Fish catch in rivers has been related to length (km) and drainage area (km2) by Welcomme (1974, 1985). Welcomme found a good correlation between drainage basin area of the river system in km2 and the catch obtained from it in tonnes for 20 rivers all of which were moderately to heavily fished.
The equation was (excluding catches from large flooded areas):
Catch = 0.03 Area 0.97 (r = 0.91).
As total length and basin area are also simply related, the equation relating catch (t) to main channel length (km) was:
Catch = 0.0032 Length 1.98 (r = 0.90).
Fish catch in floodplains was related to floodplain area (km2) for 25 tropical floodplains of which 14 were African floodplains (Welcomme, 1985: Tables 7.12 and 7.13).
Catch = 4.23 Floodplain Area 1.005 (r not given)
Fish yields in floodplain rivers have also been related to fishing effort (Welcomme, 1985: Figure 7.13).
Evaluation of the data set of 37 rivers using the information given by Welcomme (1985, pers. comm.) revealed that only some 19 rivers could be used for the models (Table 6). Most of the catches of these rivers are only known as single-year figures (see Table 6). The first five rivers in Table 5 were not in the list of rivers used by Welcomme (1985: Table 7.11) for calculating the above equations and six rivers in that list were excluded: the Nile (underexploited) and Zambezi, Benue, Oueme, Senegal and Rufiji/Ruaha (all with extensive floodplains). The other rivers of the original set of 37 rivers were either underexploited or had extensive floodplains.
African floodplains were selected from the data set of 25 floodplains used by Welcomme (1985) (Table 7).
The equations given by Welcomme (1985) were recalculated using the data sets of African rivers and floodplains given in Table 6 and 7. Plots of the relationships of drainage area and length against total catch for the rivers and floodplain area against total catch for the floodplains are given in Figures 4, 5 and 6.
The new models for the rivers are:
Catch (t/y) = 0.048 Drainage Area 0.93 (r = 0.95)
Catch (t/y) = 0.01 Length 1.81 (r = 0.92)
and the new model for the floodplains is:
Catch (t/y) = 8.78 Floodplain Area 0.90 (r= 0.93).
The data sets and models for the African rivers and floodplains are quite similar to those given by Welcomme (1985). This can easily be explained by the fact that several papers by Welcomme (1972, 1975, 1979, 1985) were used as key references for the Source Book. Only limited, reliable new information on rivers and no new information on floodplains became available with the publication of the Source Book.
