Effects of feed composition on life history developments in feed intake, metabolism, growth and body composition of European eel, Anguilla anguilla

To examine the effect of feed composition on changes in feed intake and subsequent feed utilization with age, five populations of European eel, with an average initial body weight of 5 g each fed a different diet, were monitored for 302 d. The five feeds differed in their content of crude protein (33-63% DM), crude fat (6-28% DM) and calculated carbohydrates (NFE; 15-42% DM) such that five levels of digestible protein/digestible energy (DP/DE) were realised: 13, 16, 21, 28 and 29 g MJ- 1. At three points in time, with three size groups, nitrogen and energy balance studies were conducted in which next to feed intake and growth also digestibilities of dry matter, protein, fat, NFE and energy as well as O2 consumption and NH4-N excretion were measured. Due to the distinct life history of the semelparous, in the present study predominantly male eel, a well-defined goal in terms of mature size and composition could be inferred, presumably to maximize their lifetime reproductive output. In order to reach this goal the animal needs to survive and to grow and voluntary feed intake of the eels could be adequately described with the feed intake model 'eating to requirements subject to constraints', where voluntary feed intake is considered to originate from a requirement for maintenance (survival) and a requirement for growth. Live weight gain is almost completely based on protein deposition (PD) and eels, like other animals, strive to reach a genetically determined growth potential (PDmax) thought to be driven by the difference from the mature protein mass (Ptmax). Body lipid content increases with size and varied with diet from a minimum of 25% at high DP/DE ratios to a maximum of 33% at low DP/DE ratios, at body weights of 130-140 g. Preferable allocation of dietary protein to PD (protein sparing action of non-protein energy) was confirmed as marginal efficiency of protein utilization increased with decreasing DP/DE ratio from 0.29 to 0.54. Marginal energetic efficiency of PD, k(p) was 0.54 and marginal energetic efficiency of LD, k(f) varied from 0.67, indicating de novo lipid synthesis (from dietary protein) at high DP/DE ratios, to 0.93, indicating direct lipid synthesis (from dietary lipid) at low DP/DE ratios. Marginal efficiencies did not differ from those of other fish or other farm animals. Differences between fish species in feed intake and utilization of feeds differing in macronutrient composition, as well as life history developments in feed intake and feed utilization are therefore based on differences in growth rate, in turn with mature weight (Ptmax), and body composition (LD/PD ratio).