut, lemon sole, plaice, sole, turbot.

halibut flat fish
Halibut Flat Fish

Oily fish

Oily fish are those fish which have oils throughout the fillet and in the belly cavity around the gut, rather than only in the liver like white fish.

Oily fish generally swim in mid-waters or near the surface (the pelagic zone). Oily fish are a good source of Vitamins A and D as well as being rich in Omega 3 fatty acids.

For this reason the consumption of oily fish has been identified as more beneficial to humans than white fish. Amongst other benefits, studies suggest that the Omega 3 fatty acids in oily fish may help sufferers of depression, reduce the likelihood of heart disease and improve inflammatory conditions such as arthritis.

However, oily fish are predators, high up in the food chain, and thus more likely to contain toxic substances than white fish.

Oily Fish


Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater.



Crustaceans are cladocerans if they have 4–6 pairs of (thoracic) legs, lack any paired eyes, swim with their second pair of antennae, and have at least the head not covered by a carapace. Crustaceans are some of the most important marine life to humans—crabs, lobsters, and shrimp are widely fished and consumed around the world.



The molluscs (soft bodied animals) belong to the large and diverse phylum Mollusca, which
includes a variety of familiar animals well-known as decorative shells or as seafood. These
range from tiny snails, clams, and abalone to larger organisms such as squid, cuttlefish and
the octopus.



Gastropoda is the largest molluscan class with about 35,000 extant species.
The gastropods are torted asymmetrical molluscs and usually possess a coiled shell. The
soft body normally consists of head, foot, visceral mass and the mantle.

The shell in this subclass is typically coiled with an opening
at the ventral end known as aperture. The aperture is covered by operculum which closes
the opening of the shell.



There are about 10,000 living bivalve species. The bivalve as the name implies, possesses two valves (shells) lying on the right and left sides of the body. Bilateral symmetry is a characteristic feature. The shell is mostly composed of calcium carbonate. Umbo is the first formed part of the valve and is above the hinge. The soft body of the bivalve is covered by the mantle comprising two lobes. The foot is muscular and is ventral.



Cephalopods are purely marine in habit, and there are about 600 living species. They are considered as the fastest marine invertebrates. Head is highly developed. The cuttlefishes come under the order Sepioidea and are characterised by the presence of a shell (chitinous or calcareous), 10 circum oral appendages and the tentacles are retractile into pockets. Suckers have chitinous rings. Posterior fin lobes are free and not connected at midline. The cuttlebone is internal and located dorsally underneath the skin.


Spoilage Of Fish  Reasons And Signs:   

  • Main bacterial spoilage is due to micrococcus, pseudomonas, corney bacteria, bacilli, achromo and flavo bactor. 
  • These bacteria grow at higher pH i.e. more than 7; around a temperature of 15-20 deg. C.
  • Parasitic infections also cause early spoilage.
  • The bacterial growth can be eliminated by proper control of pH and temperature. Spoilage also occurs in the fish when lactic acid content in the fish gets reduced due to struggle and stress before death. TMA (trimethilamin) oxide is an important compound of marine fish. After death of the fish this compound is broken down due to oxidation and causes bad smell. 

The other reasons for the spoilage are as follows: 

  • Enzymes in dead fish because of microbes and chemicals bring about changes. Faulty method of catching and handling Personal hygiene. 

Signs of spoilage  

  •   Such fish is liable to float in the water.
  • Eyes are seen shrunken and collapsed.
  • The gills are either gray or brown in color.

  • Emit unpleasant and offensive odor.

  • Scales are soft and easily rubbed off.

  • Flesh is slimy and pits on pressure.

  • Flesh is detached easily from bone structure. Unpleasant and offensive smell from abdominal cavity. 

  • A pink color line along the back bone is the sign of decomposition.

Assessment Of Period Of Freshness Of Fish  

  • Fish is fresh when the eyes are bright and bulging with red gills.
  • In case the eyes are sunk to the level of eye brim, it may be considered 24 to 48 hours old.
  • In case the eyes are sunk deep inside, it may be considered 48 hours or beyond.

Selection and judgment of fish   

 Fresh fish is ideal to procure. it depends upon the location, weather condition, high standards of hygienically maintained shops with proper infrastructure, and temperature control.  

