Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 145

Lipid levels from 10 to 20% of the dry weight of the diet (~20% for juveniles, 10% for adults) are usually considered sufficient for many fish to use protein without depositing excess lipid (Sargent et al. 2002). Excessive fat levels or inappropriate levels of fatty acids can lead to fatty infiltration of the liver, a common chronic health problem in aquarium fish.

Essential lipids can include cholesterol, phospholipids, inositol, and fatty acids (NRC 2011). Some fish can synthesize cholesterol endogenously. Phospholipids, the general term for lipids containing phosphorus, includes sphingomyelin, phosphatidyl‐choline, ‐ethanolamine, ‐serine, and ‐inositol. Dietary phospholipid inclusion can improve survival and growth in larval and juvenile fish. Optimal requirements can be as high as 6% of diet dry matter for larval fish (NRC 2011). Inositol (which may also be categorized with vitamins) is a component of phosphatidylinositol in cell membranes. Inositol can be synthesized endogenously, but often not at high enough levels to meet needs. Young fish, particularly larval fish, appear to require higher levels than older fish. Fishmeal is a rich source of inositol, so diets containing fish or fishmeal generally do not require additional supplementation (NRC 2011).

Fatty acids are the preferred energy source in fish and play a key role in yolk sac and larval development, maintaining cellular membranes, and modulating immune responses (Tocher 2003). All fish species studied are known to require linoleic acid (18:2n6; an omega‐6 fatty acid) and linolenic acid (18:3n3; an omega‐3 fatty acid) (NRC 2011). These requirements can generally be met with terrestrial animal‐ and plant‐based lipid sources. Many freshwater fish species are able to produce polyunsaturated fatty acids (PUFAs), including eicosapentaenoic acid (EPA, 20:5n3) and docosahexaenoic acid (DHA, 22:6n3) (Hixson 2014). However, many carnivorous and benthivorous marine fish require dietary long‐chain omega‐3 PUFAs and higher omega‐3:omega‐6 ratios (Ahlgren et al. 2009; Hixson 2014). In general, cold‐water species are thought to have higher requirements for omega‐3 PUFAs compared to warm‐water species (NRC 2011). Omega‐3 PUFAs are generally provided from marine‐based sources such as fish meal and fish oils, but also can be found in algae. Vegetable oils are generally poor sources of these fatty acids, although bioengineered algae may prove useful in the future. Another PUFA, arachidonic acid (AA, 20:4n6), may also be required or recommended for optimal performance in fish. It has been shown to improve egg quality and survival, larval growth and survival, and adaptation to different salinities in several fish species (Bell and Sargent 2003).

Total lipid content and fatty acid profiles of food items vary considerably with species, life stage, and diet, but values for common seafood items are reported in Table A4.2. The same variability is seen in zooplankton, which is commonly raised for feeding larval fish, with large amounts of EPA in wild water fleas (Daphnia spp.), and large amounts of DHA in wild marine copepods (Ahlgren et al. 2009). These levels can be manipulated through the diet.