The test fishing method proposed is adapted from the Drottningholm sampling method, developed by the Institute for Freshwater Research of Drottningholm in Sweden, to study the effects of liming on oligotrophic, acidified lakes in Sweden. The method is based on a careful standardization of fishing gear and procedures to minimize method-related variation in catch figures. It is used to estimate the number of fish species present, assess fish population changes on a whole reservoir basis, and estimate relative abundance of the dominating species.
The Drottningholm method uses benthic and pelagic multimesh monofilament nylon gillnets (made by Lundgrens Fiskeredskapsfabrik AB, Storkyrkobrinken 12, 11128 Stockholm Sweden) consisting of 14 randomly distributed panels of various mesh sizes in a near exponential series. Each panel is 3 m long, and hence the total length of each net is 42 m. The benthic nets are 1.5 m while the pelagic nets are 6 m deep. The pelagic nets are marked at 3 m depth so that the catch can be separated into the upper and lower 3 m portions. The total area of the benthic nets is 63 m2 while that of the pelagic nets is 252 m2. For a detailed description of the nets see Hammar & Filipsson (1985).
To increase the precision of the catch per effort estimates, the water body is stratified into depth zones. The number of benthic nets required in each depth zone (0–3m;3–6m; 6–12m) depends on the volume (surface area and max. depth) of the water body (Table A.; Nyberg & Degerman, 1988; Nyman, 1990).
The catch from each pelagic net consisted of two efforts, one from the 0–3 m zone and the other from the 3–6 m zone. The pelagic net area per effort (126 m2) is twice that of benthic net area per effort (63m2).
The benthic nets were set randomly within each depth zone and with respect to angle to shoreline. When setting the benthic nets care was taken not to cross depth zones, i.e. in depth zone 3–6 m nets were set at 4.5–6 m depth, in depth zone 6–12 m nets were set at 7.5–12 m depth. In any case the overlap in depth zone did not exceed 0.5 m.
In a preliminary adaptation for southern African context, benthic nets were not set in depth zones where dissolved oxygen concentration was less than 3 mg/1, since it is unlikely that fish are present under these conditions.
Table A. Number of bottom multimesh gillnets to be set in different depth zones in relation to surface areas and maximum depths of reservoirs. Minimum of nets set in any lake is 8 (from Nyberg & Degerman. 1988).
| <20ha | ||||
| Max depth (m) | <6 | 6–11.9 | 12–19.9 | 20–34.9 |
| Nets set at (m) | 1.5–6 | 7.5–11.9 | 13.5–19.9 | 21.5–34.9 |
| ∑efforts | 8 | 8 | 16 | 16 |
| Depth zone (m) | nets | nets | nets | nets |
| <3 | 4 | 3 | 4 | 4 |
| 3–5.9 | 4 | 3 | 4 | 3 |
| 6–11.9 | 2 | 4 | 3 | |
| 12–19.9 | 4 | 3 | ||
| 20–34.9 | 3 | |||
| 21–50ha | ||||
| Max depth (m) | <6 | 6–11.9 | 12–19.9 | 20–34.9 |
| Nets set at (m) | 1.5–6 | 7.5–11.9 | 13.5–19.9 | 21.5–34.9 |
| ∑efforts | 8 | 16 | 16 | 24 |
| Depth zone (m) | nets | nets | nets | nets |
| <3 | 4 | 5 | 5 | 5 |
| 3–5.9 | 4 | 6 | 5 | 5 |
| 6–11.9 | 5 | 3 | 5 | |
| 12–19.9 | 3 | 5 | ||
| 20–34.9 | 6 | |||
| 50–100ha | ||||
| Max depth (m) | <6 | 6–11.9 | 12–19.9 | 20–34.9 |
| Nets set at (m) | 1.5–6 | 7.5–11.9 | 13.5–19.9 | 21.5–34.9 |
| efforts | 16 | 24 | 24 | 32 |
| Depth zone (m) | nets | nets | nets | nets |
| <3 | 8 | 8 | 7 | 7 |
| 3–5.9 | 8 | 8 | 7 | 7 |
| 6–11.9 | 8 | 5 | 9 | |
| 12–19.9 | 5 | 6 | ||
| 20–34.9 | 3 |
