Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 156

Live plants in aquatic systems can provide a supplemental source of nutrition for herbivorous fish. Many species of freshwater plants have been used as food sources, including Cabomba spp. (fanwort), Egeria (Elodea) densa and Elodea canadensis (water weed, elodea), Limnophila spp. (ambulia), Myriophyllum mattogrossense, Rotala indica (toothcup), Hygrophila polysperma, Nymphea spp. (water lily, lotus), and Lemna spp. (duckweed). Consideration should be given to the substrate, lighting, and carbon dioxide levels necessary for photosynthesis and growth. Limited information is available on the nutritional value of freshwater plants, as research has primarily focused on weed control and alternative feed sources for domestic animals. Some research has looked at Lemna sp. as a potential high‐protein feed source for cultured fish. When grown under ideal conditions and harvested regularly, it has 5–15% fiber, 35–43% crude protein, and ~5% polyunsaturated fat on a dry matter basis (Leng et al. 1995).

The largest use of microalgae (phytoplankton) in fish feeds is in aquaculture, where over 40 species are used. The most common are Chlorella, Dunaliella, Scenedesmus, and Spirulina spp. They are commonly used in marine herbivores, marine fish larvae, and fingerlings. Considerable attention has been given to the concentrations of omega‐3 fatty acids in microalgae, especially EPA and DHA. Microalgae can also have a high fraction of β‐1,3‐glucan, which can act as an immune‐stimulant in fish (Dalmo et al. 1996; Dalmo and Bøgwald 2008). Many microalgae are good sources of carotenoids, including astaxanthin in Haematococcus spp., β‐carotene in Dunaliella spp., and β‐carotene and zeaxanthin in Spirulina spp. Spirulina spp. are also a good source of thiamine, riboflavin, and cobalamin (Thajuddin and Subramanian 2005). However, microalgae show high variation in chemical composition depending on species and growth conditions (Ben‐Amotz et al. 1985; Reitan et al. 1994). Microalgae can be fed directly, provided in commercial fish feeds, or gut‐loaded into an intermediate food source such as rotifers.

Several macroalgae (e.g. seaweeds) are used as fish feed, including Porphyra spp. (nori, a red algae), Gracilaria verrucosa (ogonori, a red algae), Undaria pinnatifida (wakame, a brown algae), Ulva spp. (sea lettuce, a green algae), and Cladophora glomerata (a filamentous green algae). Carbohydrate and fiber content of select algae species have been reported by Mišurcová et al. (2010).

The benefits of incorporating algae into fish diets have been shown for a variety of fish: Chlorella or Scenedesmus spp. fed to Nile tilapia; Chlorella spp. fed to olive flounder (Paralichthys olivaceus); Undaria, Ascophyllum, Porphyra, or Ulva spp. fed to red seabream (Pagrus major); Gracilaria or Ulva spp. fed to European bass (Dicentrarchus labrax); Ulva spp. to striped mullet (Mugil cephalus); Ulva spp. fed or Pterocladia spp. fed to gilthead seabream (Sparus aurata); and Porphyra or a Nannochloropsis–Isochrysis spp. combination fed to Atlantic cod (Gadus morhua), although many of these studies fail to identify the nutritional factors responsible for these benefits (Yone et al. 1986; Mustafa et al. 1995; Wassef et al. 2001; Wassef et al. 2005; Valente et al. 2006; Badwy et al. 2008; Walker et al. 2009; Walker and Berlinsky 2011; Rahimnejad et al. 2017).

The limitation of algae as feed is the digestibility of the cell wall in non‐herbivorous species. Studies suggest that only 10–15% of dietary protein requirements of non‐herbivorous fish can be met by algae without compromising growth. The type and quantity of extracellular polysaccharides, which are abundant in certain algae, can also interfere with nutrient absorption. Finally, some algae contain trace element concentrations that can be detrimental.