Following a steadily increasing demand from the aquaculture industry for raw materials for feed production, there is an increasing interest in harvesting lower trophic level organisms as a new source for marine protein and lipids. Before a potential commercial fishery on lower trophic level species can be started, we need to establish sound scientific knowledge about the targeted ecosystem and assess the potential consequences posed by the harvest to manage such a resource within a sustainable framework.
Economically viable harvest most often requires some sort of aggregation of biomass (i.e. spawning grounds, fish schools, overwintering area). One of the most promising aggregation of unexploited marine biomass is the mesopelagic layer, often referred to as the 'deep scattering layer' by the way it is observed on echo sounders. However, a fishery targeted at the mesopelagic layer would necessarily imply a mixed fishery as the layer consists of a wide range of species. The main goal of LOTROMIX have been to quantify the species composition, distribution and total biomass in the Norwegian Sea, and further analyse the quality of the catch for feed production and assess potential ecological consequences of mixed fishery on this unexploited resource.
One of the goals of LOTROMIX has been the use of towed submersible platforms that enable us to use multi-frequency categorization in the deep scattering layers. Because we deploy the acoustic instruments on a towed body, we also get usable results from the higher acoustic frequencies, offering improved performance when compared to results obtained via the hull mounted transducers. We are now better able to assess the vertical distribution of organisms in the macrozooplankton size spectrum from shallow to deep depths, and we're able to use the results from the acoustic categorisation to assess broad taxonomic composition of the deep scattering layers. Since we're able to decompose the layer compositions and acoustic contributions from the different acoustic categories, these results will also allow us to improve acoustic biomass estimates obtained from the hull mounted equipment. The deep-towed platform also provides us with direct in situ target strength profiles, meaning we can now convert acoustic profiles obtained from both the hull mounted and towed equipment to profiles of actual organism densities more accurately.
Since the start of the project, existing data have been gathered and quality controlled. This data material is used together with new data obtained during LOTROMIX cruises in 2015 and 2016. A novel approach to estimating total biomass has been developed. Besides being able to estimate total biomass and spatial distribution, we are also able to calculate the uncertainty of our biomass estimates.
Total lipids extractions have been completed for all the dominant species found in the trawl samples. These total lipids extraction has also been used in analyses by gas chromatography to evaluate the total content of various fatty acids and fatty alcohols in the samples. Pooled samples from the mesopelagic layer had an average lipid contents of about 9% of wet weight. The fatty acid composition contained favourable amounts of both PUFA and DHA+ EPA in all samples. A mixed catch from the mesopelagic layer is thus well suited to constitute a part in salmon feed.
Following a steadily increasing demand from the aquaculture industry for raw materials for feed production (Tacon et al., 2012), there is an increasing interest in harvesting lower trophic level organisms as a new source for marine protein and lipids. Before a potential commercial fishery on lower trophic level species can be started, we need to establish sound scientific knowledge about the targeted ecosystem and assess the potential consequences posed by the harvest in order to manage such a resource within a sustainable framework.
Economically viable harvest most often requires some sort of aggregation of biomass (i.e. spawning grounds, fish schools, overwintering area). One of the most promising aggregation of marine lower trophic level biomass is the mesopelagic layer, often referred to as the 'deep scattering layer' by the way it is observed on echo sounders. However, a fishery targeted at the mesopelagic layer would necessarily imply a mixed fishery as the layer consists of a wide range of species. The main goal of LOTROMIX will be to quantify the species composition, biomass distribution and production on a temporal and spatial scale in the Norwegian Sea and further analyse the quality of the catch for feed production and assess potential ecological consequences of mixed fishery on this unexploited resource.