Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 146

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.