in certain algae, can also interfere with nutrient absorption. Finally, some algae contain trace element concentrations that can be detrimental.
Live Foods
Ambush and highly visual predatory fish (e.g. frogfish, seahorses) generally require live foods to trigger an eating response. Other predatory fish that are not readily eating harvested food may require live foods while being converted onto a managed diet. Most larval fish also require live foods in the early stages of development.
The most popular live foods cultured in aquariums are brine shrimp (Artemia spp.) for brackish or marine fish and rotifers (particularly Brachionus spp.) for freshwater, brackish, or marine fish.
Brine shrimp are typically obtained as cysts or live cultures and reared in salt water. Cysts hatch into nauplii that develop into adults through several molts. Alternatively, cysts can be decapsulated to feed without worrying about removing unhatched cysts or shells. Post‐hatch nauplii contain an internal yolk sac and, if no exogenous food is available, can survive on these yolk reserves for up to five days post‐hatch (Treece 2000). However, the nutritional quality of nauplii declines rapidly as the yolk sac is consumed. Nauplii should either be fed out within 24 hours post‐hatch or receive supplemental feeding and/or gut‐loading. Brine shrimp are nonspecific feeders, as long as food size is small (<50 μm for nauplii and <60 μm for adults), and a variety of products can be used (e.g. algae, microparticles, omega‐yeast, or marine oil emulsions, which are particularly rich in PUFAs). Products should have taurine, manganese, and omega‐3 fatty acids with a DHA:EPA ratio of 2:1, as these promote growth, increase survivability, and improve stress resistance (Sargent et al. 1997, 1999; Woods 2003; Nguyen et al. 2008). Administration of probiotics to brine shrimp improved survival of larval European bass (Touraki et al. 2012).
Rotifers are typically obtained as cysts or live cultures and maintained in freshwater or brackish water, although some can be cultured in salt water. Providing supplemental feeding for two to four days and gut‐loading for two to three hours prior to feeding out both improve nutrient concentrations (Romero‐Romero and Yúfera 2012). These nutrients appear stable for at least eight hours after rotifers are placed in fish tanks (Dhert et al. 2014). Supplements should contain taurine, omega‐3 fatty acids, vitamin C, vitamin E, manganese, zinc, iodine, and selenium (Hamre et al. 2008; Matsunari et al. 2013a, b). The levels of vitamin C and E in copepods (500 and 115 mg/kg, respectively) may provide an initial target, and these levels can be achieved in rotifers with supplementation (Kolkovski et al. 2000). The effectiveness of iodine supplementation depends on the source; sodium iodide is mostly lost within two hours of enrichment while thymol iodine is still present after 18 hours (Srivastava et al. 2012). Selenium enrichment of rotifers is also feasible, but the concentration and source must be considered to avoid toxicity concerns; inorganic selenium sources have the potential to be more toxic than Se‐methionine sources (Penglase et al. 2010).
While brine shrimp and rotifers are the most common live foods, many other organisms are routinely available for purchase or culture. Other marine food items include mysids (Mysis spp.), copepods, amphipods, and fiddler crabs (Uca spp.). Other brackish or freshwater food items include water fleas (Daphnia and Moina spp.), ghost/glass shrimp (often Palaemonetes spp.), bloodworms (midge larvae), blackworms (Lumbriculus spp. oligochaetes), mosquito larvae, goldfish, swordtails (Xiphophorus hellerii), and guppies and mollies (Poecilia spp.). Many terrestrial invertebrates can also be easily cultured and fed to fish, e.g. crickets, earthworms, mealworms, grindal worms, and vinegar eel nematodes (Panagrellus redivivus (Turbatrix aceti)).
With live foods, consideration must be given to animal acquisition, transport, housing, and feeding. Live foods should always be housed and fed in a manner that meets their behavioral and physiological needs and practices should be sustainable and minimize stress to the feeder animals. How live prey are fed affects their health and their nutritional value to the end consumer. Without good feeding practices, many of these live foods are nutritionally incomplete. Gut‐loading may be used to further increase their nutritional value. Gut‐loading refers to the diet provided for a specific period of time prior to feeding out, with nutrients designed to fill the GI tract as opposed to being absorbed.
Live foods may carry infectious diseases that are transmissible to other fish or invertebrates. To minimize this risk, live foods are best cultured in‐house and closely monitored for morbidity or mortality. Incoming live foods should go through a quarantine period prior to feeding out. This quarantine period may include necropsy examinations to look for potential pathogens.
Further information on husbandry requirements of feeder animals can be found in Schaeffer et al. (1992), NRC (1996), and Spencer and Spencer (2006).
Vitamin and Mineral Supplementation
Most wild fish consume a varied diet that changes with location, season, and life stage. Variation of items fed to managed fish is recommended to minimize the risk of deficiencies. Additional vitamin and/or mineral supplementation may be used to compensate for potential dietary deficiencies and loss of nutrients during food storage and thawing and from leaching once in water. Many commercial supplements are available for fish, with combinations of vitamins, minerals, and fatty acids. The delivery methods range from liquids added to the water or used to soak foods, to capsules or tablets that are added to the food.
Liquid supplements can leach away before consumption, be removed by filtration, and/or negatively impact water quality. It is also hard to control the supplement dose. The efficacy of this type of delivery system for supplements is largely untested. In general, capsules or tablets are more reliable than liquid supplements. Elasmobranchs are routinely supplemented with a multivitamin and mineral tablet within their seafood, and several commercial multivitamins are available, e.g. Mazuri® Shark and Ray II tabs, Sea Tabs® for Birds, Turtles, Fish and Sharks, Vetafarm® Elasmo Tabs, International Zoo Vet Group Elasmobranch Tablets (Janse 2003; Hoopes 2017).
Since many essential vitamins are unstable in the presence of light, heat, oxygen and/or water (e.g. A, B1, C, and E), careful storage of supplements is important (Lešková et al. 2006). Independent nutritional testing is recommended to validate product labels and storage conditions. Regular evaluations of supplementation are suggested as animals, diets, and the information available change over time.
Feeding a portion of the total diet as high‐quality pellet, flake, or gel may help ensure delivery of vitamins and minerals, since these products are often formulated using water‐ and heat‐stable vitamin and mineral mixtures. This is particularly useful in species that are small and/or where it is difficult to deliver supplementation in tablet form.
Feeding Behavior, Amount, and Frequency
The feeding behavior of fish is complex. Fish possess several chemosensory systems which include gustation (taste), olfaction (smell), and chemical sensory and chemoreceptor cells, with the acceptance or rejection of food being primarily attributed to inputs from chemoreception (Lall and Tibbetts 2009). Hunger is the cue for feeding behavior in fish, with the primary stimulus being gut fullness and/or metabolite levels in the circulatory system (Jobling and Wandsvik 1983). Numerous abiotic and biotic factors affect food intake in fish including how the diet is presented, diet composition, hormonal and biological state, environmental conditions, fish health and developmental state, and system design (Lall and Tibbetts 2009).
It is generally accepted that fish feed to satisfy their energy requirements. Fish fed a diet low in energy content are forced to increase their consumption rate and gastric evacuation rate to compensate for a low‐calorie diet. Correct food size is also important. Food items should be sufficiently small to be physically ingested, while large enough to be consumed without expending too much energy in the process (Lall and Tibbetts 2009). In European eels (Anguilla anguilla), optimal particle size was ~40–60% of mouth width (Knights 1983). Trout showed a similar optimal particle size, although particles up to 100% of mouth width could be consumed, particularly when the particle was smooth (Knights 1983).
Determining the optimal feed amount