Utilization of the polychaete Hediste diversicolor (O.F. Millier, 1776) in recycling waste nutrients from land-based fish farms for valueadding applications'
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- Institutt for biologi 
Aquaculture has become one of the world’s fastest growing food production industries to meet the nutrient demand of the growing human population. Atlantic salmon, which accounts for 4% of the global marine fish production, is the most produced fish species in aquaculture. Increasing salmon hatcheries and expectation for larger-size smolt in land-based farming result in increased production of sludge, which causes nutrition waste and ecological problem. Furthermore, a boost in fish production will lead to increased demand for fishmeal and fish oil that contain high protein, balanced amino acids and omeg-3 long chain polyunsaturated fatty acids. While the wild marine origin fish for aquafeed is limited, so a continuing demand for sustainable high-quality feed ingredients has been a big concern for intensive aquaculture. It is necessary and practical to take into consideration both nutrient wastes and demand in aquaculture. It has been studied that polychaete showed good growth performance and efficiently recycled valuable fatty acids (FA), like EPA and DHA, when integrated with fish farming. Meanwhile, polychaete is considered as the indispensable maturation feed or feed supplement for several fish and crustacean broodstock. These make polychaete become the promising candidate to reduce the waste while supply feed ingredients. To realize the utilization of polychaete in recycling waste while producing feed ingredients, the current thesis was designed based on two-fold: the first objective was to verify the ability of the polychaete Hediste diversicolor to utilize (i) sludge from salmon smolt aquaculture; and (ii) solid digestate from biogas production for growth and conversion into nutrient-rich polychaete biomass. The second objective was to explore artificial breeding including (i) the gametogenesis, embryonic development and larval nutrition; and (ii) the effect of external factors on oogenesis in order to develop a controlled life cycle of H. diversicolor. H. diversicolor showed a specific growth rate of around 0.9-1.2 d-1 after feeding with smolt sludge, solid biogas digestate. The protein and lipid content in polychaete was 52 - 60% and 12.4 - 15.7% of dry weigh, respectively, which can meet the requirement of fish and shrimp together with balanced amino acids and fatty acids profiles. Gametes were obtained by dissection, and in vitro fertilized eggs (»196-200 μm) developed to the swimming trochophore stage after 5 - 6 days at 11℃, and became the 3-setiger stage larva in 12 days. The vitellogenesis can be induced through a transfer of the female from long-day to short-day exposure with suitable temperature, or decreasing temperature with long day exposure in autumn. A shortened winter photoperiod transition can accelerate the oocyte growth rate. Increasing temperature can synchronize oocyte maturation in spring. In conclusion, the polychaete H. diversicolor can successfully grow on smolt sludge and solid biogas digestate by recycling waste streams from fish aquaculture and biogas production into biomass. The biochemical composition of the worms suggests they can be alternative protein and lipid sources to fish feed. While by manipulation of photoperiod and temperature, the oogenesis of the worm showed difference in developmental speed. The embryonic and larval development trial can provide basic information on practical worm reproduction. Through exploration in oogenesis, embryonic and larval development, basic knowledge makes larvae production all year-round possible. The polychaete H. diversicolor can be promising and sustainable feed ingredients.