Use of Hydroacoustic Methods to Assess Ecological Status Based on Fish: A Case Study of Lake Dejguny (Poland)

Abstract

The idea of the article was to verify the thesis that acoustic studies of ichthyofauna in lakes provide sufficient information to determine the necessity (or lack thereof) of carrying out a full procedure for assessing the ecological status based on fish. The manuscript compares acoustic data collected in 2008 and 2021 in Lake Dejguny (Poland) as a result of acquisition carried out with the same EY-500 echosounder, on the same routes (transects), and the echogram analyses were carried out using software dedicated to this echosounder. Target strength (TS) distribution was determined in the lake and in individual water layers with a thickness of 2 m, as well as, using the mapping method, changes in the distribution of fish in selected water layers. Large changes in TS distribution in the vertical profile were demonstrated between both years of the study, as well as a clear reduction in the space available for fish in waters below 24 m in 2021, which was associated with a reduction in the concentration of dissolved oxygen below this depth to a value lower than 2.5 mg L−1. It was suggested that it could be possible to develop a simple index enabling the assessment of changes in fish structure, based, similarly to the Large Fish Index, on the ratio of the number of large fish to small fish. The obtained results confirmed that the proposed methods of acoustic data analysis can be the basis for the decision on the necessity (or lack thereof) of conducting an assessment using the Polish national LFI-EN method based on the results of one-off fishing with Nordic multimesh gillnets.

Repeatability of Hydroacoustic Results versus Uncertainty in Assessing Changes in Ecological Status Based on Fish: A Case Study of Lake Widryńskie (Poland)

“…The method of analyzing acoustic data proposed in a previous study [19], which may enable the detection of significant changes in the structure of ichthyofauna in lakes and on this basis a change in the ecological state, requires determining the uncertainty of the results. An attempt to determine such uncertainty based on the results collected in Lake Widry ńskie was the aim of this study.…”

“…In a previous study, which described this method using the example of data from Lake Dejguny, the limits for TL Large were ≥30 cm and for TL All they were ≥10 cm [33]. As indicated by Hutorowicz et al [19], the equation that best described the relationship between target strength (TS) and total fish length (LC) indicated that the corresponding target strength values were −38 dB and −47 dB, respectively. The index was determined according to the following equation (in a previous study [19], other TS ranges were mistakenly provided):…”

“…Hydroacoustic data were processed according to the scheme proposed in a paper describing changes in Lake Dejguny [19]. The data were processed using the Simrad EP500 system ver.…”

“…Based on the essence of the large fish indicator, the IND index was calculated [19]. According to the idea of the large fish indicator [32], the IND value is simply the ratio of the sum of the number of fish with a total length greater than the assumed threshold that defines "large fish" (TL Large ) to the number of all fish (larger than TL All ).…”

“…where n TS is the number of fish with a specific target strength (TS). According to this assumption, the comparison of IND values at the beginning and at the end of a given period should allow for an effective and successful assessment of changes in ecological status [19].…”