Emmanuel Mashonjowa

The implications of a changing climate on the Kapenta fish stocks of Lake Kariba, Zimbabwe

The influence of climatic variables (rainfall, temperature and evaporation rates) and lake water levels on the stocks of the sardine fish species Limnothrissa miodon (Boulenger), commonly known as Kapenta in Lake Kariba, was investigated. Secondary data of the climatic variables, water levels and fish catches recorded from 1963 to 2008 were analysed to determine their trends over time as well as the relationships among them. The analyses showed that rainfall is decreasing at a rate of 0.63 mm per year around Lake Kariba, while evaporation rates have increased by 31% at an average rate of 2.77 mm per year since 1963. The temperatures around the Kariba area have been rising since 1964; with the maximum range increasing at a faster rate than the minimum temperatures. Kapenta fish production has decreased significantly (R 2 =0.85, P≤0.05) since 1974 at an average rate of 24.19 metric tons per year. This pattern of decrease was also observed in the artisanal fish catches that have declined at an average rate of 37.26 metric tons per year between 1974 and 2003. All the climatic factors as well as the water levels could explain variations in the Kapenta fish catches with the water levels exerting the greatest influence (R 2 =0.84, P 0.05); followed by maximum temperature (R 2 = 0.72, P≤0.05), evaporation and rainfall. In turn, water levels are largely influenced by climate with temperature and rainfall explaining a significant portion of the variation in the water levels (R 2 =0.99, and R 2 =0.93, P≤0.05) in that order. This suggests that both climate (maximum temperature in particular) and nutrients, which are influenced by water levels, are the primary determinants of Lake Karibas Kapenta production. Concerning are the possibilities that a changing climate in and around the lake may continue to adversely affect water levels, the stratification cycle, nutrient fluxes and the Kapenta fish production in the lake.

Measurement and Simulation of the Ventilation Rates in a Naturally Ventilated Azrom-Type Greenhouse in Zimbabwe

A simple greenhouse ventilation model, based on the stack and wind effects (the main driving forces for natural ventilation) was adapted, calibrated, and validated using measured air renewal rates in a three-span naturally ventilated Azrom-type greenhouse in Zimbabwe. Crop transpiration rates were monitored using stem heat balance sap flow gauges installed on the main stems of rose plants to continuously monitor whole-plant transpiration (WPT). This allowed continuous and automatic determination of full scale air renewal and leakage rates using the water vapor balance method. The model was fitted to experimental data of ventilation rates, and discharge and wind effect coefficients were determined. The results show a good fit between measured and predicted values (R2 = 0.80 and 0.82 for winter and summer, respectively), although there is a general over-estimation of the greenhouse air renewal rates, particularly during the night. The model, nevertheless, adequately describes the natural ventilation process in the greenhouse all year round. The model can be used as a design tool to evaluate and optimize the effects of different ventilation configurations and strategies on greenhouse air renewal rates, and as a component in a greenhouse climate model in order to further evaluate the effects of ventilation strategies on the inside greenhouse and crop microclimate, and thus lead to better greenhouse climate control.

The decline of Kapenta fish stocks in Lake Kariba – a case of climate changing?

curve fitted to Figure 1 by Ndebele-Murisa et al. (2011; their Figure 13) fails to fit to this upward trend, but continues the decreasing trend of 1998 to 2000. In fact, they specifically state that the trend is one of “decline (1992–2002)” (their page 111), and ignore the upward trend for the last two years. I would challenge such an oversimplification of the data shown from 1992 to 2002 in Figure 1. Whether this reversal from the year 2000 is significant and sustained can obviously be debated in the absence of relevant data, but I would argue that the entire trend seen in Figure 1 can be attributed to direct human activities and human-induced environmental actions with no necessity for any climatic influence. The writer does not deny that possible climatic change is an important issue, and its long-term effect on the environment and human survival requires constant assessment. However, the extremely biased interpretation of (for example) these fish catch data illustrate a paranoia and pre-occupation with this issue to the exclusion of the many other contributing factors, and is detrimental to a scientific evaluation of its effect, and also on the perception incorrectly generated by the public. The conclusions reached in the paper by Ndebele-Murisa et al. (2011) are a disservice to science because they isolate scientific data from the social complexities and weed invasion — the real World. Human intervention is necessary on all time-scales, and it appears (Tumbare, 2008) that the Zambezi River Authority is undertaking an active and holistic management programme that has immediate and short-term effects, the consequences of which deserve more recognition over the more purely academic interpretations.

Design, fabrication and testing of a low cost Trunk Diameter Variation (TDV) measurement system based on an ATmega 328/P microcontroller

Abstract A fast-responding inexpensive dendrometer is needed for measuring daily or hourly plant growth responses to water stress. This study reports on a low-cost microcontroller driven Trunk Diameter Variations (TDV) measurement system that was designed and constructed for automating the measurement and recording of plant stem diameter. The TDV system comprises of an ATmega 328/P microcontroller, which forms the heart of the controlling circuit, a real time clock for time stamping measurements, liquid crystal display for displaying purposes and an external secure digital card and shield for storing measured data. The stem diameter variations are measured by a caliper-type sensor based on a full bridge strain gauge that is attached to a flexible arm of mild steel. When bearing bending strain, the excited full bridge strain gauge outputs a voltage directly proportional to the bending strain, hence linear displacement. The TDV sensor displacement calibration procedure was carried out using an inside micrometer from a calibration kit. The calibration of the sensor indicated a linear relationship between the displacement and the sensor’s output voltage with a high coefficient of determination value of 0.998, calibration multiplier of 0.426 mm V−1 and an offset output voltage of 0.688 V. The TDV sensor was mounted on a smooth cylindrical dried wood away from direct sunlight and rain while air temperature was measured close to the TDV installation. Temperature sensitivity test results show that the accuracy of the sensor above 20 °C is 0.031 mm and for temperatures below 20 °C the accuracy is above 0.050 mm. The TDV system was tested on a tomato plant for 42 days, Fountain tree for three weeks and on a Citrus tree for one week under greenhouse and open-field conditions in order to evaluate its performance and suitability under different plant species and climatic conditions. The TDV system proved to be robust and produced valid results on the plant’s physiological measurements, with a good storage of the measured data over the experimental period. We concluded that the TDV system is suitable for stem diameter variations measurements and water and fertilizer stress monitoring on herbaceous and woody stem plants due to its fast response (hourly or less), ease of construction and installation, and low cost (