As in the Konnevesi experiment 1980 the two-boat mid-water trawls were supplied by the Institut für Fangtechnik, Hamburg. Staff of the Langenargen institute, under the advice of Dr. E. Dahm, operated the trawls and skippered the two boats. For details on the Commercial Vendance Trawl (C.V.T.) and the Young Fish Trawl (Y.F.T.) which were identical to those used during the Konnevesi experiment (Bagenal et al., 1982), and for information on the fishing procedures, reference is made to Steinberg and Dahm (1975) or Dahm (1980). The distance between headrope and footrope and the speed-depth relationship were measured during preliminary trials in August 1983. Speed was measured by a float-and-line log, and calibrated against throttle positions. Throttle position was then used during the exercise as the main speed control and, together with towing warp lenght, as the depth control. Occasional soundings of the trawl with the echo-sounding boat during the first days of the exercise confirmed the assumptions of trawl depth and opening height originating from the previous trials. The wing spread of the trawls was controlled by keeping a marked line between the two boats as tight as possible during the towing.
Each catch was processed immediately on board of the R. V. KORMORAN.
The fishes were grouped into 3 main categories: whitefish (Coregonus lavaretus), perch (Perca fluviatilis) and others, consisting of 363 fish: 244 Rutilus rutilus, 69 Gasterosteus aculeatus and 39 Leuciscus leuciscus (Table 1) and some others.
Table 1
Trawl catches curing the III EIFAC Intercalibration Exercise
| Haul No. | Date | Time from - to | Trawling depth (m) | Distance Towing Boats (m) | Net Type | Catch | Total |
| 1 | 24/10/83 | 19.10 – 19.40 | 9,0–12,0 | 64 | C.V.T | 622 perch, 40 whitefish, 4 roach, 1 dace | 667 |
| 2 | 24/10/83 | 20.20 – 20.50 | 9,5–12,5 | 64 | Y.F.T. | no catch because of technical defects | - |
| 3 | 24/10/83 | 21.06 – 21.36 | 9,5–11,5 | 64 | Y.F.T. | 227 perch, 2 whitefish 1 dace | 230 |
| 4 | 24/10/83 | 22.02 – 22.32 | 9,5–11,5 | 64 | Y.F.T. | 8 perch, 9 whitefish | 17 |
| 5 | 24/10/83 | 22.51 – 23.31 | 9,0–12,0 | 64 | C.V.T. | 29 perch, 37 whitefish, 2 dace | 68 |
| 6 | 25/10/83 | 18.33 – 19.03 | 9,2–12,0 | 64 | C.V.T. | 465 perch, 11 whitefish | 476 |
| 7 | 25/10/83 | 19.50 – 20.20 | 5,0– 8,0 | 64 | C.V.T. | 5 whitefish, 16 dace | 21 |
| 8 | 25/10/83 | 20.42 – 21.12 | 3,5– 5,5 | 64 | Y.F.T. | 0 perch, 6 dace | 16 |
| 9 | 25/10/83 | 21.48 – 22.18 | 3,5– 5,5 | 50 | Y.F.T. | 1 perch, 1 whitefish | 2 |
| 10 | 25/10/83 | 22.50 – 23.20 | 9,0–12,0 | 64 | C.V.T. | 1 perch, 3 whitefish | 4 |
| 11 | 26/10/83 | 18.30 – 18.57 | 14,0–17,0 | 64 | C.V.T. | 997 perch, 9 whitefish, 1 dace, 34 roach, 2 bream, 1 stickleback | 1044 |
| 12 | 26/10/83 | 19.25 – 19.55 | 3,5– 5,5 | 50 | Y.F.T | no catch because of technical defect | - |
| 13 | 26/10/83 | 20.36 – 21.03 | 3,5– 5,5 | 50 | Y.F.T | 19 perch, 2 dace, 11 roach | 32 |
| 14 | 26/10/83 | 21.37 – 21.53 | 0 – 2,0 | 50 | Y.F.T. | 180 perch, 1 whitefish, 3 dace 150 roach 1 pike-perch, 1 bream, 63 stickleback | 339 |
| 16 | 27/10/83 | 19.06 – 19.14 | 14,0–17,0 | 64 | C.V.T. | 2526 perch, 6 whitefish, 31 roach, 2 bream | 2 565 |
| 17 | 27/10/83 | 19.58 – 20.26 | 14,0–17,0 | 64 | C.V.T. | 109 perch, 20 whitefish, 1 dace, 1 pike-perch | 131 |
| 18 | 27/10/83 | 20.48 – 21.18 | 0 – 2,0 | 50 | Y.F.T. | 7 whitefish, 4 dace, 4 stickleback | 15 |
| 19 | 27/10/83 | 21.57 – 22.27 | 14,0–17,0 | 64 | C.V.T. | 31 perch, 66 whitefish, 2 dace, 1 stickleback | 100 |
C.V.T. = Commercial Vendance Trawl
Hauls not going deeper than 8 m are called surface hauls, in contrast to the deep ones. According to water depth (inshore - offshore), the hauls were grouped into 3 sets: near the 50 m - depth line, 60 – 150 m, and near the 200 m - line. To compare the gear efficiency of C.V.T. and Y.F.T up to three paired hauls could be utilized: nos. ⅓, 5/4, and 7/8. Summarized catches of the Y.F.T. are biased towards surface catches, and vice versa, because 6 out of 8 Y.F.T.s were made in the 0 – 8 m layer.
Fish length is that of Lt, without final stretching of the caudale. For example, the range 25.5 – 26.4 cm is calculated as 26.0 cm. Mean lengths of fluviatilis are calculated for the size range 12 – 20 cm (probably age group I), thus two smaller perch (probably AG 0) of two subsamples were excluded. Age was not determined directly, but deduced from the rather regular length frequencies (Figure 2), which were compared with earlier results on the growth of the fishes of Lake Constance (Hartmann, 1978).
The numbers of fish were converted into weight on the basis of the trawl catches, Florin (1978), and the raw data of Hartmann (1983).
The perch of haul nos. 6, 11, 14, and 16 (Table 1) as well as the sticklebacks of haul no. 15 were subsampled (n = 180, 128, 144, 146 and 32). The size distributions of the totals were then estimated proportionally.
The statistical treatment of the catch data was limited by the diversity of the 18 trawls with respect to water column, trawling depth, type of gear, and region of the Lake (see Table 1).
