Do plant-based diets for Atlantic cod (Gadus morhua L.) need additions of crystalline lysine or methionine

Atlantic cod (Gadus morhua), initial weight 15 g, were fed ten experimental diets for 15 weeks. The diets were based on a mixture of plant proteins (PP) and fish meal (FM), where PP constituted 65% of dietary protein. PP mixtures were chosen to reach as low levels of lysine and methionine as possible. The diets were supplemented with increasing amounts of lysine (19.2-31.9 g kg⁻¹ diet) or methionine (9.4-12.3 g kg⁻¹ diet), in a regression design. No growth difference among diet groups was found in the plant-based diets. Increased dietary lysine resulted in decreased liver size, plasma triacylglycerol concentration (TAG) and lipid productive value (LPV). Methionine additions did not result in changed Hepatosomatic index (HSI), LPV or plasma TAG. Feed conversion ratio (FCR) and protein utilization were neither affected by lysine nor methionine. Plasma and muscle concentrations of free lysine and methionine correlated with dietary levels 5-h post feeding. Overall conclusion was that cod maintain growth rates in plant-based diets if dietary protein was high, without additional supplements of crystalline lysine or methionine. Lysine intake significantly influenced lipid metabolism, showing the necessity to add lysine in plant protein-based diets to hinder increased lipid deposition. No such effects were found because of lack of methionine additions.