Jon Øvrum Hansen defended his thesis on April 29th 2011

Ane Gro Siri Skjelfjord

Antarctic krill (Euphausia superba) as a feed ingredient for salmonids with focus on the shell fraction and fluoride

Abstract from the thesis - PhD thesis 2011:21

Antarctic krill (Euphausia superba) may be the largest biomass of a single species worldwide and is a promising ingredient for use in fish feeds. Krill have a balanced amino acid profile, and krill lipids are rich in phospholipids with a high concentration of n-3 polyunsaturated fatty acids. In addition, krill are highly palatable to fish. The limitations in availability of fish meal worldwide, and the increasing decline in pelagic fish populations increase fish meal prices. Use of krill in fish feed must be funded on science-based, sustainable krill harvest. The main objective of this thesis was to enhance understanding of Antarctic krill as a feed ingredient for salmonids. The sub objectives were to increase knowledge regarding: 1) nutritional effects of krill shell in fish feeds; 2) response to different dietary levels of krill meal in Atlantic salmon (Salmo salar); and 3) uptake and accumulation of fluoride in salmonids reared in fresh or salt water.

Atlantic salmon reared in salt water, fed whole krill meal (WKM) at the expense of fish meal, grew at a lower rate during the first feeding period compared to fish fed fish meal or partly deshelled krill meal (PDKM). In a second experiment, increasing levels of a PDKM:pea protein concentrate (PPC) mixture (3.5:1) in diets for Atlantic salmon showed an optimum growth rate at a PDKM/PPC level of 400 g kg-1. In both experiments there were larger differences in growth rates and/or weight gain between the krill fed fish and the fish meal control group during the early part of the experiment compared to the total feeding period.

Complete replacement of fish meal with WKM in diets for Atlantic salmon, reared in salt water, tended to reduce lipid digestibility compared to a replacement with PDKM. Increasing dietary level of a PDKM/PPC mixture, on the other hand, gave a linear increase in lipid digestibility. Salmon fed WKM or an increased level of PDKM/PPC had lower digestibility of several amino acids compared to those fed fish meal as the sole protein source. The high dietary copper level in the WKM and PDKM were excreted through the salmon faeces. There was no increase in copper accumulation in liver or whole body of salmon fed these diets.

Histological changes in the kidney where observed in salmon fed diets with WKM or PDKM. These changes were described as mild to moderate degeneration, apoptosis and/or necrosis of renal tubule cells. Similarly, feeding increasing levels of a PDKM/PPC mixture to salmon produced dose dependent histological changes in the kidney. Despite the low dietary bioavailability of fluoride in salt water reared fish, we hypothesized that the high dietary fluoride level in the krill meals caused the observed kidney changes. This was based on findings in other animals. High dietary fluoride fed to salmon reared in freshwater did, however, not induce similar kidney changes as seen in the krill fed salmon reared in salt water. Freshwater reared salmon fed diets with a fluoride level at 3,500 mg kg-1 from sodium fluoride (NaF), however, had crystalline material within distal tubules and/or collecting ducts in the kidney, which probably are small ureterolithiasis (commonly called kidney stones).

Dietary fluoride in the form of NaF was quickly absorbed and reached a peak in plasma fluoride at three hours in rainbow trout (Oncorhynchus mykiss). Trout fed fluoride from krill shell did not show the same clear peak in plasma concentration, and the estimated cumulative absorption of fluoride was reduced by nearly that half compared to the NaF fed fish. Supplementation of dietary calcium nearly prevented the fluoride uptake in fish fed the NaF diet, while adding calcium to the diets with krill shell reduced the uptake to nearly the half.

Atlantic salmon reared in freshwater and fed a low-fluoride control diet or two diets with fluoride levels of 150 or 350 mg kg-1 with krill shells as fluoride source had higher faecal excretion of fluoride compared to salmon fed fluoride levels of 1,500 and 3,500 mg kg-1 from NaF. Salmon fed the two krill shell diets showed no difference in fluoride accumulation in whole body, bone, muscle, liver, or kidney compared to the low-fluoride control fed fish. Fish fed the NaF diets showed increased fluoride accumulation in both liver and kidney, and especially in the bones.

The major implications of this research for practical feed production are that when using a high inclusion of krill in diets for Atlantic salmon, a partly deshelled krill meal is preferred to avoid possible negative effects of the shell fraction on growth rates and nutrient digestibility. Diets containing 150 and 350 mg fluoride kg-1 from krill did not cause any histological changes in gills, liver or kidneys of Atlantic salmon reared in freshwater. The high concentration of calcium and magnesium in salt water is probably the main cause for the strong limitation in fluoride uptake in salmon.

Updated: 17.06.11
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