Why are fish more efficient at converting protein than terrestrial livestock?

Fish have the ability convert a greater proportion of the protein and energy they receive from feed and turn it into human food (i.e. fillet) than what terrestrial livestock can (refer to Table below to see comparisons).

There are two reasons why fish are more efficient at converting feed into human food than terrestrial livestock:

1. Fish are able to convert feed into human food more efficiently (i.e. lower food conversion) because they expend less energy on maintaining bodily processes than terrestrial livestock:

  • Fish don’t need to maintain their internal body temperature, whereas livestock are warm-blooded like humans and must use energy to maintain their internal body temperature of 37°C;
  • Fish live in water so they expend less energy to maintain upright in the water column, whereas livestock must contend with gravity; and
  • Fish expend less energy by excreting a more simple form of urine (ammonia), whereas livestock must use more energy converting ammonia into a more concentrated form of urine (urea).

2. A high proportion of a fish can be eaten by humans compared to terrestrial livestock, e.g. 68% of an Atlantic salmon can be eaten, compared to only 38% of a lamb.

 Product yield, energy and protein retention in edible parts of Atlantic salmon, pig, chicken and lamb
  Atlantic salmon Pig Chicken Lamb
Harvest yield (%)a 86.0 72.5 65.6 46.9
Edible yield (%)b 68.3 52.1 46.1 38.2
FCRc 1.15 2.63 1.79 6.3
Energy retention (%)d 23 14 10 5
Protein retention (%)e 31 18 21 5

 a Harvest yield is yield of gutted and blend animal

b Edible yield is ratio of total body weight that is normally eaten, muscle, body adipose tissue and liver, lung, and heart for pig. Skin is excluded from all animals

c FCR = (kg feed fed)/ (kg body weight gain)

d Energy retention = (energy in edible parts)/ (gross energy fed)

e Protein retention = (kg protein in edible parts)/ (kg protein fed)

Table data from: Bjorkli, J. Protein and energy account in salmon, chicken pig and lamb. M.Sc. Thesis, Norwegian University of Life Sciences (UMB), Norway (2002).