Predicting the Nitrogen and Crude Protein Content of Fish from Wet Weight

(*see Ramseyer, LJ 2002 
Predicting Whole-Fish Nitrogen Content from Fish Wet Weight Using Regression Analysis, North American Journal of Aquaculture 64:195-204)

    It was recently discovered that the nitrogen (N) and crude protein content of fish can be predicted from fish weight.  A study by *Ramseyer  (2002) found a strong linear relationship between N content and body weight for log-transformed data for 68 species of fish and hybrids (see graph).   Although a strong relationship between body composition and body weight has previously been reported for a few species, this study is the first to demonstrate that this relationship exists for all species for which data was available.  This page provides an introduction to the study, and makes available a downloadable database (Excel 7.0a, 457 kb) of 2800 observations of fish N and crude protein content.  It also provides linear regression variables to facilitate the calculation of fish N content from body weight for 68 species and hybrids.  This information will be of major utility for applications where fish N or protein information is needed, including animal diet design, fish hatchery management and water quality monitoring.  

Click on a Question

1. How was the body composition-body weight relationship determined?


2. How can the study findings be put to practical use?


3. How do I calculate the N and crude protein content of whole fish from wet weight?


4. Are N and crude protein values for fish available in tabular form?


5. Do similar body composition-body weight relationships exist for other kinds of animals?


6. I have raw data.  How can I contribute to this database?

1.  How was the body composition-body weight relationship determined?

2.  How can the study findings be put to practical use?

1. Determining the nutritional value of whole fish fed to other animals.  Fish are a main food source for seals, sea lions, porpoises, certain birds and even other fish, and are an important seasonal delicacy for bears preparing for winter hibernation.  Zookeepers may use the regression equations to estimate the amount of protein fed to their fish-eating charges.  Similarly, biologists may use the regression equations to estimate the protein intake of wild animals after observing the size and species of fish eaten.

 

2. Studying the N budget of a stream.  Nitrogen is often the first limiting nutrient for algae growth in water.  The decomposing carcasses of spent migratory fish such as salmon may be an important source of N enrichment in some streams.  Since fisheries managers frequently count the number of returning spawners in streams, the contribution of spawner carcasses to the N budget of streams can be estimated using regression relationships once average spawner weight is determined.
3. Measuring or predicting N in fish hatchery effluents using Mass Balance Models. The N content of fish of various sizes is a key component in mass balance models.  Mass balance models are used to:
   1. Calculate how much N is in hatchery effluents. This is done to ensure that a hatchery is within its legal limits for waste discharge.  Although N can be measured directly in effluent samples using chemical methods, effluents must be sampled continuously to provide an accurate measurement of N loss.  Mass balance calculations may be a more economical way to estimate N in effluents.

   2. Calculate maximum fish production rate at a hatchery which is possible without exceeding N discharge limits.
   3. Locate sites for future aquaculture facilities.  Sites may be located based on predicted N concentrations in hatchery effluents and a knowledge of legal waste discharge limits.

3.  How do I calculate the N and crude protein content of whole fish from wet weight?

Slope and intercept values for 68 species of fish and hybrids are listed in Table 1. *NOTE* that calculations are only valid when the fish for which you are determining N content is within the same weight range as the group of fish originally used to derive the slope and intercept. Permissible weight ranges are listed in Table 1.

Example calculation: How much N is in a 379 g common carp?

Step 1  Check weight range
A 379 g common carp is within the permissible weight range (0.0078 - 1450 g) for common carp listed in Table 1.

Step 2  Get slope and intercept
For common carp, the slope is 1.03 and the intercept is -1.69 (Table 1).

Step 3  Calculate N content
The log of 379 is 2.5786.
Log N = (2.5786) x (1.03) - 1.69
Log N = 0.966
Antilog N = 9.247 g. Therefore, a 379 g common carp contains approximately 9.25 g N.

Step 4  Calculate protein content
Nitrogen values can be converted to crude protein using the formula: crude protein = (N x 6.25). Therefore, a 379 g common carp contains approximately (9.247 g x 6.25) = 57.8 g crude protein.

4.  Are N and protein values for fish available in tabular form?

5.  Do similar body composition-body weight relationships exist for other kinds of animals?

6.  I have data.  How can I contribute to this database?