Limnology and fisheries potential of Lake le Roux

Abstract

The fisheries potential of the reservoir in the Orange River has been determined by the combined studies of ichthyologists and limnologists. The combination has been synergistic. It has been shown that the physical structure of the reservoir, coupled with its short retention time of less than one year and the rapid attenuation of light by the high suspensoids, are together responsible for the low summer temperature of the lake. Because of this the overall metabolism of the lake is low. It is argued that this is the primary factor in control of the reservoir's production of fish. Included in this is the effect of rapid light attenuation upon the photosynthesis of phytoplankton and therefore the production of immediately assimilable plant cells such as diatoms which are grazed by filter feeding zooplankton. Dense blooms of Microcystis aeruginosa do occur during the second half of summer but they do not seem to increase the standing stock of zooplankton upon which the principal fish with a pelagic life history stage, the smallmouth yellowfish, feed in the subadult stage. This important food fish selects the large zooplankters individually, depending on vision to catch its prey. The maintenance of high suspensoid concentrations therefore reduces the efficiency of foraging. At age 2+ years a return to a more inshore feeding mode is made when considerable natural mortality, up to 80% by the end of the 3rd year, occurs. Annual breeding takes place regularly, confined almost exclusively to the flowing Orange River at the lake's upper end. In contrast the other important food fish, the Orange River labeo, spawns irregularly depending on more local rainfall; therefore strong year classes are only produced in some years. Total annual yield is provisionally assessed at 150 - 250 tons, giving, at R 1/kg, a minimum value of R 150 000 per annum. It is therefore recommended that a commercial gill-net fishery be established, primarily for yellowfish, with a mesh size chosen to optimize the capture of subadults up to 30 cm long to harvest these before they die of starvation. Larger fish would remain and form the basis of a recreational angling fishery. This management proposal is unorthodox but essential if the productivity of the lake is to be used economically. Recommendations include the careful monitoring of the commercial catch in conjunction with continued assessment of the response of the stocks to this policy by fisheries biologists of the Cape Department of Nature and Environmental Conservation.