Carbohydrates (sugars, starches, and fiber) are poorly digested by most fish but they can be mobilized to meet energy demands. They play the largest role in the nutrition of herbivorous fish. Carbohydrates in the form of starch are generally present in commercial fish diets at 5–25% (Hua and Bureau 2009).
Starch digestibility is variable in fish but is usually low. In general, cold‐water carnivorous fish appear to have the lowest capacity to digest starch, while warm‐water herbivorous or omnivorous fish are more capable of starch digestion (Hemre et al. 2002). Digestibility is routinely improved by heat and pressure treatment of the starch which results in gelatinization; this occurs during commercial diet production or heating of food items prior to feeding (Kumar et al. 2006; Hua and Bureau 2009). The carnivorous Hong Kong grouper (Epinephelus akaara) showed reduced growth rates when fed >8% dietary carbohydrate (non‐gelatinized) (Wang et al. 2016), while herbivorous blunt‐snout bream (Megalobrama amblycephala) were able to adapt to diets containing as much as 42% carbohydrate (dextrin) (Li et al. 2016). Due to the wide variation in digestibility, it is probably advisable to limit dietary starch whenever practical, particularly for cold‐water carnivorous fish.
Non‐starch polysaccharides (NSPs) are relatively indigestible by fish and can lead to alterations in digesta viscosity such that nutrient absorption is impaired (Sinha et al. 2011). Carbohydrase enzymes can be added to diets to hydrolyze these NSPs, which can improve growth rates and feed use when carbohydrates are fed (Castillo and Gatlin 2015).
Fermentable carbohydrates are likely to be important substrates for microbial fermentation in the caudal GI tract in herbivorous, omnivorous, and planktivorous fish. In three species of herbivorous fish, microbial fermentation in the posterior portions of the GI tract resulted in short‐chain fatty acid (SCFA) synthesis at rates that were equivalent to terrestrial vertebrates and marine reptiles, despite colder environmental temperatures (Mountfort et al. 2002). It is likely that most of the carbohydrates used for microbial fermentation are hemicellulose and cellulose (Mišurcová et al. 2010; Templeton et al. 2012). The microbial communities found in the piscine GI tract are likely critical for proper health and nutrition and should be considered when designing proper nutrition programs.
Vitamins
Vitamins serve as cofactors, hormones, and regulators of cellular differentiation. They are generally required from the diet in trace amounts. Vitamins may be categorized as fat‐soluble (vitamins A, D, E, K) or water‐soluble (B vitamins including thiamine [B1], riboflavin [B2], niacin [B3], pantothenate [B5], pyridoxine [B6], biotin [B7], folate [B9], cobalamin [B12]; vitamin C [ascorbic acid]; choline). Requirements are affected by size, age, growth rates, environmental factors, and inter‐relationships with other nutrients (NRC 2011). Larval fish nutrition presents some unique concerns related to vitamin nutrition and is discussed in more detail later. Estimated vitamin requirements are presented in Table A4.3. As with other nutrients, these recommended levels are generally determined for optimal performance of aquaculture species and so should be used as a guideline rather than an instruction.
Fat‐soluble vitamins A, D, E, and K may be supplemented in diets, but present a higher risk of toxicity since they are stored in liver and adipose and are harder to excrete in excess.
Vitamin A is involved in cellular differentiation and vision. Deficiency in larval fish is associated with incomplete eye migration, abnormal pigmentation, and skeletal abnormalities, while in juvenile fish, skin and liver hemorrhages, exophthalmos, and twisted gill opercula have been described (NRC 2011; Hamre et al. 2013). Carotenoids are pro‐vitamin A compounds (and pigments) that occur naturally in many wild‐type food items. Carotenoids are generally considered less toxic since the conversion of carotenoid to retinol is regulated in response to body vitamin A levels. In several freshwater and saltwater fish species, β‐carotene, astaxanthin, and canthaxanthin can be converted to vitamin A, although their conversion seems most efficient when vitamin A levels are low (Moren et al. 2002). The use of astaxanthin and canthaxanthin as precursors to vitamin A is somewhat unique – most birds and mammals studied are not able to convert these carotenoids to vitamin A (Goodwin 1986).
Table A4.2 Total lipid and fatty acid composition of seafood items commonly fed to fish.
Source: Gruger et al. (1964). © John Wiley & Sons.
| Pacific herring, fillet (Clupea pallasii) | Atlantic mackerel, fillet (Scomber scombrus) | Menhaden, whole (Brevoortia tyrannus) | Coho salmon, fillet (Oncorhyncus kisutch) | Lake herring (Coregonus artedii) | Rainbow trout (Oncorhyncus mykiss) | Lake whitefish (Coregonus clupeaformis) | Blue crab (Callinectes sapidus) | Littleneck clam (Protothaca staminea) | Pacific oyster (Crassostrea gigas) | |
|---|---|---|---|---|---|---|---|---|---|---|
| Tissue lipid content (%) | 12.8 | 3.2 | 15 | 7.5 | 2.5 | 2.5 | 2.2 | 2.1 | 0.5 | 2.5 |
| Lipid fatty acid composition (%) | ||||||||||
| 16:0 palmitic acid | 15.1 | 28.2 | 28.9 | 10.2 | 17.7 | 8.2 | 10.5 | 15.2 | 23.8 | 21.4 |
| 18:1n9 oleic acid | 16.9 | 19.3 | 19.3 | 18.6 | 18.1 | 19.8 | 27.2 | 17.6 | 10.8 | 8.5 |
| 18:2n6 linoleic acid | 1.6 | 1.1 | 1.1 | 1.2 | 4.3 | 4.6 | 5.5 | 1.9 | 1.4 | 1.2 |
| 18:3n3 linolenic acid | 0.6 | 1.3 | 1.3 | 0.6 | 3.4 | 5.2 | 3.7 | 1.2 | 1.6 | 1.6 |
| 20:5n3 EPA | 8.6 | 8.6 | 7.1 | 12 | 5.9 | 5 | 6.4 |
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