Balancing protein and energy in aquafeeds at every life stage

Protein is the costliest part of an aquafeed formula and the nutrient most easily wasted. A fish short of digestible energy does not hold protein back for growth. It deaminates the amino acids, burns them for fuel and excretes the nitrogen through the gills.

The opposite mistake costs money too. A diet with more energy than its protein can match fills the fish before it has eaten enough protein, and the surplus is stored as fat. The right balance moves with species, life stage and the energy sources in the formula.

Protein, energy and what goes wrong

SRAC's overview of fish nutrition calls protein the most expensive dietary component and notes that excess protein increases nitrogenous waste. Carnivorous fish in particular use protein efficiently for energy, because the ammonia from deaminated protein leaves across the gills at little energy cost. That efficiency is the problem: protein burned for maintenance was paid for but never became fillet.

The fix is protein sparing: supply enough non-protein energy from lipid and, where the species can use it, digestible carbohydrate, so amino acids go to tissue synthesis. Lipid is the dense option, as SRAC gives average gross energy values of 5.65 kcal/g for protein, 4.15 for carbohydrate and 9.45 for lipid, roughly 23.6, 17.4 and 39.5 MJ/kg. The FAO profile for Atlantic salmon notes that raising dietary fat, up to 24%, improves the efficiency of protein utilization.

The energy side fails in two directions:

• Too little energy. FAO's training manual on fish and shrimp nutrition notes that low dietary energy density makes the animal use nutrients for energy rather than for growth, and burning protein adds to the nitrogen excreted.
• Too much energy. The same manual warns that excess dietary energy may reduce nutrient intake or cause excessive fat deposition. Fish eat mainly to meet their energy needs, so an energy-rich diet can satisfy them before they have eaten enough protein.

Formulate on digestible protein and amino acids

Crude protein on a feed tag says little about what the fish can use, since two feeds at the same figure can differ in digestibility and in the energy behind the protein. The working target is digestible protein per unit of digestible energy, written as DP:DE in g/MJ.

The FAO profile for Atlantic salmon gives DP:DE ratios for maximum growth on practical diets of about 23 g/MJ for fingerlings, 20 g/MJ for smolts, 19 g/MJ for 0.2 to 2.5 kg growers and 16 to 17 g/MJ for 2.5 to 4 kg fish. For trout on high-energy diets, SRAC suggests about 20 g/MJ. A ratio well above that may mean excess or poorly digestible protein, and one well below it may mean excess fat that affects flesh quality and dress-out.

Amino acid balance matters more than the crude protein figure. Fish need the 10 indispensable amino acids rather than protein as such, and SRAC notes that a deficiency of any one can limit protein synthesis. The FAO salmon profile names lysine, methionine and arginine (or threonine) as the most limiting when fishmeal is reduced and plant proteins increase, and warns that imbalances such as arginine against lysine can cause deficiency.

Reference points for lysine and methionine:

• Nile tilapia: FAO lists lysine at 5.12% and methionine at 2.68% of protein, with cystine present; FeedOptima's typical ranges are lysine 1.4 to 1.6% and methionine plus cystine 0.9 to 1.0% of diet.
• European sea bass: FeedOptima's typical reference values are at least 4.8% lysine and 2.3% methionine in dietary protein.
• Common carp: FeedOptima's typical values for juvenile carp at about 38% crude protein are lysine 2.2% and methionine plus cystine 1.0% of diet.

A formula can meet its crude protein specification and still fall short on lysine or methionine, so check both on a digestible basis whenever fishmeal comes out.

How requirements shift with life stage

Small fish need the most protein relative to energy, and in salmon the balance moves toward lipid as they grow. The FAO profile notes that cutting dietary protein and raising lipid lowers DP:DE and improves energy use, which is how its stage targets fall with size.

Standard trout feeds make a smaller shift. SRAC's trout guide says fry and fingerlings need more protein and energy than larger fish, with feeds at about 50% protein and 15 to 20% fat, while feeds for larger fish typically carry 38 to 45% protein and 10 to 18% fat. The extra lipid comes with high-energy trout diets, at 45 to 50% protein and 18 to 24% fat.

