3.2. Anatomy and physiology
3.3. Growth and reproduction
Fish are generally defined as aquatic vertebrates that use gills to obtain oxygen from water and have fins with variable number of skeletal elements called fin rays (Thurman and Webber, 1984).
Five vertebrate classes have species which could be called fish, but only two of these groups - the sharks and rays, and the bonyfish - are generally important and widely distributed in the aquatic environment. The evolutionary relationship between the various groups of fish is shown in Figure 3.1.
Fish are the most numerous of the vertebrates, with at least 20 000 known species, and more than half (58 %) are found in the marine environment. They are most common in the warm and temperate waters of the continental shelves (some 8 000 species). In the cold polar waters about 1 100 species are found. In the oceanic pelagic environment well away from the effect of land, there are only some 225 species. Surprisingly, in the deeper mesopelagic zone of the pelagic environment (between 100 and 1 000 m depth) the number of species increases. There are some 1 000 species of so-called mid- water fish (Thurman and Webber, 1984).
Classifying all these organisms into a system is not an easy task, but the taxonomist groups organisms into natural units that reflect evolutionary relationships. The smallest unit is the species. Each species is identified by a scientific name which has two parts the genus and the specific epithet (binominal nomenclature). The genus name is always capitalized and both are italicized. As an example, the scientific (species) name of the common dolphin is Delphinus delphis. The genus is a category that contains one or more species, while the next step in the hierarchy is the family which may contain one or more genus. Thus the total hierarchical system is: Kingdom: Phylum: Class: Order: Family: Genus: Species.
The use of common or local names often creates confusion since the same species may have different names in different regions or, conversely, the same name is ascribed to several different species, sometimes with different technological properties. As a point of reference the scientific name should, therefore, be given in any kind of publication or report the first time a particular species is referred to by its common name. For further information see the International Council for the Exploration of the Sea "List of names of Fish and Shellfish" (ICES, 1966); the "Multilingual Dictionary of Fish and Fish Products" prepared by the Organisation for Economic Cooperation and Development (OECD, 1990) and the "Multilingual Illustrated Dictionary of Aquatic Animals and Plants" (Commission of the European Communities, 1993).
The classification of fish into cartilaginous and bony (the jawless fish are of minor importance) is important from a practical viewpoint, since these groups of fish spoil differently (section 5) and vary with regard to chemical composition (section 4).
Figure 3.1 Simplified phylogenetic tree of the fishes. (Examples of food-fish, using common English names are shown in parantheses). (SOURCE: N. Bonde (1994), Geological Inst., Copenhagen)
Furthermore, fish can be divided into fatty and lean species, but this type of classification is based on biological and technological characteristics as shown in Table 3.1.
Table 3.1 Classification of fish
|Scientific grouping||Biological characteristics||Technological characteristics||Examples|
|Cyclostomes||jawless fish||lampreys, slime-eels|
|Chondrichthyes||cartilaginous fish||high urea content in muscle||sharks, skate, rays|
|Teleostei or bony fish||pelagic fish||fatty fish (store lipids in body tissue)||herring, mackerel, sardine tuna, sprat|
|demersal fish||lean (white) fish (store lipids in liver only)||cod, haddock, hake grouper, seabass|
Being vertebrates, fish have a vertebral column - the backbone - and a cranium covering the brain. The backbone runs from the head to the tail fin and is composed of segments (vertebrae). These vertebrae are extended dorsally to form neural spines, and in the trunk region they have lateral processes that bear ribs (Figure 3.2). The ribs are cartilaginous or bony structures in the connective tissue (myocommata) between the muscle segments (myotomes) (see also Figure 3.3). Usually, there is also a corresponding number of false ribs or "pin bones" extending more or less horizontally into the muscle tissue. These bones cause a great deal of trouble when fish are being filleted or otherwise prepared for food.
