The protein requirement for haddock fed isocaloric diets is reported to be 50% (as-fed basis). Preliminary results indicate that optimum growth and feed utilisation is achieved with diets containing high levels of dietary protein (55%) and relatively low levels of dietary lipid (10%). Unlike salmonids, the added cost of higher dietary lipid levels is not necessary as it only leads to increased liver growth (Tibbetts et al. 2001).
In summary tests in feeding fry, fingerling, and yearling fish have shown that gross protein requirements are highest in initial feeding fry and that they decrease as fish size increases. To grow at the maximum rate, fry must have a diet in which nearly half of the digestible ingredients consist of balanced protein; at 6-8 weeks this requirement is decreased to about 40% of the diet for salmon and trout and to about 35% of the diet for yearling salmonids raised at standard environmental temperature. Given the high concentration of protein required in fish feeds, there is considerable dependency on fishmeal as a protein source and there are issues about substituting this, especially in diets for carnivorous fish.