a food item is consumed or rejected.
Certain compounds in poor‐quality feeds are known deterrents, including highly oxidized oils, trimethylamine (produced in decaying fish flesh), and aflatoxins (produced by molds). Improper storage leading to rancidity or mold growth depresses feed intake and can result in death if consumed in significant amounts (Lall and Tibbetts 2009).
Amino acids appear to be important feeding stimulants for fish. In omnivorous fish, betaine, glycine, alanine, and mixtures of L‐amino acids are important feeding stimulants (Lall and Tibbetts 2009). In the herbivorous redbelly tilapia (Tilapia zillii), glutamic acid, aspartic acid, serine, and lysine are major feeding stimulants (Johnsen and Adams 1986; Adams et al. 1988). However, adding these stimulatory materials to the food has had limited practical success (Adams et al. 1988). The best practice to stimulate fish to feed is to deliver food types similar to the wild diet. For example, the feeding stimulants identified for redbelly tilapia are abundant in romaine lettuce.
Various supplements and medications have been used to encourage feeding within the hobbyist, zoo, and aquarium communities. These have included vitamin B/C/E, garlic, diazepam, mirtazapine, megestrol acetate, low‐dose dexamethasone, dronabinol, capromorelin, and levothyroxine. However, the supporting literature is scarce and anecdotal reports have shown that these supplements are largely ineffective. Of these, garlic shows the most potential, although high doses can lead to oxidative damage (Ashdown and Violetta 2004; Lee and Gao 2012).
Assisted Feeding
Assisted feeding includes tube feeding (gavage) and force feeding. Deciding when to assist feed requires an understanding of the normal feeding periodicity of the species and the health, metabolic state, and history of the individual. Some species of fish exhibit normal periods of inappetence ranging from two to six months (e.g. moray eels). More metabolically active species require frequent feeds and may develop pathology without any visual decrease in body condition.
Tube Feeding (Gavage)
The prior diet of a fish (e.g. seafood, pellets, soaked flake) can often be made into a slurry or gruel for tube feeding. It is essential that gruel be well‐blended and smooth, so heads, spines, fins, skin, and/or shells usually need to be removed prior to using a blender. Commercial tube‐feeding formulas may also be useful (e.g. Oxbow® Carnivore Care, Mazuri® Shark/Ray gel). The gruel or formula, or a mixture, should be blended with water or an electrolyte solution so that it is just thin enough to pass through a syringe and appropriately sized tube. It should not be any more dilute, as this will reduce the caloric concentration. A good starting point is a 60:40 ratio of food to water. This ratio can be adjusted based on the resulting gruel consistency and thickness. The volume for tube feeding will often start at 0.5–1% of fish body weight and increase to 2–3% as necessary.
Tube feeding can be performed under manual or chemical restraint. For small fish, red rubber tubes, avian tracheal tubes, or IV catheters can be used. Luer‐lock syringes are useful as they hold the tubing more securely than Luer‐slip syringes. For large fish (e.g. >10 kg), small or large animal gastric tubes are useful. The tubing should be cut to an appropriate length by approximating the length from the mouth to the middle of the pectoral fins (the approximate location of many fish stomachs). Beveling the leading edge of the tube and filing any rough edges may reduce the risk of trauma.
It is usually best to fill the syringe immediately before feeding to reduce the risk of clogging. Air pockets should be removed prior to feeding. Most fish are easiest to gavage in ventral recumbency, while small elasmobranchs are often easiest to gavage in dorsal recumbency and large sharks may be easiest in slight lateral recumbency. If a gag is needed, PVC tubes and syringe cases work well. The tube should be gently inserted through the mouth along the midline into the esophagus. If the tube is not centered correctly, it will emerge from one of the gill openings or the spiracle. If this happens, the tube should be retracted and replaced along the midline. It is normal to feel the pharyngeal teeth when placing a gastric tube in a bony fish. Once the resistance changes or the predetermined length has been reached, gentle pressure can be used to deliver the mixture into the stomach. Once the appropriate volume has been provided, or if any regurgitation is seen, the tube should be removed. Fish should be monitored for any subsequent regurgitation as this is common. If the syringe or tubing becomes clogged, it is usually best to remove the gavage tube and try again with a new setup. Serious problems such as gastrointestinal rupture or accidental administration into the swim bladder in physostomous fish are rare but can occur, so suitable restraint and gentle pressure are important.
When tube feeding is needed, it often needs to be repeated regularly to keep an individual on a positive nutritional plane. The frequency will depend upon the condition of the fish, its sensitivity to handing stress, practicalities of regular handling, and the animal's natural feeding periodicity. Gastrostomy tubes could be used to reduce handling requirements in some species, with one report in a green moray eel (Gymnothorax funebris) (Kizer 2005). Food should continue to be offered to the fish during tube feeding to encourage free feeding.
Force Feeding
For medium to large fish, whole or cut seafood may be force fed. For example, in rockfish (Sebastes spp.) under chemical restraint, the fish can be held in a vertical position and food gently placed into the cranial esophagus using forceps. In sharks and batoids, an appropriate‐diameter PVC tube may be placed in the mouth and prey may be pushed through on a feeding pole or plunger. It is easier to force feed firm dietary items, e.g. partially thawed butterfish (Stromateidae).
Larval and Broodstock Nutrition
Larval fish nutrition is a relatively new area of focus for both aquaculture and conservation efforts. Larval fish grow rapidly, with reports of 30–100% growth/day in some species, and their nutrient requirements are likely different from adult and juvenile stages (Hamre et al. 2013).
The composition of the egg from which a larval fish emerges is clearly significant for long‐term success. While egg composition varies dramatically between species, a few generalizations may be made based on the limited data available. Egg lipid levels tend to correlate positively with egg incubation times (Kaitaranta and Ackman 1981; Jaroszewska and Dabrowski 2011). Free amino acids, which are found in relative abundance in eggs, tend to be higher in eggs from pelagic fish than demersal fish (Jobling 2016). Larval fish usually depend on their yolk sac for three to five days post‐hatch, but this period may last up to 60 days in some species (Jaroszewska and Dabrowski 2011). Appropriate egg and yolk nutrition is modulated through the broodstock. Following the yolk‐sac period, larval fish transition to exogenous diet items. Mixed feeding, in which larval fish are offered food items while yolk‐sac nutrition is still available, is recommended to improve overall nutrition and stimulate gut development (Jaroszewska and Dabrowski 2011).
Options for feeding larval fish include live food items and commercial diets. Diets for small fish larvae are typically based on brine shrimp nauplii and rotifers (particularly Brachionus spp.). They may also be fed commercial microparticulate diets with hydrolyzed protein (a source of free amino acids), as the larvae may not possess the anatomy or physiology to break down more complex food particles (Kaushik et al. 2011; Langdon and Barrows 2011). However, these are associated with several issues, including high rates of nutrient loss due to large surface area, clumping, and poor uptake by larval fish (Langdon and Barrows 2011; Hamre et al. 2013). For larger larvae, live food items may include adult brine shrimp, copepods, and a variety of commercial diets. Live foods should always be enriched prior to feeding as this can significantly improve larval survival (Hamre et al. 2008; Dhert et al. 2014).
Nutrition of the broodstock impacts larval success. For example, vitamin E supplementation improved spawning quality in several fish species, and recommended levels were >150–190 mg D,L‐α‐tocopherol/kg diet (165–209 IU vitamin E/kg diet) (Fernández‐Palacios et al. 2011). Tryptophan also seems to be required at higher levels for reproduction than for growth (Jobling 2016). And in cod, egg fatty acid concentrations, particularly AA, DHA, and EPA concentrations, were positively