## 2 Materials and Methods
### Methods  
To compare the fish density and target strength distribution attained by two echo sounders Simrad EK15 and Simrad EY-M, records were made along ten 1500-2300 m long transects, obliquely crossing the lake at daytime (11:00-16:00) in late May 2016 (Figure 1) in a speed of 1.6 to 1.9 m s\({}^{-1}\). Both transceivers were powered by 12 V batteries.  
Daytime was chosen because the fish are distributed on greater depth than at night [12; 13], so a higher number of fish will be detected (in the approximately coned sound beam) at daytime, giving a more representative TS distribution. [PERSON] et al. [10] recommended recording in darkness when recording echoes of TS \(<-\)44 dB, (i.e., fish length \(<\) 15 cm), at high densities (\(>\) 1000 fish per ha), due to schooling behavior during daylight. In Lake Storsjeen, density was lower (\(<\) 1000 fish per ha), and the fish larger (30-40% of the echoes had TS \(\geq-\)44 dB).  
Figure 1: Map of Lake Storsjeen with transects Tr 1–10 recorded in May 2016, and the test transect Trr where six transects were performed to test the effects of the ping repetition frequency (PRF) and pulse duration (PD) on density estimates, in September 2017.  
The EK15 and EY-M echo sounders transmit sound at different frequencies, 200 kHz and 70 kHz, respectively, and this made it possible to run both systems simultaneously. Another difference is that the transducer of EK15 has a beam angle of 9\({}^{\circ}\) whereas EY-M has an angle of 11.2\({}^{\circ}\), so the latter covers a slightly larger water volume. The fish density was analyzed in two relatively narrow depth layers of high concentration of fish (Figure 2), thereby making it possible to explore potential effects of different systems on different depths. The equipment was calibrated from the boat when anchored up above 15 m depth with the sphere at 8 m depth. For the EY-M calibration, a 32.1 mm copper sphere (corresponding to TS = \(-\)39.4 dB at temperature 5 \({}^{\circ}\)C), and for EK15 a 38.1 mm wolfram carbide (WC) sphere (corresponding to TS = \(-\)39.2 dB, at water temperature 5 \({}^{\circ}\)C) was used.  
Simrad EY-M operates with a fixed pulse duration (PD) of 0.60 ms, and a ping repetition frequency (PRF) of 1.5 or 3.0 pings s\({}^{-1}\), whereas the Simrad EK15 has several options; PD of 0.08, 0.16, 0.32, 0.640 and 1.280 ms, and PRF up to 40 pings s\({}^{-1}\). PD was set to 0.32 ms, assumed to give higher resolution of schools than the higher PD alternative of 0.64 ms. The water temperature in May was 4-5 \({}^{\circ}\)C (salinity \(<\) 2 ppm), giving a sound speed in water of 1422-1427 m s\({}^{-1}\). PRF was chosen 1.5 s\({}^{-1}\) for EY-M and 1.33 s\({}^{-1}\) for EK15, and the slightly lower PRF was chosen for EK15 because parts of some transects were difficult to analyze with PRF = 1.5, as the (second) bottom echo met the next signal and produced noise on both systems. A 500 m long stretch in the second crossing of the lake was therefore omitted in the analysis of both echo sounders. This divided a crossing in two, forming transect 2 and 3. With 1.33 pings per second, the echo met the next signal at \(>\)50 m depth where the detections were sparse.