Background
Today’s fish feeds are produced from a large variety of feed ingredients. These have different properties in feed production, resulting in considerable variation in the physical quality of commercial feeds. Feed represents more than 50% of the costs in Norwegian fish farming, and the total yearly cost of feed is approximately 7 billion NOK. Thus, even small fractions lost or poorly utilised will amount to large sums.

The feed is commonly transported from a storage silo to the sea pens using a pneumatic conveying system, where the feed pellets are carried through pipes by an air stream. The collisions between pellets and pipe walls cause pellet breakage and formation of small particles, which represent loss, and the degree of breakage depends on the physical pellet quality. Consequently, high physical quality of the feed is demanded. However, the physical properties of the feed may influence the nutritional value of the feed. Suboptimal feed intake or feed utilisation resulting in reduced growth of the fish is difficult to quantify, but may cause considerable economic loss for the fish farmer.

Two trials were performed 2008. In the first trial, breakage during feed conveying was investigated, whereas in the second trial, the effect of physical feed quality on nutritional values was investigated.

Methods
In the first experiment, three different 12 mm commercial salmon feeds (denoted A, B and C) were tested in a pneumatic conveying system (AkvaMarina CCS Feed System, Akvasmart, AKVA group, Bryne, Norway), using different airspeeds (25, 30 and 35 ms-1) and feeding rates (9, 18 and 36 kg min-1). Samples of 2 kg were run through the system, sieved with different screen sizes, and each fraction weighed to quantify the formation of small particles. The physical quality of the feeds was measured with several different tests. The aim was to investigate the effect of air speed and feeding rate on degradation of three extruded fish feeds, and, if possible, to find a value for physical pellet quality that can serve as a steering parameter to adjust the settings of the conveying system.

In the second experiment, the aim was to compare the effect of different physical feed qualities on the nutritional value. Two feeds were fed to rainbow trout kept at constant or fluctuating environment (temperature, salinity and oxygen saturation). One feed had high water stability, whereas the other disintegrated rapidly in water (Figure 1). The nutrient digestibility was estimated by analysing feed and faeces collected from an experiment previously performed by Nofima in collaboration with BioMar.

Figure 1. After 4 hr shaking in water, the pellets with high water stability (left) were almost intact, whereas the pellets with low water stability (right) were highly disintegrated.

Results and discussion
Feed quality was the main factor in pellet degradation in the conveying system in the first experiment. From the feed qualities used in this trial, no single parameter was found that could predict the pellet degradation pattern, and thus be used to optimise the settings of the system. However, in general, high air speed increased the formation of small feed particles, whereas high feeding rate protected the pellets from breakage. As become evident in this trial, the relation between feed ingredients, extruder parameters, pellet breakage during transportation and physical quality parameters of the feeds is poorly understood, and more research is required in this area.

The results already available for the second experiment showed that, briefly, the feed intake was highest in trout fed the diet with low water stability, and when kept at constant environment. The growth tended to follow the same pattern, although the experiment was too short to produce significant differences in growth. Separated water and oil were also found in the stomachs of trout with the highest feed intake, which could potentially lead to fat belching. The apparent digestibility (%) of nutrients that was examined in the present project was in general highest in the diet with high water stability. This may be related to the feed intake, being lowest in fish fed this feed, and maybe having the longest transit time and thus being most effectively absorbed from the intestine. These results clearly showed that the physical properties of the feed affect its nutritional value, and that more attention should be paid to this relation, both in research and in commercial fish farming.