## 4 Discussion  
Density estimates based on two echo sounders, Simrad EK15 and Simrad EM Y, were quite comparable for echoes of TS \(\geq-54\) dB, i.e., for fish lengths \(>5\) cm, according to the TS - FL regression of [PERSON] and [PERSON] [20]. The TS distribution was comparable for TS \(\geq-48\) dB, i.e., for fish lengths approximately 10 cm and larger, according to the regression. This means that density estimates of the pelagic fish in lakes with Simrad EK15 should be comparable with earlier estimates with Simrad EY-M. The estimates from nighttime in September 2015 compared with the records at daytime in May 2016, showed that the spatial distribution of fish might lead to underestimation of density. The gillnetting in August 2015 showed that smelt were more abundant in benthic than in pelagic habitat as compared with whitefish. Although there were no gillnet catches from May 2016, the echo sounding suggested a high abundance of echoes with TS = \(-46\) to \(-44\) dB, which according to the above mentioned regression, were fish between 12 and 20 cm of length, and corresponded to the peak of smelt caught in both types of gillnet in 2015. Length groups larger than 30 cm, corresponding to TS \(>-40\) dB, dominated in the whitefish catches, and the proportion of those were lower in the density estimate of 2016 than in the earlier years. The test recordings in September 2017 showed that the fish moved higher in the water column in the afternoon, probably to feed on plankton at night. The recordings at daytime in May, may be assumed comparable across years, and the estimates of 1986, 1988 and 2013 were rather similar, with the exception of the appearance of TS \(<-50\) dB in 2013, probably due to the occurrence of smelt, as one-year old whitefish are about 10 cm of length [12], giving slightly stronger echoes (\(\approx\)\(-48\) dB). The bimodal length distribution in the fish samples from 2015, with smelt contributing most to a peak between 12-20 cm length, and with a peak between 32-40 cm of whitefish suggested that the dramatic density increase was due to smelt. A tendency of higher proportion of single fish in the EY-M estimates in the survey in 2016, might be due to different settings for single fish detection in the two systems. Echo duration \(<1.3\times\) PD (default) was the threshold for single fish detection in EK15, and \(<2\times\) PD for EY, so the latter could include more SED. The fact that EY-M transmits sound at a lower frequency, 70 kHz compared with 200 kHz of the EK15, may also have affected the results [21]. The lower PRF chosen for EK15, 1.33 compared with 1.50 pings s\({}^{-1}\) on EY-M, may have contributed to the lower single fish detection, as there was a close to  
Figure 6: Estimated total density at 2–50 m depth from six surveys in Lake Storsjeen during 1986 to 2016. All surveys except *2015 were done during daytime in May (* = night in September). significantly positive effect of PRF on the single fish density. This was in part compensated for with the shorter PD on the EK15, 0.32 ms compared with 0.64 ms, as the single fish proportion was negatively related to PD in the experimental test. The total density estimate was negatively affected by PD = 0.64, but was also close to the significantly positively affected by PRF. The effects of lower PD and lower PRF on EK15, is difficult to judge as they counteract each other. Nevertheless, the estimates of the total density were quite comparable. Annual recording in a lake with the same equipment may show considerable variation, not necessarily reflecting population fluctuations, but rather demonstrating different spatial distribution of fish between years [13]. Though, when more pronounced differences are recorded, such as when the density is doubled, or more, from previous estimates [22; 23], the changes are significant and must be expected to affect the lake ecology. In Lake Storsjeen, the introduction of smelt, first time recorded in 2008, appeared in large amounts in test fisheries in 2015 and 2016 [12].