Protein levels in aquaculture feeds generally average
18-20% for marine shrimp, 28-32% for catfish, 32-38% for
tilapia, 38-42% for hybrid striped bass. Protein requirements
usually are lower for herbivorous fish (plant eating)
and omnivorous fish (plant-animal eaters) than they are for
carnivorous (flesh-eating) fish, and are higher for fish
reared in high density (recirculating aquaculture) than low
density (pond aquaculture) systems.
Protein requirements generally are higher for smaller fish.
As fish grow larger, their protein requirements usually
decrease. Protein requirements also vary with rearing environment,
water temperature and water quality, as well as
the genetic composition and feeding rates of the fish.
Protein is used for fish growth if adequate levels of fats and
carbohydrates are present in the diet. If not, protein may
be used for energy and life support rather than growth.
Proteins are composed of carbon (50%), nitrogen (16%),
oxygen (21.5%), and hydrogen (6.5%). Fish are capable of
using a high protein diet, but as much as 65% of the protein
may be lost to the environment. Most nitrogen is excreted
as ammonia (NH3) by the gills of fish, and only 10% is lost
as solid wastes. Accelerated eutrophication (nutrient
enrichment) of surface waters due to excess nitrogen from
fish farm effluents is a major water quality concern of fish
farmers. Effective feeding and waste management practices
are essential to protect downstream water quality.