Application of simultaneous trawl-mounted optic and acoustic methods to study the mesopelagic ecosystem

Mesopelagic fishes likely dominate the global fish biomass. Estimates based upon acoustic methods are on the order of 10¹⁰ tons, 10 times larger than previous estimates based upon net sampling. Acoustic methods may oversample mesopelagic fishes due to uncertainties in resonance, the proportion of backscatter due to siphonophores and population characteristics. On the other hand, nets may under sample due to trawl avoidance and poor retention of small individuals. In the work presented, a stereo camera system (Deep Vision) was used in combination with a self-contained echosounder (SIMRAD WBAT) as a tool to compare catch and acoustic estimates and improve our understanding of the mesopelagic ecosystem.

Oblique hauls were conducted between the surface and maximum of 350 m depth in fjords in southwest Norway, yielding catches of crustaceans, cnidaria, cephalopods and bony fishes. The trawl has a very large opening (measured 18 m high by 25 m wide) and is constructed with a series of overlapping conical liner panels of 16 mm (stretched light opening) mesh from mouth to codend. These panels are attached only at the leading (front) edge so that they flap gently to prevent organisms from becoming enmeshed in or lying flat against the meshes. The WBAT acoustic system was mounted to the headline while the Deep Vision camera was placed in the rear of the trawl. Estimates from the WBAT (counting acoustic targets) and the Deep Vision (counting the organisms in the pictures) show similar patterns in vertical distribution and densities over time for the mesopelagic organisms encountered. The identification and counting of organisms in pictures is straight-forward for the portion of the haul when the net descends through the water column and vertical distributions and volume densities show similar patterns to the acoustic estimates. Counts from the camera from the ascending portion of the oblique trawl hauls appear to be less reflective of the actual distributions, likely due to enmeshed individuals which are flushed back in the trawl as it ascends towards the surface. In conclusion, the combination of the WBAT and Deep Vision images from the downcast can improve the sampling method currently used to survey mesopelagic species by verifying acoustic targets in situ and establishing their vertical distribution.