A novel upward-looking hydroacoustic method for improving pelagic fish surveys

Abstract

For ethical reasons and animal welfare, it is becoming increasingly more important to carry out ecological surveys with a non-invasive approach. Information about fish distribution and abundance in the upper water column is often fundamental. However, this information is extremely hard to obtain using classical hydroacoustic methods. We developed a rigid frame system for pushing upward looking transducers of the scientific echo sounder (38 and 120 kHz) in front of the research vessel. The efficiency of the new approach for monitoring juvenile fish at night was investigated by comparing the results with a quantitative fry trawl in the Římov Reservoir in the Czech Republic. The experimental setup enabled comparisons for the 0–3 m and 3–6 m depth layers, which are utilized by almost all juvenile fish in summer. No statistically significant differences in the estimated abundance of juveniles were found between the two sampling methods. The comparison of abundance estimates gathered by the two frequencies were also not significantly different. The predicted mean lengths from acoustic sampling and the trawl catches differed by less than 10 mm in all comparisons. Results suggest that mobile hydroacoustic upward-looking systems can fill the methodological gap in non-invasive surveying of surface fishes.

Quantification of Chaoborus and small fish by mobile upward-looking echosounding

“…These species frequently occur in open water of the reservoir during the first year of life (Jůza et al, 2009(Jůza et al, , 2013. Chaoborus larvae are not abundant (Řiha et al, 2015) and were observed to form scattering layers in the hypolimnion during the day ascending to the surface at night (Čech and Tušer, personal communication).The pelagic habitat of the reservoir was investigated using mobile hydroacoustics and trawling over the course of two nights, 23/24 July (N1) and 8/9 August (N2) in 2014 The acoustic part of the study was performed using a newly developed method based on a mobile upwardlooking acoustic system (Baran et al, 2017). A research vessel (11 m long with a 210 HP engine) was equipped with two 12 m long submersible arms on either side that held a tiltable platform with attached transducers between their front ends.…”

“…In stratified lakes, targets of interest often occur only a few centimeters to a few meters under the surface and, for this reason, the downward-looking approach does not provide reliable data near the surface and is often replaced by horizontal echo sounding. Upward-looking system makes it possible to record small targets in the near surface layer and to accurately determine their size (Baran et al, 2017).…”

Enhancing the Scientific Value of Industry Remotely Operated Vehicles (ROVs) in Our Oceans

“…ROVs could also deploy single-/splitbeam echosounders ( Figure 4E) to evaluate biota in the water column and these have previously been mounted on and near O&G platforms to estimate numbers of fish around the structures (e.g., Wilson et al, 2006). Alternatively, multibeam systems or sector side-scan sonars ( Figure 4F) can acquire data from the water column above or below the ROV to map fauna targets (e.g., schools of fish or megafauna) or natural/anthropogenic structures (Baran et al, 2017). Longterm datasets from ROV-deployed acoustic Doppler current profilers (ADCPs; Figure 4G) can provide oceanographic information pertinent to O&G structures, such as currents, temperatures, acoustic backscatter, or the detection of internal waves (Li et al, 2009).…”

“…Single-or split-beam echosounders ( Figure 3E and Table 1) can be used to estimate fish biomass around the infrastructure and how distance from the structure influences the assemblage Wilson, 1996, 1998;Wilson et al, 2006). ROVs with mounted echosounders could perform short-term surveys of the water column around infrastructure (Baran et al, 2017; Figure 2F). The detection and quantification of internal waves using upward-looking echosounders (Li et al, 2009) provides valuable information for structural design and maintenance (Osborne et al, 1977), but also the transport and vertical distribution and density of plankton (Benoit-Bird et al, 2009), a fundamental food source in the ocean.…”

Comparison of three sampling methods for small-bodied fish in lentic nearshore and open water habitats

“…Use of disparate sampling techniques has led to research focused on describing fish assemblages within individual zones (e.g., limnetic: McQueen et al, 1986;Gido & Matthews, 2000;littoral: Weaver et al, 1993;Ruetz et al, 2007) rather than whole-waterbody assemblages based on representative sampling with a single gear type across ecotones. Therefore, specific sampling gears are often selected based on habitat characteristics, such as depth, sampling area, water temperature, methodology limitations, or target species characteristics (e.g., Pierce et al, 1990;Bonar et al, 2009;Baran et al, 2017). For instance, the beach seine is commonly used to collect fish from a diversity of standing and flowing waters and is widely used to support research and monitoring for small-bodied fishes (Bonar et al, 2009;Mandrak et al, 2006;Poos et al, 2007).…”