FeedOptima's reference ranges show the salmon pattern clearly. Typical crude protein for Atlantic salmon runs from 50 to 56% in fry feeds to 34 to 40% in finisher feeds, while lipid climbs from 16 to 20% to 32 to 38%. Nile tilapia follows the protein trend, from 40 to 45% down to 28 to 32%, but lipid stays at 5 to 10% throughout. The chart plots the midpoints by stage: protein falls at every step for both species, and salmon lipid rises toward it.

Digestible energy tracks lipid. Salmon feeds typically rise from 18 to 20 MJ/kg DE for fry to 21.5 to 23.5 MJ/kg for finishers, while tilapia feeds sit at 12 to 15 MJ/kg throughout. A larger pellet usually brings a different protein and energy specification, so check the DP:DE of each diet in the sequence, and revisit it when water temperature shifts, which the FAO salmon profile lists among the factors setting the energy requirement.

Carnivores and omnivores use carbohydrate differently

SRAC notes that carnivorous species use protein and lipid efficiently but carbohydrate less well, partly because their natural diets contain little of it. Prepared feeds for carnivorous fish usually contain less than 20% soluble carbohydrate, against 25 to 45% for omnivores. For salmon, the FAO profile adds that raw starch is essentially unavailable, that cooking during extrusion improves its digestibility and that salmon regulate blood glucose poorly under an excessive carbohydrate load.

Carbohydrate is not a straight swap for lipid in a carnivore. In a rainbow trout trial published in PLOS One, fish fed to satiation ate less digestible energy when carbohydrate replaced fat as the main non-protein energy source. Growth fell sharply on the low protein diet, at about 14 g/MJ DP:DE, but was not significantly lower on the high protein diet at about 26 g/MJ. In salmonids, lipid does most of the protein sparing, and cooked starch is kept moderate, where it adds some energy and, as the FAO salmon profile notes, helps bind the pellet.

European sea bass, though carnivorous, uses starch better than the salmonid pattern suggests. In a juvenile sea bass trial published in Aquaculture, adding 25% starch let dietary protein fall from 63% to 48% with no loss of growth or of protein and energy retention. The best results came from a mix of raw and gelatinized starch; replacing all the raw starch with gelatinized starch reduced feed intake and growth.

Tilapia and carp go further. FAO's summary for Nile tilapia allows up to 40% carbohydrate and notes that carbohydrate use appears to decline in smaller fish, so fry feeds lean on protein while grow-out feeds can carry more digestible starch. FeedOptima's typical grower ranges show the result.

Species	Grower weight (g)	Crude protein (%)	Crude lipid (%)	DE (MJ/kg)
Atlantic salmon	300 to 2,500	38 to 44	28 to 35	20.5 to 23
Rainbow trout	150 to 1,000	40 to 45	20 to 30	19 to 22
European sea bass	50 to 350	44 to 48	16 to 20	18 to 20
Nile tilapia	50 to 300	30 to 34	5 to 8	12 to 14
Common carp	50 to 500	30 to 35	5 to 8	13 to 15

Salmon and trout feeds reach their energy targets mainly with lipid, while sea bass feeds carry less lipid and more protein. Tilapia and carp feeds carry little lipid, draw much of their energy from carbohydrate and, with less protein, cost less to formulate.

In practice

1. Ask your feed supplier for the digestible protein and digestible energy of each diet, not only guaranteed crude protein and fat, and calculate DP:DE in g/MJ.
2. Compare each diet in your feeding sequence with a stage target; for Atlantic salmon, FAO's values run from about 23 g/MJ for fingerlings to 16 to 17 g/MJ for 2.5 to 4 kg fish.
3. Check digestible lysine and methionine plus cystine whenever protein sources change, against the species requirement as a percentage of protein.
4. For salmonids, supply non-protein energy mainly as lipid and keep soluble carbohydrate below about 20% of the diet, as cooked starch.
5. For sea bass, tilapia and carp, use digestible starch to spare protein once the protein requirement is met, and keep tilapia fry feeds higher in protein.
6. If fillet fat rises or dress-out falls, check whether the finisher diet's DP:DE has drifted below the stage target.
7. After moving to a more energy-dense diet, track intake and growth together, since excess energy can cut protein intake.

To check your own diets against FeedOptima's stage ranges, run an optimization with the free analysis.