Lake Constance was covered with 21 hydroacoustic transect-lines (Figure 1). These transects were randomly divided into 30 subsamples (Table 3). An echogram (Figure 5) illustrates even distribution of whitefish in the central part of the Lake. Each sample is made up from 500 to 2600 pings, dependent on depth and noise in the area. Particularly in the South East region of the lake, Rohrspitz-Lindau, gas bubbles which rose from the bottom caused some noise problems. Multiple echos recorded in shallow areas are assumed mainly to be caused by perch and roach. The density of whitefish with target strength (TS) > -41 dB have been calculated from areas with depth >/30 meter.
A logarithmic transformation of the data was used to calculate the 95% confidence limits of the population mean. The area populated with whitefish in the region South East of Kippenhorn (Immenstaad) was measured to 20 000 ha. Sixteen samples were taken from this area and we derived a mean density of 121 fish/ha. The 95% confidence limits are 172 fish/ha and 85 fish/ha.
The low density region found in the area West of Kippenhorn and in the Überlinger See added an area of 18 000 ha. Eightsamples from this part of the lake gave a mean fish density of 4.9 fish/ha. The calculated 95% confidence limits are 11 fish/ha and 1.8 fish/ha.
The estimate of the total whitefish population based on the hydroacoustic technique (which excludes the shore area and the surface layer) is found by adding the numbers from these two areas together. In October 1983 this population of whitefish should be:
2.5 × 106 whitefish
with 95% confidence limits of 1.7 × 106 and 3.7 × 106.
Table 3
Results from the hydroacoustic survey in Lake Constance
| Sample | Transect | Total density f/ha | “Whitefish” TS -41dB. f/ha | Remarks |
| 1 | 1 | 606 | 40 | |
| 2 | 2 | 3 854 | 389 | shore area |
| 3 | 3 | 261 | 55 | |
| 4 | 3b | 600 | 164 | |
| 5 | 3c | 473 | 109 | |
| 6 | 4 | 2 431 | 208 | shore area |
| 7 | 4b | 308 | 170 | |
| 8 | 4c | 240 | 153 | |
| 9 | 5 | 191 | 53 | |
| 10 | 6 | 8 105 | 151 | perch shoal |
| 11 | 6b | 121 | 6 | |
| 12 | 7 | 151 | 17 | |
| 13 | 8 | 1 250 | 131 | |
| 14 | 8b | 32 972 | 319 | perch shoal |
| 15 | 9 | 1 008 | 135 | |
| 16 | 10 | 864 | 222 | shore area |
| 17 | 11 | 528 | 109 | |
| 18 | 11b | 949 | 39 | |
| 19 | 12 | 2 410 | 292 | |
| 20 | 12b | 779 | 470 | shore area |
| 21 | 13 | 175 | 9 | Überlinger See |
| 22 | 14 | 34 | 4 | " |
| 23 | 15 | 154 | 1 | " |
| 25 | 17 | 114 | 9 | " |
| 26 | 18 | 51 | 0 | " |
| 27 | 19 | 496 | 180 | |
| 28 | 20 | 188 | 100 | |
| 29 | 21 | 249 | 71 | |
| 30 | 21b | 528 | 172 |
When the Lake Constance experiment started the main goal was to estimate the pelagic whitefish population. However, we soon found that the shore areas in the South East region of the lake were populated with dense shoals of perch. We did not have time to make a survey of all the areas populated with perch, but we made a small separate grid with 5 transects between Lindau and Bregenz to get some information on perch densities. Due to a very contagious distribution the estimate of perch in the Bregenzer Bucht must be very crude (Figure 6).
Five transects gave the following counts in the target strength range -42dB, -48dB.: 513 fish/ha, 3997 fish/ha, 1 884 fish/ha, 1 437 fish/ha and 17 760 fish/ha. The population mean of the perch in the pelagic part of the area probably lies (P=0.95) between 0.6 × 106 and 15 × 106 with a derived mean of 3 × 106.