Figure 3.2 Skeleton of bonyfish (Eriksson and Johnson, 1979)
Muscle anatomy and function
The anatomy of fish muscle is different from the anatomy of terrestrial mammals, in that the fish lacks the tendinous system connecting muscle bundles to the skeleton of the animal. Instead, fish has muscle cells running in parallel and connected to sheaths of connective tissue (myocommata), which are anchored to the skeleton and the skin. The bundles of parallel muscle cells are called myotomes (Figure 3.3).
Figure 3.3 Skeletal musculature of fish (Knorr, 1974)
All muscle cells extend the full length between two myocommata, and run parallel with the longitudinal direction of the fish. The muscle mass on each side of the fish makes up the fillet, of which the upper part is termed the dorsal muscle and the lower part the ventral muscle.
The fillet is heterogenous in that the length of the muscle cells vary from the head end (anterior) to the tail end (posterior). The longest muscle cells in cod are found at about the twelfth myotome counting from the head, with an average length around 10 mm in a fish that is 60 cm long (Love, 1970). The diameter of the cells also vary, being widest in the ventral part of the fillet.
The myocommata run in an oblique, almost "plow-like" pattern perpendicular to the long axis of the fish, from the skin to the spine. This anatomy is ideally suited for the flexing muscle movements necessary for propelling the fish through the water.
As in mammals, the muscle tissue of fish is composed of striated muscle. The functional unit, i.e., the muscle cell, consists of sarcoplasma containing nuclei, glycogen grains, mitochondria, etc., and a number (up to 1 000) of myofibrils. The cell is surrounded by a sheath of connective tissue called the sarcolemma. The myofibrils contain the contractile proteins, actin and myosin. These proteins or filaments are arranged in a characteristic alternating system making the muscle appear striated upon microscopic examination (Figure 3.4).
Figure 3.4 Section of a cell showing various structures including the myofibrils (Bell et al., 1976)
Most fish muscle tissue is white but, depending on the species, many fish will have a certain amount of dark tissue of a brown or reddish colour. The dark muscle is located just under the skin along the side of the body.
The proportion of dark to light muscle varies with the activity of the fish. In pelagic fish, i.e., species such as herring and mackerel which swim more or less continuously, up to 48 % of the body weight may consist of dark muscle (Love, 1970). In demersal fish, i.e., species which feed on the bottom and only move periodically, the amount of dark muscle is very small.
There are many differences in the chemical composition of the two muscle types, some of the more noteworthy being higher levels of lipids and myoglobin in the dark muscle.
From a technological point of view, the high lipid content of dark muscle is important because of problems with rancidity.
The reddish meat colour found in salmon and sea trout does not originate from myoglobin but is due to the red carotenoid, astaxanthin. The function of this pigment has not been clearly established, but it has been proposed that the carotenoid may play a role as an antioxidant. Further, the accumulation in the muscle may function as a depot for pigment needed at the time of spawning when the male develops a strong red colour in the skin and the female transport carotenoids into the eggs. The latter seems to depend heavily on the amount of carotenoids for proper development after fertilization. It is clearly seen that the muscle colour of salmonids fades at the time of spawning.
The fish cannot synthesize astaxanthin and is thus dependent on ingestion of the pigment through the feed. Some salmonids live in waters where the natural prey does not contain much carotenoid, e.g., in the Baltic Sea, thus resulting in a muscle colour less red than salmonids from other waters. This may be taken as an indication that the proposed physiological function of astaxanthin in salmonids explained above may be less important.
In salmon aquaculture, astaxanthin is included in the feed, as the red colour of the flesh is one of the most important quality criteria for this species.
