Abstract
Though mesopelagic fish respond to natural light (e.g., diurnal vertical migration), few studies have looked at how they respond to artificial light and if artificial lights could be used in commercial operations to improve catchability of mesopelagic fish. Here we present a preliminary study on how mesopelagic organisms respond to blue and green spotlights, as well as red and white diffuse lights in Masfjorden (Norway; max depth of 480 m). The response of organisms in each of the three sound scattering layers (SSLs) was observed when a) artificial lights were positioned in a layer or b) moved with a constant speed (generally 0.03 ms−1) towards a layer. The artificial lights were attached to a rig with a self-contained echosounder, which recorded the vertical and horizontal avoidance of organisms in each SSL to different artificial lights. Net hauls (MIC-net) and video footage confirmed that Maurolicus muelleri and siphonophores were present in the upper layer (100–150 m), while Benthosema glaciale were present in the deeper layers (∼200 m and ∼300 m to seabed). Our findings suggest that M. muelleri (SSL1) horizontally avoid blue spotlight and white diffuse light, while B. glaciale (SSL2 and SSL3) mainly avoid the same lights downwards and can be herded downwards over 250 m. Though this study should be regarded as preliminary, the observed avoidance/herding response suggests artificial lights could be applied to improve existing fish capture methods for mesopelagic fish.
Highlights
With a growing world population, the demand for seafood is ex pected to increase
This study showed that mesopelagic fish (and maybe siphonophores, Aquaculture and Fisheries xxx (xxxx) xxx see below) respond to blue, green and white artificial lights but not red artificial lights, nor the rig alone
No attraction to light was revealed by the acoustics
Summary
With a growing world population, the demand for seafood is ex pected to increase. The supply from traditional capture fisheries has been rather stagnant around 85 million tons over the last 30 years, while aquaculture production has been steadily increasing (FAO, 2018). Mesopelagic fish are found globally in the mesopelagic zone between 200 and 1000 m depth They are mainly distributed in relatively scat tered layers while in some waters, like the Arabian Sea and the Gulf of Oman, they occur in more dense concentrations (Gjøsæter, 1984). The global biomass of mesopelagic fish has earlier been estimated at about 1000 million tons (Gjøsæter & Kawaguchi, 1980; Lam & Pauly, 2005). Artificial lights could be used to herd or attract mesopelagic fish to increase catchability and selectivity in mesopelagic fisheries. The blue - and green spotlights were made to match biolumi nescence light (Haddock & Case, 1999; Kampa & Boden, 1954) and the visual pigments of M. muelleri (see Fig. 3b in de Busserolles et al, 2017) and Benthosema spp. The artificial lights were positioned on the rig so that they pointed either downwards, sidewards (horizontally) or upwards (Table 1)
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