Introduction
Integrated multi-trophic aquaculture (IMTA), involves farming of multiple marine species at different trophic levels at the same aquaculture site. In the AURORA IMTA project , integrated aquaculture of Atlantic salmon ( Salmo salar) and the brown macroalgae called sugar kelp ( Saccharina latissima ) is realized and developed at a full-scale sea-based facility in the North of Norway. Possible benefits of IMTA are currently investigated along with developing robust solutions for both biological production and structural design The main idea is that the kelp can benefit from absorbing dissolved inorganic nutrients from the salmon farming. In addition, their joint production may be an effective shared use of sea area.
A part of the current RnD -work includes developing knowledge to ensure a safe and effective design of such IMTA-farms. Currently there is little knowledge on expected loads from wave s and currents (hydrodynamic loads) on seaweed cultivation lines (Endresen et al., 2019; Kool et al., 2022), and seaweed weight (hydrostatic loads). Th is results in application of unnecessarily high safety factors in design, which may result in sub-optimal and overly expensive farm designs. In addition, increased knowledge of possible displacements and movements of cultivation lines is of great interest, a s this may affect growth (through e.g. light, temperature and water quality) and lead to line entanglement and seaweed abrasion.
Materials and methods
Five cultivation lines with sugar kelp have been instrumented with two depth sensors and a load chackle each from April 25th to July 10th 2024. In addition, the mass density of the kelp was measured once or twice a month to facilitate weight calculation.
Initial numerical analysis indicated that cultivation lines were horizontal at high hydrodynamic loads, while they were sagging in calm water. Based on these findings, the experiment was designed to investigate the following hypotheses:
Since the kelp would easily decompose during unfavourable storing conditions, our tests were done in the field using newly harvested seaweed kept in fresh seawater.
Results
All instruments, including five load chackles and ten depth sensors, worked well and produced continuous measurement data except during two periods of power failure. The findings supported all four hypotheses , also indicating that the developed methods for measuring of load, position and mass density worked well .
Vertical kelp line positions varied with several meters, and large sagging coincided with low measured loads. Peaks in load measurements coincided with h orizontal lines (similar depth measurements over a line) po sitioned at relatively shallow waters.
A method and procedure to measure mass density of seaweed (hypothesis 4) was developed and employed. The results showed that the kelp had a mass density of 920 ± 86 kg/m3, based on 14 separate measurements. This is lower than seawater, and it was observed that the kelp was floating (without visible air-bubbles). P revious literature has indicated a mass density of about 1100 kg/m3 , i.e. slightly higher than water ( Vettori & Nikora, 2017; Norvik, 2017). Possible reasons for this may be that the density may depend on site specific conditions like nutrients , kelp size and carbon content, morphology (Buck & Buchholz, 2004) or different treatment and storing of the kelp after harvesting .
References
B. H. Buck, and C. M. Buchholz, “Response of offshore cultivated Laminaria saccharina to hydrodynamic forcing in the North Sea,” Aquaculture, vol. 250, no. 3, pp. 674-691, 2005.
Endresen et al., 2019. Current Induced Drag Forces on Cultivated Sugar Kelp. OMAE2019-96375.
Kool et al 2022 (Wageningen notat). Measuring standing crop on offshore seaweeds using drag forces.
Vettori D, Nikora V, 2017. Morphological and mechanical properties of blades of Saccharina latissima. Estuar Coast Shelf Sci 196:1–9
C. Norvik, “Design of Artificial Seaweeds for Assessment of Hydrodynamic Properties of Seaweed Farms,” Master’s thesis, NTNU, 2017. Norvik et al.,