Elsevier

Fisheries Research

Volume 238, June 2021, 105907
Fisheries Research

New way to investigate fish density and distribution in the shallowest layers of the open water

https://doi.org/10.1016/j.fishres.2021.105907Get rights and content

Highlights

  • Quantitative and nondestructive sampling of surface layers of waterbodies represent great challenge.

  • Investigations show that most of the fish stay in upper meter of the water column barely accessible to traditional acoustic surveying.

  • Mobile upward looking deployment of echosounder showed distribution of fish similar to Nordic multimesh gillnets.

  • The upward-looking acoustic system is a promising approach to study the fish community in the neglected surface layer.

Abstract

Information about fish distribution and abundance in the upper part of the water column are often fundamental for both research and management. However, this information is extremely hard to obtain using conventional hydroacoustic methods. For this reason, the mobile hydroacoustic upward-looking system (38 kHz split-beam echosounder) in combination with a passive sampling method (gillnets) was tested to investigate the fish community (fish larger than 8 cm total length) in the upper 3 m of water column of the Římov Reservoir (Czech Republic) during the growing season. We found most fish located in the depth layer closest to the surface down to 1 m – 50–78 % by acoustics (layer 0.3; 1 m) and 55-–1 m) and 55–71 % by gillnets. The size structure of both methods was generally similar, but the acoustic results contained a higher proportion of small fish (<12 cm TL). The upward-looking system is a promising approach to study the fish community in the neglected surface layer, but more studies of its efficiency for large fish monitoring are needed.

Introduction

Open water represents the largest volumes of larger lakes and reservoirs. Quantitative fish sampling of these volumes still represents a challenge (Kubečka et al., 2012). Hydroacoustics is an obvious option for covering large areas without disturbing fish and causing sampling-related mortality. However, most fish utilize shallow water closest to the surface (Jarolím et al., 2010; Kubečka and Wittingerová, 1998; Vašek et al., 2009) and it is difficult to obtain sound estimates with conventional downward-looking acoustics due to nearfield near transducer and narrow beam width close to water surface. Only a small proportion of the fish stock can be assessed with downward-looking echosounding in eutrophic waters (Kubečka and Wittingerová, 1998). Another option for pelagic fish assessment, the horizontal beaming is influenced by unwanted interference of multipath reflections (Balk et al., 2017). Therefore, reliable quantitative assessments of fish community in the shallowest layers of the open water are rare.

An alternative approach of using a mobile upward-looking surveying was developed to mitigate this unsatisfactory situation. This approach was shown to provide very clear data recordings of small fish and even invertebrates (Baran et al., 2017 and 2019). Upward-looking echosounding has been used previously for surveying of fish populations at fixed location (Axenrot et al., 2004; Čech and Kubečka, 2002; Jarolím et al., 2010), but rarely in mobile mode (but see Probst et al., 2009; Říha et al., 2017; Grow et al., 2020).

Upward-looking surveys are more reliable when done at night (Baran et al., 2017 and 2019). During the day Baran et al. (2017 and 2019) observed that fish are likely to react to the survey vessel (see also Rakowitz et al., 2012 and Muška et al., 2013). Daytime is also not a suitable period for surveying because most reservoir fish perform sinusoidal swimming to search for zooplankton more efficiently (Čech and Kubečka, 2002; Jarolím et al., 2010). While performing sinusoidal swimming, the fish body aspect exposed to the upward-looking transducer is very difficult to define as the fish can have any aspect within the range of + 30 to –30 degrees tilt (Čech and Kubečka, 2002) leading to poorly defined TS to fish length relationships.

In this study, we explored whether night mobile upward-looking acoustic surveys represent a reliable tool for community assessment of yearling-and-older fish. During the summer season in the Římov Reservoir (Czech Republic), we used the upward-looking acoustics and CEN multimesh gillnets simultaneously to enable comparison of size distributions obtained from the two gears. CEN multimesh gillnets (CEN, 2015) are relatively free of size selective biases for a wide range of fish sizes larger than 8 cm (Prchalová at al., 2009) and smaller than 30 cm standard length (Šmejkal et al., 2015). If the two size distributions agree, we suggest that large fish do not differentially avoid our upward-looking acoustic system. Further, we analyzed vertical micro-distribution of fish within the uppermost 3 m of the water column.

Section snippets

Study area

This study was conducted in the Římov Reservoir (48°50′N, 19°30′E, 471 m above sea level, Fig. 1), 170 km south of Prague, Czech Republic. The reservoir was constructed on the Malše River in 1978. It is a canyon-shaped reservoir with a length of 12 km (on original riverbed), a maximum volume of 33 × 106 m3, a surface area of 2.1 km2, and an average and maximum depth of 16 m and 43 m, respectively. The trophic state of the reservoir is mesotrophic to eutrophic with well-developed thermal

Comparison of total catches and records

A total of 699 fish larger than 8 cm were caught in the 36 gillnet sets (451 bleak, 84 common bream, 120 roach, 26 European perch (Perca fluviatilis) and 18 individuals of other species). During hydroacoustic surveys, 463 targets bigger than 8 cm (−45.5 dB) were recorded and 15,526 m3 of water was sampled.

Vertical fish distribution

In pelagic gillnets we captured between 55–71 percent of all caught fish within 1 m of the surface (Fig. 3). The percentages of acoustic targets detected in the 0.3–1 m depth layer represented

Discussion

The current study demonstrates that mobile upward-looking acoustic surveys have potential to be a reliable tool in fish community assessment in the epilimnion of stratified lentic waters. Most studied parameters of the acoustic assessment including overall size distribution, and vertical distributions corresponded well with the same parameters obtained by gillnets.

Both methods revealed that fish had an affinity to the topmost 1 m of the water column. Given this vertical distribution, it is

Conclusions

The new sampling method circumvents to a large extent the disadvantages of horizontal and downward looking hydroacoustic transducers when sampling near the surface (upper 3 m). We found majority of fish to occupy water layer very close to the surface (0.3−1 m) and these would be mostly missed by down- or side-looking acoustic sampling. Comparison with gillnets showed that upward-looking records provided similar fish size distribution. Evaluation of upward-looking mobile echo sounding could be

CRediT authorship contribution statement

Roman Baran: Conceptualization, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Petr Blabolil: Data curation, Formal analysis, Investigation, Visualization. Martin Čech: Conceptualization, Investigation. Vladislav Draštík: Data curation, Formal analysis, Investigation, Software, Supervision. Jaroslava Frouzová: Supervision, Validation. Michaela Holubová: Investigation. Tomáš Jůza: Conceptualization, Investigation. Ievgen Koliada: Investigation. Milan Muška:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Dagmar Kubečková – Synková, A.D. Jayasinghe, L. Kočvara and Z. Prachař for help with data collection, as well as the FishEcUteam(www.fishecu.cz) for critical reading of the manuscript.

The work was supported by ERDF/ESF project Biomanipulation as a tool for improving water quality of dam reservoirs (No. CZ.02.1.01/0.0/0.0/16_025/0007417) and project CZ.1.05/3.1.00/10.0214 technology transfer both administrated by the Ministry of Education, Youth and Sports of the Czech Republic. R.

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