Know Your Fish Feed

Each potential ingredient in your fish’s feed is unique.  Each source of protein, fat, or carbohydrate has specific characteristics that differentiate it from other proteins, fats, or carbohydrates.  You may think that feeds with similar proximate analyses are essentially equal, but the fish may disagree.  Feeds with the same proximate analyses can be quite different in terms of the growth and other measures of performance shown by the fish consuming the feeds.  If the specific ingredients and the methods of manufacturing result in a feed that is not palatable, the fish may refuse to eat it.  If they eat it, but cannot digest it, and/or assimilate it, there will be problems with growth, as well as potential health problems.  You need specific information.

Conducting scientifically valid research on fish nutrition poses additional sets of problems if the specific ingredients are not known.  Even if the basic ingredients are known, research results may not be replicable if the digestibility of each ingredient and the way it was processed during manufacturing is not known.  It is essentially impossible to conduct meaningful, replicable, scientifically valid research using closed formula feeds (specific ingredients not identified).  Major feed mills typically conduct their own research on their feeds and feed ingredients, but consider the results to be proprietary information.  Smaller mills and nutrition researchers need that information, but typically lack access to it.  These are the basic reasons that scientists from the U.S. Department of Agriculture (USDA) Agriculture Research Service (ARS) and the Department of the Interior (DOI) Fish and Wildlife Service (USFWS) have developed a Nutrient Digestibility Database.

The National Research Council established the Committee on Nutrient Requirements of Fish back in the early 1970s.  The 1^st edition of Nutrient Requirements of Fish was published in 1973.  The Committee’s scope of responsibilities was expanded later to include shrimp.  Although this series has provided comprehensive summaries of current knowledge about nutrient requirements of fish and shrimp, and supporting nutritional science, these reports have not included the ingredient digestibility data provided in the new Nutrient Digestibility Database.  Given the fact that aquaculture now provides more than half of the total human seafood consumption, it is essential that aquaculture production be as efficient as possible, nutritionally, as well as, economically. 

Apparent Digestibility Coefficients (ADCs), identify the percentage of nutrients in each feed ingredient that is actually available to the fish.  The ADC’s vary for each ingredient and each species of fish.  Accurate information on ADC’s is needed by researchers, producers, and feed mills to formulate feeds that provide the fish with enough essential nutrients, but not too much.  Excess nutrients would be of no value to the fish, would increase the cost of feed, and cause potential effluent problems.  ADC’s are also necessary for determining the nutritional and economic value of alternative ingredients.

Nutrition data from many different laboratories have been compiled in publications such as the NRC Nutrient Requirements of Fish and Shrimp, but often show extreme variability.  There has been no standard set of methods for determining ADCs.  Peer reviewed literature is scarce, at best. Such results as are available are often not comparable because of the lack of standard methods.  Results have been influenced by such factors as variability in basal diet formulation, method of feed manufacturing (e.g. cooking versus cold formation), fecal collection method, and various other factors.  Each laboratory has used different combinations of methods to determine their set of ADCs.  As a result, the data could not be compared accurately.

The digestibility project was initiated by USDA-ARS, with collaboration by the USFWS, in 2005 to determine ADC’s for an array of traditional feed ingredients and selected novel ingredients.  ADC data on macro-nutrients, amino acids, and minerals are available for rainbow trout and hybrid striped bass only at this time.  Standardized conditions were used to the extent practical for each species (e.g. optimal temperatures had to be adjusted for each species).  Feeds were manufactured using standard commercial processes.  The standardized methods and conditions allow for comparisons among ingredients and over time.

Another aspect of the study involved testing non-traditional, or novel, ingredients.  Traditional feeds for trout and other farmed fish have been based on an assumption, partially supported by experience, that captive fish require feeds containing fish meal or animal products.  However, studies conducted in this digestibility project, and in others, have shown that fish require specific nutrients, not specific ingredients.  Identification of the specific nutrients that fish require, plus information about the presence and availability of these nutrients in plant products allows feed manufacturers to supplement the plant products with the specific nutrients that each fish requires.  As demands for seafood increase, the demand will have to be met by greater aquaculture production.  Traditional feeds based on “lower value” fish and fish products will not be sufficient to meet the demand.  Efficient, nutritionally balanced feeds will be based increasingly on non-traditional, but sustainable ingredients.

The results from the Nutrient Digestibility studies are now available online.  Researchers, small feed mills, and producers can obtain information on the nutrient content and availability for a wide variety of ingredients that can be used in fish feeds.  The information is provided in the form of Excel spreadsheets, which can be downloaded and/or printed off.  The content of macro-nutrients, amino acids, and minerals in each ingredient is provided under the label, Nutrient Composition. The ADC’s for each ingredient and the specific nutrients in it are labeled as “ADC – Trout”, or alternatively, “ADC -HSB” for hybrid striped bass. The anti-nutrient composition of selected ingredients is also provided under the label “Anti-nutrients”.

The database is a work in progress and will be updated as additional ingredients are evaluated.  Although the results have not yet been published in peer-reviewed journals, the methods used are described in the database and can be evaluated by those using it.  While the database is not all-inclusive, listing every nutrient for all potential ingredients, it does provide standardized data for a broad range of potential ingredients.  Readers can examine the database and obtain more detailed information on the data by going to the following site: www.ars.usda.gov/Main/docs.htm?docid=21905

The contents are organized under the following headings:  Project Description, Procedures, Reference Diet Formulations, Nutrient Composition of Ingredients, Apparent Digestibility Coefficients (ADC’s) Trout, ADC’s Hybrid Striped Bass, Total and Digestible Nutrients Trout, Total and Digestible Nutrients Hybrid Striped Bass, ADC’s grouped by type of Ingredient for Trout, Anti-nutrients for some ingredients, Acknowledgements, and Contacts for further information.

— John G. Nickum

Readers with specific questions can obtain additional information and guidance on the use of the Nutrient Digestibility Database by contacting Rick Barrows, Research Physiologist (Fish), Lead Scientist and Nutritionist at Rick.Barrows@ars.usda.gov and Mike Bonman, Supervisory Research Plant Pathologist, Research Leader, Plant Pathologist at Mike.Bonman@ars.usda.gov.