Muscle contraction starts when a nervous impulse sets off a release of Ca + + from the sarcoplasmic reticulum to the myofibrils. When the Ca + + concentration increases at the active enzyme site on the myosin filament, the enzyme ATP-ase is activated. This ATP-ase splits the ATP found between the actin and myosin filaments, causing a release of energy. Most of this energy is used as contractile energy making the actin filaments slide in between the myosin filaments in a telescopic fashion, thereby contracting the muscle fibre. When the reaction is reversed (i.e., when the Ca + + is pumped back, the contractile ATP-ase activity stops and the filaments are allowed to slip passively past each other), the muscle is relaxed.
The energy source for ATP generation in the light muscle is glycogen, whereas the dark muscle may also use lipids. A major difference is, further, that the dark muscle contains much more mitochondria than light muscle, thus enabling the dark muscle to operate an extensive aerobic energy metabolism resulting in CO2 and H2O as the end products. The light muscle, mostly generating energy by the anaerobic metabolism, accumulates lactic acid which has to be transported to the liver for further metabolization. In addition, the dark muscle is reported to possess functions similar to those are found in the liver.
The different metabolic patterns found in the two muscle types makes the light muscle excellently fitted for strong, short muscle bursts, whereas the dark muscle is designed for continual, although not so strong muscle movements.
Post mortem the biochemical and physiological regulatory functions operating in vivo ceases, and the energy resources in the muscle are depleted. When the level of ATP reaches its minimum, myosin and actin are interconnected irreversibly, resulting in rigor mortis. This phenomenon is further described in section 5.
The cardiovascular system
The cardiovascular system is of considerable interest to the fish technologist since it is important in some species to bleed the fish (i.e., remove most of the blood) after capture.
The fish heart is constructed for single circulation (Figure 3.5). In bony fish it consists of two consecutive chambers pumping venous blood toward the gills via the ventral aorta.
Figure 3.5 Blood circulation in fish (Eriksson and Johnson, 1979)
After being aerated in the gills, the arterial blood is collected in the dorsal aorta running just beneath the vertebral column and from here it is dispersed into the different tissues via the capillaries. The venous blood returns to the heart, flowing in veins of increasingly larger size (the biggest is the dorsal vein which is also located beneath the vertebral column). The veins all gather into one blood vessel before entering the heart. The total volume of the blood in fish ranges from 1.5 to 3.0 % of the body weight. Most of it is located in the internal organs while the muscular tissues, constituting two- thirds of the body weight, contain only 20 % of the blood volume. This distribution is not changed during exercise since the light muscle in particular is not very vascularized.
During blood circulation the blood pressure drops from around 30 mg Hg in the ventral aorta to 0 when entering the heart (Randall, 1970). After the blood has passed through the gills, the blood pressure derived from the pumping activity of the heart is already greatly decreased. Muscle contractions are important in pumping the blood back to the heart and counterflow is prevented by a system of paired valves inside the veins.
Clearly, the single circulation of fish is fundamentally different from the system in mammals (Figure 3.6), where the blood passes through the heart twice and is propelled out into the body under high pressure due to the contractions of the heart.
Figure 3.6 Blood circulation in fish and mammals (Eriksson and Johnson, 1979)
In fish, the heart does not play an important role in the transportation of blood from the capillaries back to the heart. This has been confirmed in an experiment where the impact of different bleeding procedures on the colour of cod fillets was examined. No difference could be found regardless of whether the fish had been bled by means of cutting the throat in front of or behind the heart before gutting, or had not been cut at all before slaughter.
In some fisheries, bleeding of the fish is very important as a uniform white fillet is desirable. In order to obtain this, a number of countries have recommended that fish are bled for a period (15-20 min) prior to being gutted. This means that throat cutting and gutting must be carried out in two separate operations and that special arrangements (bleeding tanks) must be provided on deck. This complicates the working process (two operations instead of one), time-consuming for the fishermen and increases the time-lag before the fish is chilled. Furthermore it requires extra space on an otherwise crowded working deck.
