(gobies) (Day et al. 2011). This loss does not appear to impose dietary constraints, as stomachless fish cover the entire trophic spectrum, including carnivory, omnivory, herbivory, and detritivory (Horn et al. 2006; German et al. 2010). Intestinal length is variable in fish, but carnivorous fish generally have shorter intestines than herbivorous species. Many fish possess pyloric caeca. These are finger‐like diverticula of the intestine at the posterior end of the stomach which secrete digestive enzymes and increase the absorptive surface area. Pyloric caeca are better developed in carnivorous than herbivorous fish (Clements and Raubenheiner 2006; Ballantyne 2014). Elasmobranchs and non‐teleost ray‐finned fish (bowfin, gar, sturgeon, and lungfish) have a spiral valve. This is a distal section of intestines with multiple layers in conical, scroll, or ring shapes, which increase the absorptive surface area.
Gut passage rates in fish are dependent on species, animal size, gut morphology, temperature, food type, meal size, and feeding rate. In general, gut emptying times range from <10 hours for most herbivores to 10–158 hours for teleost carnivores (Horn 1989). Elasmobranchs have slower gut passage times than similarly sized teleost carnivores at comparable water temperatures, with reported values of one to eight days (Cortés and Gruber 1992; Wood et al. 2007). Gut passage times in carnivorous teleosts and elasmobranchs are affected by the type of prey. For example, gastric emptying in sandbar sharks (Carcharhinus plumbeus) varied by as much as 20 hours between crab and teleost prey (Medved 1985).
Assimilation efficiency assesses how efficiently animals convert ingested food into energy. Assimilation efficiency is high in adult fish (80–90%), with a wider range in larval fish (67–99%) (Govoni et al. 1986). Food type, ration size, and gut morphology likely play a role in some of these differences. For example, Atlantic herring larvae (Clupea harengus) fed high rations of brine shrimp nauplii (Artemia spp.) defecated live nauplii, suggesting that large rations may result in low assimilation efficiency, especially in larvae with straight alimentary canals (Werner and Blaxter 1980). Assimilation efficiency in elasmobranchs has been studied in a single species (lemon shark, Negaprion brevirostris) and was comparable to that measured in teleosts (73%) (Wetherbee and Gruber 1993).
As in other vertebrates, the GI tract of fish provides habitat for a diverse ecosystem of micro‐organisms that play an important role in the health and nutrition of the host. Information on gut microflora and microbial enzyme activity in fish GI tracts is lacking. It is well‐known that terrestrial vertebrate herbivores have populations of symbiotic micro‐organisms that play a key role in breaking down plant fiber (cellulose and hemicellulose) into short‐chain fatty acids (SCFAs, primarily acetate, butyrate, and propionate). Gut micro‐organism diversity has been studied in some freshwater and marine herbivorous fish. Work by Mountfort et al. (2002) identified the contribution of gut microbiota to energy metabolism and fermentation in the hindgut of three species of temperate marine herbivorous fish, finding that microbial fermentation is an important source of energy in these fish. Thus, establishing and maintaining appropriate GI tract microbial populations is integral to maintaining appropriate nutrition.
Nutrient Requirements
Despite great diversity in food types, all fish require the same nutrients at a cellular level. Understanding the proper nutrient requirements for fish is complicated by the fact that fish can accumulate some nutrients (e.g. minerals) directly from the water. Nutrients accumulated from the water and diet can have different physiological effects and fates, which makes balancing nutrient intake and availability a challenge.
Sources of Energy
Energy is obtained from the metabolism of protein, lipid, and carbohydrates. Fish are more efficient at using protein as an energy source than birds or mammals, since ammonia synthesis does not require energy, unlike uric acid and urea (Wright and Land 1998). However, protein is very costly and the resulting ammonia excretion can be problematic for water quality. Carbohydrates may serve as an energy source either via endogenous metabolism (e.g. absorbed glucose) or via microbial fermentation producing volatile fatty acids, but the ability to use carbohydrates varies greatly by species. Carbohydrates are generally less preferred as an energy source for carnivores compared to omnivores and herbivores, and in marine fish compared to freshwater fish. Lipids are often a preferred energy source in commercial fish feeds, as they can spare the cost of protein and are used efficiently by fish. Efforts in commercial aquaculture to use lipids as an energy source and spare protein for muscle accretion have resulted in significant improvements in growth efficiency of aquaculture species (Hixson 2014). However, there is a limit to this effect; excessive energy relative to protein can reduce total feed intake (and so limit intake of essential nutrients) and can increase body lipid deposition, which may not be optimal for fish health.
The energy of a food item may be defined based on different types of measurement. Gross energy (GE) is the total energy released upon combustion of a food item and does not necessarily represent the amount of energy available to the animal. However, this is the easiest measure of energy to determine. On average, GE values for carbohydrate, protein, and lipids for fish are 4.11, 5.64, and 9.44 kcal/g, respectively (NRC 2011). Digestible energy (DE) is based on the amount of energy in a food minus the amount of energy excreted in feces after that food is consumed and is generally 10–30% less than GE (NRC 2011). Metabolizable energy (ME) is based on the digestible energy of a food minus the energy to excrete any waste products via urinary and gill losses and generally reflects another 3–6% loss of energy (NRC 2011). When evaluating the energy needs or status of an animal, it is critical that the same units are considered. For example, if the requirements are predicted in units of ME, then the diet items should be considered in similar units or adjusted based on the assumptions described above.
Protein
There are 20 amino acids. Ten are essential and cannot be produced by fish. The remainder can be synthesized by the animal, assuming there is enough dietary nitrogen. Thus, dietary sources must provide both essential amino acids and additional nitrogen. The level of dietary protein that is required by fish has been well‐documented for fast‐growing aquaculture species and ranges from 29 to 42% digestible protein (Table A4.1). This requirement has not been well‐documented for other species or other life stages. However, for larval fish, free amino acids may be more important as some lack the enzymatic capability to digest intact protein (Rønnestad and Fyhn 1993).
Essential amino acids for fish include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (NRC 2011). Arginine requirements are quite high compared to terrestrial mammals or birds, due to low activity of urea cycle enzymes. Tyrosine and cysteine are considered semi‐essential because they can be synthesized from phenylalanine and methionine, respectively. Taurine, an aminosulfonic acid, appears to be essential for some life stages, notably larval fish (NRC 2011). The relative ratios of amino acids that have been determined to be required by fish are generally proportional to the optimal dietary protein level. However, it is important to note that amino acid requirements have historically been determined with body weight gain as the predictive variable. The use of other functional end points (e.g. optimal immune function) may change the requirement. For example, recent work examining histidine requirements in Nile tilapia (Oreochromis niloticus) found that lowering histidine below previously published requirements improved weight gain, muscle fiber quality, and efficiency of protein utilization (Michelato et al. 2017). This type of refinement of nutrient requirements will continue to improve fish nutrition as well as the impact of excessive feeding on water quality and the environment.
Fish‐based protein sources most closely match the nutritional requirements of fish. However, with sustainability concerns, efforts have been made to identify other protein sources for commercial fish diets, such as marine by‐catch, terrestrial animals, plants, and microbes. Terrestrial animal protein sources lack some essential fatty acids and may be high in fat. Plant‐based protein sources may be less expensive but often lack many essential amino acids and may contain anti‐nutritional factors (discussed below).
As a final caveat, it is important