Fish are commonly classified by physical characteristics or patterns of behaviour and are usually referred to as either pelagic or demersal. The pelagic fish such as mackerel and herring are feeders of microscopic plankton found in the surface layers of the sea. Demersal fish such as grouper, cod and flatfish, live on or near the sea bed and these patterns of behaviour give rise to different fishing techniques. Generally the pelagic species of fish contain higher levels of fat content than the demersal species.

Fatty fish

Fatty fish contain most of the fat content in the body tissues and use it as a source of energy. The fat is concentrated in the flesh about the lateral line running down each side of the body and will vary considerably depending upon the time of the year. The herring for example (Clupea harengus ), may have a fat content of only 1 percent immediately after spawning and more than 20 percent at other times when feeding well. The percentage increase in fat content is at the expense of the water content with the protein level remaining fairly constant. Figure 1 shows the seasonal variation of fat and water for the mackerel (Scomber scombrus) in the UK.

Figure 1. Seasonal variation in composition of Scomber scombrus

Lean fish

Lean fish are typically bottom-living fish with low fat content. Usually less than 5 percent. The bulk of the fat is contained in the liver with the muscle having a low fat content.

Chemical composition

The chemical composition of fish depends on the fish species, the season, the condition and the feed of the fish. Main components are water, fat and protein but the low concentrations of carbohydrates, minerals, vitamins, sugars and free amino acids also present are important to flavour, odour and nutritional value. Table 1 gives the composition of the raw flesh of a number of selected species (main components only).

Table 1 Chemical composition


Scientific Name

Water (%)

Fat (%)

Protein (%)

Cod Gadus morhua




Redfish Sebastes sp. 73-79 3.2-8.1 16.8-19.7
Herring Clupea harengus 60-80 0.4-22.0 16-19
Mackerel Scomber scombrus 56-74 1.0-23.5 16-20
Hake Merluccius merluccius 80 0.4-1.0 17.8-18.6
Sole Solea solea 78 1.8 18.8
Albacore Thunnus alalunga 59-72 4.3-16.1 21-27

Specific and latent heat

Specific and latent heat values of fish are dependent upon the chemical composition of the fish but for lean fish with a water content of about 80 percent, the specific heat is 0.9 kcal/kgC for temperatures about -1C, the temperature at which lean fish starts to freeze. As freezing does not take place at a particular temperature latent heat and specific heats are removed simultaneously. Figure 2 shows the relationship of total heat content for lean fish taking an arbitrary zero of -40C.


Figure 2. Total heat content of lean fish

The heat content of fatty fish, which has a lower water content than lean fish, will be reduced. A fish with a fat content of 20 percent may start to freeze at -2C and have a heat content lower by 22 percent. Since the oil content varies seasonally the figures for lean fish may be taken as representing the maximum cooling requirement.


When a fish dies spoilage begins due to bacterial, enzymatic and chemical action. On death the bacteria present in the surface slime, gills and intestines, which do no harm to the living fish due to its natural resistance to them start to multiply and penetrate the tissues, the enzymes in the stomach and intestines start to penetrate the belly lining and oxidation of the fat in the flesh occurs. The rate of spoilage is dependent upon the holding temperature and is greatly accelerated at higher temperatures, due to increased bacterial action at the higher temperatures. Freshness is to a certain degree subjective but it can be "measured" against an agreed scale by assessment of appearance, odour and-taste. Figure 3 shows how freshness of a lean fish deteriorates with time at selected holding temperatures.

Figure 3. Deterioration of lean fish

Fatty fish will deteriorate far quicker than lean fish due to the higher rate of oxidation of the fat. Herring for example will be unacceptable after five or six days when held at 0C and after only 30 h when held at 15C.

Spoilage can be reduced by washing, gutting, gilling and above all by keeping the fish cool by the use of ice.

Table 2 lists the shelf lives of selected species.

Table 2 Shelf life

Temperate Water

Tropical Water


Shelf-life (days)


Shelf-life (days)



Snaper (Brazil)


Haddock 12-15 Tuna (USA) 29
Whiting 9-12 Synagric japonicus (India) 27
Hake 8-10 Bonga (West Africa) 20
Redfish 13-15 Sea bream (West Africa) 26
Herring 5-6 Burrito (West Africa) 22
Mackerel 7-9 Tilapia (West Africa) 28

Weight-length relationship

The relationship between weight and length of fish may be of interest in the design of fish processing machinery and fish handling equipment. Figure 4 shows the relationship for cod, snapper, flounder, mackerel and trout.

Figure 4. Weight-length relationship

Age-weight relationship

The age-weight relationship of a fish is of particular interest in the husbandry and harvesting of farmed fish and for any given species will depend on the type of feed, the water temperature and quality, the holding method and sex of the fish. As a comparison Figures 5 and 6 show the approximate relationship for salmon, halibut, turbot, plaice, grey mullet, hake and red mullet.

Figure 5 Age-weight relationship for salmon

Figure 6 Age-weight relationship for flat and round fish