All values are on a dry matter basis.
a Carbohydrate calculated by difference.
Gel Diets
Gel diets are designed using gelatin or some other gelling agent. They can be purchased as gels or as a dry powder to which warm water is added. Gel diets offer several advantages including relatively high protein levels, particularly if gelatin is the gelling agent chosen, and high moisture levels and therefore higher palatability than dry diets. An additional benefit is the ease of supplementation: supplements to enhance nutrient consumption and oral medications can easily be added. Gel diets generally sink (with some exceptions) and once prepared should be handled like raw fish, so preparation and storage space should be considered when using these types of diets.
Harvested Fish and Invertebrates (Seafood)
Commercially harvested fish (particularly capelin, herring, mackerel, smelt, and silversides) and invertebrates (particularly mollusks and crustaceans) are commonly fed to fish in zoos and aquariums. It is essential to consider the wild‐type diet and feeding ecology of the target species when making seafood choices. For example, fatty fish (e.g. herring, mackerel, sprat) should not be fed to macro‐invertebrate or bottom‐foraging species (e.g. dasyatid rays). Sustainability of food sources should also be considered. Commercially obtained seafood should be of a quality intended for human consumption.
It is important to offer freshwater‐based food to freshwater fish and marine‐based seafood to marine fish. This distinction may seem trivial, but the essential fatty acids (DHA, EPA, and AA) and the ratio of omega‐3:omega‐6 fatty acids of freshwater and marine fish vary greatly due to the respective phytoplankton sources at the base of the food web. For example, studies on juvenile seahorses (Hippocampus sp.) suggested that they have a high requirement for DHA which may not be met by freshwater mysid shrimp, compared to saltwater mysid shrimp (Chang and Southgate 2001).
It is important to avoid feeding herbivorous fish a diet containing seafood, as this can exceed their protein and lipid requirements, leading to fat deposition in muscle, around the coelomic organs, and/or in the liver.
Plants and Algae
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