Abstract
Hydroacoustic acquisition was performed by means of two different single beam systems, the Simradmodel EK15 from 2015 and the Simrad model EY-M from the 1980s to explore potential differences of fish density and target strength (TS) distribution between EK15 and EY-M-based estimates. The oligotrophic Lake Storsjøen (48 km2) with steep shores, was chosen for the survey. The pelagic fish stock is dominated by whitefish (Coregonus lavaretus L.), and, recently, illegally stocked smelt (Osmerus eperlanus L.), and a low proportion of Arctic charr (Salvelinus alpinus L.). The total density was estimated at two depth layers (18–32 and 32–48 m), and was quite similar for TS ≥ −56 dB: 755 and 498 fish ha−1, respectively for the EK15-based estimates and similarly 766 and 490 fish ha−1 for the EY-M estimate. Target strength distributions were similar for TS > −48 dB. The proportion of single fish detected with EK15 was negatively affected by the long pulse duration. Six acquisitions from 1986 to 2016 showed a dramatic increase of density of TS = −46 to −44 dB echoes (>10 cm) between 2013 and 2016. This was due to the growth of the introduced smelt population.
Highlights
IntroductionFor the management of water resources, monitoring ecologically important fish species is crucial
For the management of water resources, monitoring ecologically important fish species is crucial.Pelagic salmonids and cyprinids predate zooplankton in temperate lakes, and affect the species composition and size structure of the zooplankton community, which in turn affects the abundance of planktonic algae [1,2], and further, influencing water quality and its suitability for public use [3,4,5].The hydroacoustic assessment is an efficient method to estimate the density and size distribution of pelagic fish, and has been used in lakes for more than three decades, and in marine environments for much longer [6,7]
Historical hydroacoustic data, recorded by means of older equipment are important for comparison with current and future surveys in a climate change perspective, and there is a need for calibration of old to new equipment
Summary
For the management of water resources, monitoring ecologically important fish species is crucial. The hydroacoustic assessment is an efficient method to estimate the density and size distribution of pelagic fish, and has been used in lakes for more than three decades, and in marine environments for much longer [6,7]. A transceiver, first presented in [8], Simrad EK15, has kept the single beam character. A few comparative studies are published, like a comparison of the single beam. This study was conducted in Lake Storsjøen in southeast Norway, and the aim was to compare density estimates and target strength distribution of echoes of the pelagic fish community based on two different single beam Simrad echosounders, an EY-M from 1982 and an EK15 from 2015. Sci. 2019, 9, 2041 different ping repetition frequency and pulse duration on EK15-based density estimates were explored, and density estimates from five former years are presented as a contribution to monitoring fish density
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