7.7 Feeding Practices

From research and production experience, most species in the production for food harvest have improved operational efficiency through better nutrition and feeding practices. Certain practices have been established for species, production systems, or life stages. Taking channel catfish (Ictalurus punctatus) as an example:

• CP @ 52% in fry. Dropping to 40% over 20g
• Fish meal is half of the fry diet and reduced with fish growth by poultry meal
• Digestible protein (DP) > 75% w/ > 5.1% lysine
• Antioxidants & fungistatic to retain palatability
• Complete minerals and vitamin premix added

When the yolk sac is absorbed, the new fish arrive at a most critical nutrition step, the first feeding stage (often termed the “swim-up stage”). First, feeding protocols must be taken to afford as much larval and fry survival as possible.

Many production facilities calculate daily feeding rates based on a percentage of biomass. Daily feeding rates are calculated on a sample of fish that are weighed and averaged across the population. The percentage is determined by the amount of feed consumed to produce satiation (enough feed to calm appetite) in the group being fed. The weight of feed to reach satiation is a percentage of the total biomass of the fish. The rate of feeding can be adjusted for growth. Outdoor ponds that use floating feeds for channel catfish may place floating barriers around a designated feeding area to prevent feed from blowing away from the feeding area in the wind.

The fry mature into fingerlings. During the time from fry to fingerlings, the feed/biomass ratio can be reduced to around 3% body weight divided into two feedings/day. Pond production of Channel catfish(CCF) fry may need 35-40 pounds of feed per surface acre, depending on stocking density and size.

The paradox of fish raising is simple: TOO MUCH FEED = Poor water quality; TOO LITTLE FEED = Poor growth/survival.

Winter & Fish

Fish do not grow well in lower temperatures. Warmwater fish are particularly sensitive to lower temperatures. Tilapia, for example, cannot survive below mid-fifties Fahrenheit. Most salmonids tolerate winter temperatures much lower but still reach a dormant level of performance in feed utilization. This dormancy is apparent in net pen rearing of Atlantic salmon in places like Maine or New Brunswick, where the winter ocean water may drop to only a few degrees above freezing. The water does not freeze because of the salinity, but the enzyme activities and appetite will drop drastically. Inappetence is a sign of temperature decline to a critical level. Survival is possible, but decreased temperature slows immune response to pathogens. Fungi are common problems in low-temperature situations.

Pelletization or Extrusion of the Diet

The feed can be half the cost of producing the fish harvest in fish-raising operations. Formulated feed is the most cost-effective means to provide a balanced diet. Concentrated nutrition into pellets was a large step toward efficiency in fish farming. The feeds are manufactured for many life stages using a pellet mill or extrusion. Choosing the process is based on the buoyancy needed for the pellets. Buoyancy alterations are done to better match a species’ feeding preference. The tilapia, catfish, and carp are examples of species with surface-feeding characteristics that are fed floating diets. Species that take better to slow-sinking pellets would be salmon and trout. Sinking feed would be prepared for shrimp and cod.

The dry mixed feed ingredients are compressed through a pelleting machine. The compression uses pressure and some moisture, and temperature to create a formed pellet. Pelletized feeds sink because of the density. None of the starch ingredients are gelatinized in a simple pelleting process to affect the density and buoyancy.

Extrusion processes are used to affect the buoyancy of formulated feeds. Besides some formulation changes, the extrusion process uses higher temperature and moisture through an extruder that controls the speed and profile. Extruded feed is done with higher moisture, pressure, and temperature to “cook” the ingredients. Extrusion parameters are controlled to determine the degree of expansion. Feed integrity increases since the gelatinized starches act as glue on the ingredients. Proteins and fibers contribute less than starch to the binding. The level of starch is the key to managing flotation. The extrusion process permits feed with high energy from high dietary fat levels of 20-40%. Heat extrusion increases protein digestibility and inactivates antinutritional components, as discussed below. The heating also increases the digestibility of many nutrients, such as proteins and carbohydrates.

When starch is added to the extruded formulation, it is gelatinized to increase feed stability and allows more lipids and fats to be added to the diet. Feeding management further controls leaching by feeding on a percentage of body weight and temperature. ^[1] The downside of heating for extrusion is the inactivation of some vitamins. Therefore vitamins are added in excess before extrusion or afterward as premixes.

Anti-nutrients

Care is taken in choosing ingredients that may contain what is called “anti-nutrients.” For example, soybean meal has a trypsin inhibitor that can block the digestion of proteins unless it is inactivated with heat. Soybean Hemagglutination Agents (HA) are a real problem for species without true stomachs to inactivate it with gastric acid. For example, the HA clumps the RBCs in carp, gobies, and blennies.

Cottonseed meal carries Gossypol, which is highly toxic to fish and tastes bad to fish.

Canola meal contains glucosinolates that interfere with thyroid function leading to goitrous conditions and tastes bad to fish. While discussing the topic of goiter, a related aspect of captive rearing is the thought that nitrate appears to cause goiter in captive sharks and other elasmobranchs. Ozone also may be in play since the ozonation in saltwater tanks can reduce iodine availability. This iodine decline is an example of how many facets need to be considered when dealing with some of the challenges that may or may not be nutritional.

Feeding Behaviors and Bad Tastes

Fish find a feed with their senses of olfaction and sight. Fish have a sensitive sense of taste. They will swallow feed based on taste and texture. Taste buds are not always or solely in the oral and pharyngeal cavities. Sensory organs may be found on the skin in some species. Most fish have yet to have their senses completely explored and described. Certain conditions appear to create bad tastes to fish. Rancid fats resulting from the oxidation of unsaturated fat are a particular problem. Putrid proteins, i.e., trimethylamines, develop from inadequate or prolonged storage. The stale feed seems easy for many fish species to identify, depending on species and age. Molds seem to be bad tasting to every species of fish. A pronounced sensitivity resulting in a rapid decline of the feed intake at early or low feed levels has become bad tasting.