## 3 Results
### Effects of Ping Repetition Frequency (PRF) And Pulse Duration (PD) On EK15-Based Estimates  
During the test crossing along the test transect (TTr)transect (Figure 1) in September 2017, the fish were concentrated in two layers. The density was highest at 18-25 m depth during the 12 first ESUs (starting at 14:48, local time). The fish gradually positioned higher in the water, and ended concentrated at 12-17 m during the last three ESUs (ending at 18:17, local time).  
This dense layer was analyzed to test the effect of varying PRF (1.33 and 1.5 pings per sec) and PD (0.16, 0.32 and 0.64 ms). The EK15-based total density N\({}_{\rm{EK}}\) ha\({}^{-1}\) ranged from 264 to 885 fish ha\({}^{-1}\), with a mean 521 (95% C.I: 481-572), across the 36 ESUs, regardless of PRF and PD, that means the  
Figure 4: **(left)**: Total length frequency distribution of smelt and **(right)**: whitefish caught in benthic nets (filled columns) and pelagic nets (open columns)  
Figure 3: **(above)**: Estimated means of total fish density of 10 transects based on Simrad EK15 (solid line) and Simrad EY-M (dotted line) at depth interval 18–32 m and **(below)**: interval 32–48 m run simultaneously in May 2016.  
95% C.I. was \(\pm\)8.7% of mean. The EY-based total density N\({}_{\rm EM}\) ha\({}^{-1}\) ranged from 106 to 556 fish ha\({}^{-1}\), with a mean 503 (95% C.I.: 462-573), and 95% \(\pm\)C.I. comprising \(\pm\)8.4% of mean. The EK-based estimate was higher, but not significantly as both estimates were within the 95% C.I. of the others. Single fish detection comprised 83.6-94.4% of the EK15 estimate and 80.7-100% of the EY-M estimate, with exception of one ESU where single fish comprised only 66.0% of N\({}_{\rm EM}\).  
The effect of PRF was non-significant in all models, though close to significantly positive for N\({}_{\rm EK}\) and N\({}_{\rm SED}\) ha\({}^{-1}\) (\(p\) = 0.052-0.064), and was excluded from the models. Then, there was a negative effect of the longest PD in all models, and also a negative effect of PRF = 0.32 on the proportion of single fish (PROPSED) (Table 2, Figure 5). N\({}_{\rm EK}\) and N\({}_{\rm SED}\) were both positively related to N\({}_{\rm EY}\). The mean TS (S.E.) across ESUs with PD = 0.16, 0.32 and 0.64 ms, was \(-\)49.2 (0.33), \(-\)47.3 (0.46) and \(-\)48.5 (0.72) dB, respectively, and was significantly higher at PD = 0.32 ms than at PD = 0.16 ms (t = 4.2, d.f. = 28, \(p\) < 0.01) and PD = 0.64 ms (t = 2.6, d.f. = 22, \(p\) < 0.05), suggesting the importance of PD specific calibration to achieve the correct TS.  
\begin{table}
\begin{tabular}{l c c c c} \hline
**Variables** & **Estimate** & **S.E.** & **T** & **p** \\ \hline N\({}_{\rm EK}\) ha\({}^{-1}\) & & & & \\ Intercept & 267.5 & 89.9 & 2.98 & **<0.01** \\ N\({}_{\rm EY}\) ha\({}^{-1}\) & 0.55 & 0.15 & 4.135 & **<0.0001** \\ PD = 0.32 & 3.90 & 42.6 & 0.09 & n.s. \\ PD = 0.64 & \(-\)119.3 & 56.6 & 2.11 & **<0.05** \\ N\({}_{\rm SED}\) ha\({}^{-1}\) & & & & \\ Intercept & 265.4 & 79.6 & 3.34 & **<0.01** \\ N\({}_{\rm EY}\) ha\({}^{-1}\) & 0.47 & 0.14 & 3.47 & **<0.01** \\ PD = 0.32 & \(-\)18.8 & 37.74 & 0.50 & n.s. \\ PD = 0.64 & \(-\)140.5 & 50.1 & \(-\)2.80 & **<0.01** \\ PROP\({}_{\rm SED}\) & & & & \\ Intercept & 94.6 & 1.69 & 56.1 & **<0.0001** \\ N\({}_{\rm EY}\) ha\({}^{-1}\) & \(-\)0.002 & 0.003 & 0.85 & n.s. \\ PD = 0.32 & \(-\)3.94 & 0.80 & 4.94 & **<0.0001** \\ PD = 0.64 & \(-\)6.52 & 1.06 & \(-\)6.15 & **<0.0001** \\ \hline \end{tabular}
\end{table}
Table 2: Results of linear modelling of EK15 based total density (N\({}_{\rm EK}\) ha\({}^{-1}\)), density of single fish detection (N\({}_{\rm SED}\) ha\({}^{-1}\)) and the proportion of single fish density (PROPSED) explained by the estimated total density by means of Simrad EY-M (N\({}_{\rm EY}\) ha\({}^{-1}\)) and ping duration (PD); S.E.: standard error, T: t-test value, p: probability of null-hypothesis to be true.