The research programme currently underway was established to provide an understanding of the seasonal changes in Kainji Lake as well as a summary of the conditions presumed to be related to potential production of fishes in the lake. In evaluating the work up to the present, it is appropriate to suggest changes in the current research programme and to suggest new projects which appear to be important to the development of the fishery of the lake.
In the preceding chapter a number of applications of the findings of this research were suggested. These suggestions were primarily intended for use by fishery scientists in interpreting and planning their own work, and hence only indirectly provide guidance for development itself. The recommendations presented in this chapter similarly relate to the strategy of further fishery research, while recommendations for management and development of fisheries are expected to follow from the fishery rather than the limnological work.
The general pattern of seasonal changes of the physical characteristics of the lake are now clear, at least for the open waters of the lake. With this background available, the number of sampling stations in the main parts of the lake can be reduced, keeping Stations 3 (lower arm), 7 (western half of the central basin), 11 and 12 (each side of Foge Island) and 15 (eastern half of the central basin).
In place of the omitted stations, stations should be added farther inshore, at the mouths of the Swashi and Kpan rivers, and over the eastern shallows near Papiri. These will provide information on inshore conditions in those areas now thought to be deficient from the conditions described for the lake as a whole.
From the analysis of the heat budget of the lake, it appears that the year-to-year variations in temperature of the lake are closely related to the amount and temperature of the inflow. More precise information on the temperature of the inflow is needed to check this finding and to provide a basis of predicting the annual differences in the thermal regime of the lake. Owing to the considerable inconvenience of taking the large research boat into the northern arm, it is suggested that the temperatures at the Rofia ferry crossing be obtained during fishing operations in that area by the fishery crew of the project. As the temperature should be obtained for at least three depths (0.5 m, mid-depth, 1 m from bottom) it would be desirable to obtain an electronic thermometer with a long (30-m) cable for the fishery programme. Such a unit would considerably simplify the present temperature measurements made by the fishery programme.
Limnological studies have shown that turbidity is the most reliable, easily measured characteristic identifying the several kinds of water masses (white and black floods, lateral inflow) which mix in the lake determining local chemical characteristics of the water. At present it is only possible to measure turbidity readily during periods of high turbidity. While a transmissometer was ordered early in the programme, the order had to be abandoned owing to difficulty with the supplier. It is recommended that another attempt be made to obtain a wide-range remote transmissometer (electric), equipped with tri-colour filters, in order to provide more complete information on changing water conditions.
The project has been cooperating with the University of Ife on both chemical studies and studies of isotopic composition of the lake water. As both programmes are of considerable significance, but are beyond the technical and logistic capabilities of the project, both should be encouraged.
The gross chemical characteristics of the lake can be followed more readily by project personnel than by personnel of the University of Ife. Occasional detailed chemical analyses, (especially of phosphate, nitrate and silica) needed to interpret the changes observed in productivity and plankton composition are best obtained by the Ife group. While a few such analyses have already been made by the University of Ife, both before and after impoundment, anticipated joint expeditions have not been possible to date. Further effort should be made to arrange such joint trips.
Samples for isotopic analysis have been collected by the project and sent to the International Atomic Energy Commission (University of Heidelberg) for analysis. The interpretation of these results is the responsibility of the University of Ife and the International Atomic Energy Commission. The results to date have been of great significance in verifying the sources of water to the lake, and may be expected to provide checks on water balance within the lake. While no changes are needed in the programme, the number and location of samples obtained could well be changed to fit the suggested changes in the stations visited during the regular limnological cruises.
The initial phase of the biological programme was largely concerned with identification of the various organisms inhabiting the lake. Quantitative work on their abundance and productivity, the second phase of the research, has only begun. These studies should be continued through at least one full year, while a second year would provide a valuable check on the reality of seasonal changes. It is suggested that priority be shifted from the present studies of plankton organisms to studies of the inshore and bottom organisms after a full year's study of the former, while maintaining a check on the plankton work completed.
It is now clear that the meteorological information obtained on the regular cruises of the lake is not very meaningful in relation to the effects of weather on the lake. The present meteorological stations are either poorly located for limnological purposes or incomplete. Plans are underway to improve the stations at Shagunu, Yelwa, and Kainji, and to add one at Papiri in connexion with the project's programme of irrigation.
Within one year of the completion of these improvements, that is, with a full year of records, a further analysis of wind and radiation balance for the lake would be desirable, as the present data do not permit reasonable assessment of the effect of change from river to lake on local conditions of wind, humidity, temperature and evaporation. While the effects of the lake are expected to be significant only within a few miles of the lake, agricultural, fishery and transport developments on or near the lake are somewhat hampered by the lack of reliable data on these conditions. If possible, the analysis should include a study of wave conditions on the lake in relation to the local winds to provide a basis for advice to fisherman on lake hazards in the central basin. While a pole for measuring wave heights was constructed for the project to be used on the routine cruises, systematic measurements of wave heights were postponed until such time as valid wind data were available.
