Nutrition and reproductive capacity in fish

Abstract

Selected text: Despite the extra nutritional requirements, many fish cease to feed during gonadal maturation (Homans & Vladykov, 1954), and in the salmon and the herring degenerative changes occur in the gut (Rutter, 1902; Greene, 1926). During this period of semi-starvation the fish may carry out a lengthy migration, prominent sexual characters may be developed (Davidson, 1935), or a profound metamorphosis may precede maturation (Okkelberg, 1921); these factors are a further nutritional drain. Fat and protein reserves are withdrawn from the muscles and the liver during maturation. Greene (1926) found that the migrating king salmon, Oncorhyncus tschawytscha, lost 51.6% of the total muscle mass; the fat content of the muscle fell from 15'5 to 2.2% at spawning and the protein content from 17 to 13'7% ; similar changes occur in other fish; thus the oil content falls from 23 to 10% in the maturing North Sea herring, Clupea harengus (Wood, 1958), and from 14 to 7% in the mackerel, Scomber scombrus (Liihmann, 1956). Changes also occur in the amino-acid composition of the flesh proteins and in the ionic and protein contents of the blood. A number of changes occur in the composition of the ovary during the process of maturation; there is an increase in the percentage protein content and dry matter (Garrod & Newell, 1958; Braekkan, 1999, and a preferential accumulation of the more unsaturated fatty acids (Channon & El Saby, 1932; Shorland, 1939). Feeding and the rate of growth affect the onset of first maturity, the fastest growing fish maturing earliest (Raitt, 1932; Simpson, 1951; Alm, 1953). An improvement in feeding has been shown to hasten the onset of maturity by I year in flounders, Pleuronectes limanda (Kandler, I 932 ; Gross, 1949), herring, Clupea harengus (Cushing & Burd, 1956) and char, Salvelinus alpinus (Runnstrom, 1951), whereas poor feeding delayed maturity in trout and perch, Perca fluviatilis (McCay, Dilley & Crowell, 1928-9; Alrn, 1953). In a laboratory population Svardson (1943) found that maturity occurred earliest in fastest-growing Lebistes ; fish of medium growth rate matured later but at a larger size and with a larger gonad, and the slowestgrowing fish were last to mature, again at a small size, and with a small gonad (Fig. I). However, in overcrowded natural conditions with very little food available for growth, stunted populations are found in which maturity occurs very early and at a small size (Alm, 1946). In Lebistes, Svardson (1943) found that, after the testes had attained a certain size, spermiogenesis was most rapid when the food supply was poor and he concluded (Svardson, 1951) that this fact might account for precocious maturation in stunted populations. However, precocious maturation has never been induced by a poor diet in differential feeding experiments with a single stock of fish, and it seems likely that earlier maturation in conditions of overcrowding would have a selective advantage when growth is very slow. The fecundity of fish may increase with better feeding; thus Simpson (1951) found that in the North Sea fast-growing plaice, Pleuronectes platessa, from good feeding areas had a higher fecundity than slower-growing fish of the same size from poor areas. When fishing reduced the numbers of the Sakhalin herring, feeding conditions improved considerably and their fecundity increased (Piskunov, I 952) ; similarly the fecundity of Caspian roach, Rutilus rutilus, of 18 cm length increased by 50% in 1946 when feeding conditions were exceptionally favourable (Chugunova, 1951). In the viviparous mosquito fish, Gambusia afinis, the brood size also increases with better feeding (Krumholz, 1948). A considerable number of eggs do not develop fully in carp, Cyprinus carpio, but are resorbed after spawning (up to 35% of the eggs in older fish), and Vasnetzov (quoted by Nikolsky, 1950) has suggested that when there is a substantial improvement in feeding conditions these eggs may develop normally, increasing fecundity. In at least some species egg size may be affected by feeding. Comparing different populations of char, Salvelinus alpinus, and of whitefish, Coregonus clupeaformis, in Swedish lakes, Maar (1949) and Toots (1951) found that populations with a high growth rate produced eggs of a large size; Maar (1950) and Nikolsky (1950) have also reported that within the same population of fish the larger fish have eggs of a larger size. The level of feeding may influence spawning frequency in fish, thus Gambusia females kept well fed in the laboratory produce broods with greater frequency than when wild (Turner, 1937). In Salvelinus fontinalis and S. alpinus a deficient diet, due to overcrowding, may prevent annual spawning (Vladykov, 1956; Grainger, 1953) and in the Great Slave Lake whitefish, Coregonus clupeaformis, with a very short growing season of 3 months, spawning may occur at intervals of 2 years or more instead of annually (Kennedy, 1953).