Limnological annual cycle inferred from physical-chemical fluctuations at three stations of Lake Tanganyika

peer reviewedTen variables were measured at least twice per month at three locations of Lake Tanganyika (East Africa) over one year (1993-94). Upwelling was observed in the south of the lake during the dry, windy season from May to September. Stratification was variable in strength but always present in the north. The lake showed a marked tilting of the epilimnion during the dry season (0-20 m in the South, 60-70 m in the North). This period was followed by oscillations of water masses towards an equilibrium when the strong winds from the south east ceased. Conductivity and pH fluctuations indicated dampened oscillations, particularly at the ends of the lake. Movements of the epilimnion toward an equilibrium position generated and/or re-inforced internal waves. These waves were inferred from fluctuations of chemical and physical characteristics of the lake. The concentrations of inorganic P and N commonly fluctuated by a factor of 3 or more in the epilimnion. The period of long-period internal waves was estimated to be ca. 28-33 days. Turbidity changes suggested pulse production caused by internal waves linked to non-random patchiness in nutrients and organisms. Turbulence resulting from highly dynamic physical events also induce random-patchiness in water composition. The lake water generally showed oligotrophic characteristics near the surface but had high concentrations of nutrients in deep water. The results showed that the trophic state of Lake Tanganyika, like that of the oceans, seems to depend largely on regeneration processes. The annual limnological cycle in Lake Tanganyika appears closely linked to the climatic conditions

The present status of the hook fishery and its impact on the fish stocks of Lake Victoria

Surveys on the hook fishery of Lake Victoria revealed that long lines are the most important gear used for catching Nile perch. Hand lines and hook and line are used to catch Nile tilapia and also Nile perch. Hooks are increasing because of their cheapness. They now total about 12 million lake-wide. Hooks are preferentially baited with live bait; they select the catch according to their sizes and bait type. Big range of fish sizes is caught. Hooks tend to reduce the size of the brood stock made up of big perch that carry millions of eggs. The use of live bait suppresses the recovery of resurging species with an adverse impact on the lake’s biodiversity. Use of the recommended hook sizes 4-9 should be enforced and overcapacity reduced for sustenance of the fish stocks. The farming of bait fish should be encouraged for standardisation of bait size to target the right size of the catch and spare the recovering species.Key words: hook fishery management, fish stocks, biodiversity, bait fish farmin

Heterogeneity in physical, chemical and plankton-community structures in Lake Tanganyika

From 28 August to 6 September 1995, we monitored the lake-wide physical, chemical and biological properties of the pelagic waters in Lake Tanganyika. The aim of this study was to examine the spatial environmental variability and its relation to fluctuations in plankton abundance and community assembly. Trade winds had triggered an overall downward tilt of the isotherms; accumulation of warm surface waters and intensified stratification towards the north along with upwelling increased mixing, combined with decreased stratification towards the south. Dissolved oxygen, turbidity, conductivity, phosphorus and chlorophyll-a were higher; whereas temperature and pH were lower in southern than in northern waters. We observed a transition zone (6 to 7°S) where vertical water column structures changed from northern to southern conditions. This did not include the river Rusizi affected regions north of 4°S. High spatial heterogeneity in nutrient supply and degree of mixing had a strong impact on the plankton community. While the northern environments appeared to be based on reduced internal nutrient loading, lower phytoplankton biomass and smaller sized zooplankton with an important role of Cyclopoids, the mixed environments in the south seemed to be based on increased phosphorus availability and larger sized zooplankton dominated by Calanoid grazers. We emphasise that wind-driven formation of a gradual change in physical and chemical properties of the productive zones along the north-south axis of Lake Tanganyika is of importance in regionally determining nutrient flows. This consequently affects plankton community structures and composition, and therefore food web structure and functioning

Fish catches from Lake Tanganyika mainly reflect changes in fishery practices, not climate

Recently it was claimed that an increase in regional temperature, related to global climate change, has resulted in substantial decline in the pelagic fish catches from Lake Tanganyika, East Africa. Surface temperatures of Tanganyika indeed show warming trends, but evidence for decreased productivity is ambiguous, and no overall decline in fish catches has been documented. In contrast, total lake-wide fish catches increased up to 1995, increased artisanal catches outweighing regional declines in industrial fisheries. We conclude that the present evidence is not sufficient to demonstrate the effects of climate change on fish stocks in Tanganyika. Evidence indicates rather that fishery intensification has been the major factor affecting fish populations

Challenges and opportunities for integrating lake ecosystem modelling approaches

A large number and wide variety of lake ecosystem models have been developed and published during the past four decades. We identify two challenges for making further progress in this field. One such challenge is to avoid developing more models largely following the concept of others (‘reinventing the wheel’). The other challenge is to avoid focusing on only one type of model, while ignoring new and diverse approaches that have become available (‘having tunnel vision’). In this paper, we aim at improving the awareness of existing models and knowledge of concurrent approaches in lake ecosystem modelling, without covering all possible model tools and avenues. First, we present a broad variety of modelling approaches. To illustrate these approaches, we give brief descriptions of rather arbitrarily selected sets of specific models. We deal with static models (steady state and regression models), complex dynamic models (CAEDYM, CE-QUAL-W2, Delft 3D-ECO, LakeMab, LakeWeb, MyLake, PCLake, PROTECH, SALMO), structurally dynamic models and minimal dynamic models. We also discuss a group of approaches that could all be classified as individual based: super-individual models (Piscator, Charisma), physiologically structured models, stage-structured models and trait-based models. We briefly mention genetic algorithms, neural networks, Kalman filters and fuzzy logic. Thereafter, we zoom in, as an in-depth example, on the multi-decadal development and application of the lake ecosystem model PCLake and related models (PCLake Metamodel, Lake Shira Model, IPH-TRIM3D-PCLake). In the discussion, we argue that while the historical development of each approach and model is understandable given its ‘leading principle’, there are many opportunities for combining approaches. We take the point of view that a single ‘right’ approach does not exist and should not be strived for. Instead, multiple modelling approaches, applied concurrently to a given problem, can help develop an integrative view on the functioning of lake ecosystems. We end with a set of specific recommendations that may be of help in the further development of lake ecosystem model