While several of the studies suggested here are basically limnological, others are suggested to assist in bridging the relation between environmental characteristics and fishery implications and hence involve other research programmes. Several are essential to the utilization of data given in this report for guiding the management of the fisheries of the lake.
7.2.1 Meteorological and Climatic Studies
7.2.2 Total Dissolved Solids and Predictions of Catch
As discussed in Section 6.1.2, a hypothesis has been advanced relating expected catch of fishes to the morphoedaphic index. To date, the value of total dissolved solids used in calculating the index for Kainji has been calculated from other data. The necessary equipment for measuring the value directly is now available at the project, A careful study of the relations among total dissolved solids, turbidity, total ionic concentration and conductivity should be undertaken and can readily be accommodated in the present limnological programme. This should provide a basis for re-evaluating the predictions made on catch and their relation with the results of the catch surveys now underway at the project.
7.2.3 Variability of Limnological Factors
While the general features of the physical limnology of the lake have now been described, there remains some uncertainty about short-term and local variation. In addition to the recommendations made, the following new studies seem both desirable and feasible within the present resources of the project.
Temperature Variation
The present measurements of temperature at selected stations have given some indication of differences from place to place on the lake which are not easily interpreted. It is recommended that a surface temperature recorder be installed in the research boat so that the changes in temperature from station to station can be interpreted in terms of data obtained between stations1.
Changes in Chemical Composition with the Floods
While the present data suggest little variation in conductivity and other measures of chemical composition over the year, data from White et al. (1965) suggest that substantial changes in the composition of the inflow occur during a period of about three weeks when the local and white floods are rising. An intensive study of this period could be readily undertaken from the station at Shagunu. The field work could be handled by the Limnological Technician using a small boat.
7.2.4 The Distribution and Feeding Habits of the Small Pelagic Fishes of the Lake
In Section 5.2.3 it was suggested that the scattering layer observed with the echosounder throughout the lake was produced by small fishes, probably the clupeids Sierrathrissa and Pellonula. If these prove to be as abundant as the echosoundings suggest, they will necessarily comprise a very significant element either of the developing fishery directly, or as food for the predators which are now being fished. It has been proposed that the counterpart limnologist should use part of his time in examining the relationship between the zooplankton of the lake, which he is currently studying, and these clupeids which presumably obtain a significant portion of their food from the zooplankton. This work should be encouraged while, in addition, it is recommended that someone with experience in interpreting echosoundings, preferably with experience with pelagic shoaling species, be invited to Kainji for several weeks to train project personnel in the use of the echosounder and to assist in interpreting the records that are now being obtained.
7.2.5 Physiological and Behavioural Studies of the Species of Fishes of Kainji Lake
The application of limnological knowledge to practical fishery management is largely dependent upon an understanding of the physiological requirements and behavioural preferences of the species which comprise a fishery. Unfortunately this latter understanding is much poorer for the species inhabiting tropical waters than for those of the temperate regions. The following recommendations are made to help bridge the gap between the limnological studies and practical fishery management:
A thorough review of the literature pertaining to ecological life histories of tropical freshwater species should be undertaken, perhaps by FAO, emphasizing the environmental requirements of those species which are commercially important in the tropics.
Universities throughout Africa should be encouraged to undertake such studies which are rather too specialized and time-consuming to be made a part of such projects as the Kainji Lake Research Project, While these studies are a direct contribution to the development of a fishing technology, they remain clearly in the category often regarded as “basic” research.
7.2.6 Long-range Changes in the Productivity of the Lake
It has been noted that gradual changes in the productivity of Kainji were most likely to follow indirectly from changes in land use in that part of Nigeria and West Africa which supplies water to Kainji Lake. The Kainji Lake Research Project is in a favoured position to integrate the findings of several kinds of disciplines as they relate to changes in fertilization, burning of grasslands, grazing lands and distribution of human populations. All these are pertinent to the future productivity of the lake, for better or for worse.
While some significant study programmes in fishery limnology are limited by available knowledge, time and equipment, the present capability of the project for limnological research seems adequate for immediate needs. It may prove advisable to engage a consultant in echosounding for a short period of time.
In relation to new limnological programmes, further emphasis should be given to cooperation with Nigerian universities. The project should retain responsibility for integrating the work of such institutions with the specific demands of fishery development.
