Nutrient (C, N and P) budgets of Hediste diversicolor (OF Müller, 1776) fed salmon smolt sludge.

Norway is the largest producer of Atlantic salmon (Salmo salar) in the world, with a yearly production of approximately 1.3 million tons. With aquaculture of this scale, a large amount of nutrient pollution occurs in the form of spilled feed and excreted nutrients. As this can have adverse effects on aquatic systems, there is an environmental and economic incentive for recycling said nutrients into valuable products. One proposed solution is integrated multi-trophic aquaculture (IMTA), where lower trophic level organisms are cultivated using the waste products of the main species as feed. The Nereid polychaete Hediste diversicolor has been shown to utilize aquaculture sludge as a feed source and is therefore a novel candidate for this purpose. This thesis aimed to investigate the bioremediation potential of H. diversicolor fed salmon smolt sludge, by establishing an individual nutrient budget containing the three macronutrients carbon (C), nitrogen (N) and phosphorus (P). This was based on a feeding experiment that investigated the feeding activity, nutrient assimilation, feces production and metabolism at two feeding levels, namely 5% and 40% of the polychaetes total N content. The oxygen consumption of the polychaetes were measured to observe the impact of feed concentration and feeding state on the respiratory rate. The polychaetes ingested a larger amount of feed in the high feeding level. However, the ingestion rate decreased with an increasing feed availability, with 64 ± 20% and 22 ± 12% of the feed added being ingested for the low and high feeding treatment, respectively. A significantly higher assimilation rate was seen in the high feeding treatment (72-85%) than in the low (50-80%) for all three macronutrients, with N being the most efficiently assimilated and P being the least. For the low feeding treatment, significantly different assimilation rates were seen for all three nutrients, but no difference was found in the high. Due to the low ingestion rate, the assimilation of the total amount of nutrients added was found to be much lower. The total assimilation was higher in the low treatment (28- 53%) than in the high (16-21%). The largest feces production was seen in the low treatment for P with 40% of the ingested nutrients. Only P showed a significant difference between the low and high treatment when investigating percentage feces production, but a similar trend was also seen in C and N. The respiratory rate of the polychaete did not differ between the two feeding levels, nor the feeding state of the polychaetes. A rate of - 0.04 μmol O2 mgDW-1 h-1 was established, giving a CO2-production of 0.84 μg CO2 mgDW- 1 h-1. It is concluded that Hediste diversicolor utilizes salmon smolt sludge as a feed source. The polychaetes efficiently assimilates ingested nutrients, with the lowest ability for assimilation found in P and the highest in N. However, the assimilation ability of the polychaete is strongly limited by its low ingestion rate at higher feeding levels. Thus, H. diversicolor has the potential of recycling nutrient from aquaculture sludge, making it a novel candidate for IMTA-applications in the future.