Following is the receiving standards for the receiving of the fish 

In case of fish purchased whole: 

  • Eyes: Should be bright, full, and not sunken, no slime or cloudiness.
  • Gills: Should be bright red, no bacterial slime.
  • Flesh: Should be firm, translucent, and resilient so that when pressed the impression goes quickly. It should not be limp ¾ Scales: Should be flat, moist, and plentiful. 
  • Skin: Should be covered with fresh sea slime, be smooth and moist with sheen and no abrasions or bruising, and no discoloration.
  • Smell: Pleasant, no ammonia or sourness
  • Small fish when held horizontally on the palm, does not drop at the tail and remain rigid.

Purchasing points of prawns and lobster:

  • Purchase alive with both claws attached to ensure freshness ¾ They should be heavy in proportion to their size.
  • The coral of the hen lobster is necessary to give the required color for the soups and sauces.
  • Hen lobsters are distinguished from hen lobsters by a broader tail.

Purchasing points of crab:

  • Buy alive to ensure freshness
  • Ensure that both claws are attached
  • They should be heavy about their size

Purchasing points of mussels:

  • The shells must be tightly closed to indicate that they are alive
  • Mussels should be of good size
  • There should not be an excessive number of barnacles attached
  • Mussels should smell fresh

Structure of the meat  

All the fishes consist of nearly 75% of moisture and also the aluminous (egg whites) consistency varies little from fish to fish (18%).  In the consistency of fat the variation is much wider; about 26% for the conger- eel family; 12% for salmon and salmon –trout; 9% for trout; 8.2% for shad and 6% for herring. 

Fish flesh does not vary much from that of the land animals. Where the fish are at an advantage is in the contents of the phosphorated compounds and in the fact that fish especially the leaner ones, are much more easily digestible and so represent an excellent food for the sedentary and the sick. 

Fish lack the connective tissue like land animals as they have no limbs and so the meat is lighter and flakier. Medium-size fish are better than large fish, which may be coarse, small fish often lack flavor. 

Fish is a useful source of animal protein as meat. Oily fish such as sardines, mackerels, herrings, and salmon contain Vitamin A and Vitamin D in their flesh, white fish such as halibut and cod, these vitamins are present in the liver. 

The bones of sardines, whitebait, and tinned salmon when eaten provide the body with calcium and phosphorus. Fish is also a good source of magnesium, copper, iron, and iodine. Since all fish contain protein, it is good body-building food and oily fish is useful for energy and as a protective food because of its vitamins. 

Owing to its fat content oily fish are not so easily digestible as white fish and is not suitable for cooking for invalids 


Store fresh fish covered with ice at 29-32°F /-2 0°C, in a self-draining perforated pan and replenish the ice as needed. Keeping the fish iced will provide a constant rinsing action that minimizes odors and maximizes shelf life up to a week. Fresh fish should always be gutted before storing and will have a longer shelf life if kept whole. Wrap fillets in plastic wrap to prevent the ice crystals from tearing the flesh of the fish. Fresh fish is best consumed within 2-3 days from delivery but whole fish can be held longer, from 1-2 weeks depending on the species and freshness and the holding method. Frozen fish should be thawed slowly under refrigeration or cold running water.

  • It is most important to maintain freshness and prevent staleness in the fish. In deep-sea fishing operations, the aim is to catch the maximum catch of edible fish in ideal conditions. 
  • Fish should not be subjected to more stress and stress after the catch.  
  • Freshness depends upon temperature control, weather control, and the quantum of fish. Immediately freezing at – 10 C is advisable while moving the seafood from deep-sea location to coast. 
  • It is advisable to store fish at temperatures ranging from –23 to –29 deg. C to conserve enzymes from destruction or keep fishes alive in tanks.  
  • While receiving fish it should be checked whether the fish is being marketed below 3 deg C or not.  
  • Fishes like tuna are stored at a temperature more than – 20 deg C 
  • Shellfish is normally stored in tanks alive after the catch, and then it is subjected to processing. 
  • Subjecting to a temperature less than – 3deg C can enhance the shelf life of certain fish It can be stored for more than three to six months. It is the case with shellfishes also.  


Fresh fish (caught less than 4 hours beforehand): The whole fish should be gut, scaled, then washed, dried with absorbent paper, wrap in foil or plastic film and freeze in a tray. 

Fish Slices: Freeze raw on a tray. 

Small fish: Gut, dry and freeze raw in a tray. 

All fish and shellfish should be stored on sealed plastic bags, preferably vacuum-packed. All frozen fish should be thawed in a refrigerator for several hours before use. Do not thaw in water as it tends to damage the structure of the fish and does not taste as good. The cooking tie for thawed fish will be a little longer than fresh fish. Once frozen fish has been thawed it should not be in any conditions be refrozen as this could prove as a major health hazard. 

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