Several researchers have questioned the necessity of handling the fish in a two-step procedure involving a special bleeding period (Botta et al., 1986; Huss and Asenjo, 1977 a; Valdimarsson et al. 1984). There seems to be general agreement about the following:
Disagreement exists as to the cutting method. Huss and Asenjo (1977 a) found best bleeding if a deep throat cut including the dorsal aorta was applied, but this was not confirmed in the work of Botta et al. (1986). The latter also recommended to include a bleeding period (two-step procedure) when live fish were handled (fishing with pound net, trap, seine, longline or jigging), while Valdimarsson et al. (1984) found that the quality of dead cod (4 h after being brought onboard) was slightly improved using the two-step procedure. However, it should be pointed out that the effect of bleeding should also be weighted against the advantages of having a fast and effective handling procedure resulting in rapid chilling of the catch.
Discoloration of the fillet may also be a result of rough handling during catch and catch handling while the fish is still alive. Physical mishandling in the net (long trawling time, very large catches) or on the deck (fishermen stepping on the fish or throwing boxes, containers and other items on top of the fish) may cause bruises, rupture of blood vessels and blood oozing into the muscle tissue (haematoma).
Heavy pressure on dead fish, when the blood is clotted (e.g., overloading of fish boxes) does not cause discoloration, but the fish may suffer a serious weight loss.
Among the other organs, only the roe and liver play a major role as foodstuffs. Their size depends on the fish species and varies with life cycle, feed intake and season. In cod the weight of the roe varies from a few percent up to 27 % of the body weight and the weight of the liver ranges from 1 to 4.5 %. Likewise, the composition can change and the oil content of the liver vary from 15 to 75 %, with the highest values being found during autumn (Jangaard et al., 1967).
During growth it is the size of each muscle cell that increases rather than the number of muscle cells. Also, the proportion of connective tissue increases with age.
Most fish become sexually mature when they reach a size characteristic of the species and is this not necessarily directly correlated with age. In general, this critical size is reached earlier in males than in females. As the growth rate decreases after the fish has reached maturity, it is therefore often an economic advantage to rear female fish in aquaculture.
Every year mature fish use energy to build up the gonads (the roe and milk). This gonadal development causes a depletion of the protein and lipid reserves of the fish since it takes place during a period of low or no food intake (Figure 3.7).
Figure 3.7 Relation between feeding cycle (percentage sample with food in stomach) and reproductive cycle (gonad development), percentage fish with ripening gonads (spawning, percentage ripe fish) of haddock (Melanogrammus aeglefinus). It should be noted that the development of the gonads takes place while the fish is starving (Hoar, 1957).
In North Sea cod it was found that prior to spawning the water content of the muscle increases (Figure 3.8) and the protein content decreases. In extreme cases the water content of very large cod can attain 87 % of the body weight prior to spawning (Love, 1970).
Figure 3.8 Water content of cod muscle (Gadus morhua) (Love, 1970)
The length of the spawning season varies greatly between species. Most species have a marked seasonal periodicity (Figure 3.7), while some have ripe ovaries for nearly the whole year.
The depletion of the reserves of the fish during gonadal development can be extremely severe, especially if reproduction is combined with migration to the breeding grounds. Some species, e.g., Pacific salmon (Oncorhynchus spp.), eel (Anguilla anguilla) and others, manage to migrate only once, after which they degenerate and die. This is partly because these species do not eat during migration so that, in the case of a salmon, it can lose up to 92 % of its lipid, 72 % of its protein and 63 % of its ash content during migration and reproduction (Love, 1970).
On the other hand, other fish species are capable of reconstituting themselves completely after spawning for several years. The North Sea cod lives for about eight years before spawning causes its death, and other species can live even longer (Cushing, 1975). In former times, 25-year-old herring (Clupea harengus) were not unusual in the Norwegian Sea, and plaice (Pleuronectes platessa) up to 35 years old have been found. One of the oldest fish reported was a sturgeon (Acipenser sturio) from Lake Winnebago in Wisconsin. According to the number of rings in the otolith, it was over 100 years old.