(After Degerman, Nyberg, Appelberg, 1988, Enderlein and Wickstrom 1989, Fjalling and Furst 1990)
The Drottningholm standardized method uses benthic and pelagic multimesh monofilament nylon gill nets made by Lundgrens Fiskeredskaps-fabrik AB (Storkyrkobrinken 12, 11128 Stockholm Sweden). This multipanel gill net consist of 14 randomly distributed panels of various mesh sizes. Each panel is 3m long, and thus the total length of each net is 42m. The benthic nets are 1.5m high while the pelagic nets are 6m high. The pelagic nets are marked at 3m depth so that the catch can be separated into the upper and lower 3 m portions. The total area of the benthic nets is 63m2 while that of the pelagic nets is 252m2.
This Drottningholm test-fishing method was developed by the Institute for Freshwater Research, Drottningholm, 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 whole reservoirs, and estimate the relative abundance of the dominating species. It has been modified for use in other climates, including Eastern Zambia in Southern Africa.
Due to the shallowness of dams studied in Botswana, no pelagic nets have been used during test fishing; only one depth zone was recognised.
In Botswana, nets were set overnight from sunset to sunrise. The nightly catch from each benthic net was considered as one effort. The number of efforts needed with benthic nets in each depth strata depends on the surface area and maximum depth of the reservoir. The benthic nets were set randomly within each depth zone (one in Botswana) and with respect to angle to shore line.
The fish caught in each net were sorted by species. Fish were measured individually to the nearest millimeter by length and the nearest gram by weight. Total catch for one night's effort, catch per effort, was calculated as the sum of all fish caught by one net in one night. Empty nets were included when calculating average catch, the catch per unit effort (CPUE). The Drottingholm method transforms the catches per net using an In (CPUE+1) transformation, in order to obtain a distribution of CPUE closer to normal. In Botswana, however, only untransformed data was used.
Test fishing exercises in October and November 1990 in the Semarule dam in Botswana were used to adjust the method to Botswana environmental conditions. All panels with 6.25 and 8 mm mesh, were over-saturated with small Barbus spp., more than 600 g of fish per panel. Because of this, the gear was modified to remove the two smallest mesh sizes, 6.25 and 8 mm bar height, from the multipanel net -- which thus became 6 m shorter. The two small-mesh panels were joined together to form one net and were used during November to December 1990 and July to October 1991. These small-mesh nets were set separately during a shorter period of time and during the day and the results extrapolated to overnight catch figures. After the onset of the rain from late December 1990 to July 1991, it was not neccessary to set the small-mesh nets for a shorter period of time.
