Although almost all necessary minerals are available in most practical fish diets and fish can also absorb minerals from the surrounding water, mineral deficiencies do, on occasion, arise in farmed fish. Almost invari- ably they are associated with a reduced bioavailability of the mineral rather
than a frank deficiency. The reduced bioavailability may be associated with a dietary imbalance or with presentation of the element in an unsuitable form. It may also be associated with interaction with other dietary ingredi- ents, such as vitamins and fibers (Tacon and De Silva 1983). The absence of an acid-secreting stomach in cyprinid fish prevents the absorption of or- ganically bound minerals, especially phosphorus, and within plant proteins the presence of phytic acid may chelate trace elements such as iron, copper, and zinc (Satohet al.1989).
Phytate is the storage form in which phosphorus is retained in seeds.
Grains and storage products used in fish diets are therefore often rich in phytate-phosphorus, but this is unavailable to fish.
Fish meals used in practical diets contain high levels of calcium, which, particularly in the case of whitefish meals, greatly reduces the bioavailability of trace elements, and clinical outbreaks of mineral deficiencies can occur.
For this reason experimental studies using whitefish meals with relatively high inherent trace element levels, but without trace element supplementa- tion, have been shown to induce typical deficiency signs of reduced growth, loss of appetite, and cataract in salmonids and common carp (Watanabe et al.1983; Yamamotoet al.1983).
8.5.1. Iodine
Fish meals contain iodine so modern diets incorporating fish meals do not normally exhibit iodine deficiencies. However, when salmonid diets were mainly of mammalian origin, goiters (thyroid hyperplasia), readily controlled by iodine therapy, were common. Originally such lesions were de- scribed as thyroid adenocarcinomata (Gaylordet al.1914) but their goitrous nature was defined by Marine and Lenhart (1910). The confusion between goiter and neoplasia is readily understandable because, as Davis (1953) has pointed out, the salmonid thyroid, unlike that of the human, is diffusely scat- tered around the ventral aorta and so, when goitrous, strongly resembles an invasive adenocarcinoma (Fig. 8.16).
8.5.2. Iron
Iron is more often a problem in relation to its toxicity at high levels in cer- tain water systems rather than as a deficiency, and the melanomacrophage system of the hemopoietic tissues plays a particularly avid role in the reten- tion of iron in the body following hemorrhage or infection (Roberts 2000).
Iron deficiency anemia has been reported in brook trout (Kawatsu 1972), yellowtail (Ikeda et al. 1973), red sea bream (Sakamoto and Yone 1978), carp and eel (Nose and Arai 1979), and catfish (Gatlin and Wilson 1986).
FIG. 8.16
Advanced thyroid “tumors” (actually iodine deficient) in a rainbow trout (Salmo gairdneriRichardson). From Halver (1972).
8.5.3. Copper
Probably because of the ease with which fish can absorb copper from water, copper deficiencyper sehas not been described, although Muraiet al.
(1981) attempted to induce it in channel catfish. They did, however, suc- ceed in showing a slight anemia when fish were fed high supplementary levels. The toxicity of high levels of environmental copper is well recog- nized and induces gill pathology similar to that of the other environmental heavy metal ions. Satohet al.(1983) reported that carp fed high-ash diets without copper supplementation developed characteristic cataracts. One particularly important feature of high copper levels, whether of dietary or environmental origin, is the facilitative effect it has been shown to have on the pathogenesis ofVibrio anguillaruminfection in salmonids (Hilton and Hodson 1983).
8.5.4. Manganese
Manganese is one of the essential elements in whose absence the usual features of poor growth and dwarfism are complemented by the occurrence
of cataracts. Satohet al.(1983) and Yamamotoet al.(1983), working with rainbow trout and carp, showed that manganese supplementation in an available form was essential where fish meals were a major component of the diet. Ishac and Dollar (1968) showed that inOreochromis mossambicus,the absence of manganese resulted in poor growth, reduced food consumption, and loss of equilibrium, but they did not report cataract development. Skele- tal abnormalities associated with manganese deficiency have been reported for rainbow trout (Ishac and Dollar 1968), carp (Ogino and Yang 1980), and tilapias (Yamamoto 1983).
8.5.5. Zinc
Zinc deficiency is normally related to the presence in the diet of zinc- binding agents such as ash, phytic acids, and calcium (Ketola 1979; Hardy and Shearer 1985). General features of zinc deficiency have been described in most farmed species, but the usual deficiency signs of poor growth and skin and fin lesions are complemented by the important feature of bilateral lenticular cataract development (Fig. 8.17). Other clinical signs of zinc defi- ciency in salmonids include erosion of fins, short-body dwarfism, and poor egg hatchability (Hardy 2000).
FIG. 8.17
Zinc deficiency cataract in a salmon showing the delicate gray translucent lens (arrowhead).
Cataract development is an important feature of many marginal defi- ciency conditions in fish. In salmonids, in particular, there is an extensive range of cataracts described. These are associated not only with zinc defi- ciency but also with riboflavin, (Phillips and Brockway 1957), methionine (Postonet al.1977), and a particularly significant but as yet undefined defi- ciency associated with withdrawal of components of animal origin, such as blood meal, from high-energy salmon diets (Wall 1998).
The pathology of each type of deficiency cataract is distinctive and Barash et al.(1982) have shown that each type also has its own characteristic protein banding in electrophoresis.
8.5.6. Phosphorus
Phosphorus is one of the few minerals which fish cannot supply from their aquatic medium. Nevertheless, phosphorus deficiency is rare because most fish diets have high levels of fish meal, which is rich in this element.
As fish meal levels are being progressively reduced, however, and replaced by plant proteins in high-energy feeds, both for economic reasons and to reduce the phosphorus excretion levels from fish farms, it is becoming more frequent. Aside from the normal deficiency features of anorexia and dark coloration, phosphorus-deficient fish also show signs directly related to the role of phosphorus in bone mineralization, principally softening and defor- mities of the head, vertebrae, and ribs (Ogino and Takeda 1976; Shearer and Hardy 1987; Skonberg 1997; Baeverfjordet al.1998).
8.6