A study of the biological and economic factors affecting the kapenta (Limnothrissa miodon) population and yield in Lake Kariba
Mahere, Tafadzwa Stephen
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This study investigated some of the variables that influence the fishery of Limnothrissa miodon (kapenta) in Lake Kariba, namely, biological (limnological factors, zooplankton composition and densities) and economic (fishing effort and profitability in the industry). The findings were then assessed to identify relationships with kapenta catches. There were indications that the Lake had warmed up since limnological findings made in previous studies. The mean depth profile temperature recorded for July 2011 of 22.9ºC was 4.3ºC higher than the mean depth profile temperature recorded for the same month from 1987-1999. The mean depth profile temperature value also represented a 5.1ºC increase over the mean depth profile temperature recorded in 1967-1968. Thermoclines and oxyclines in the open water stations were established at depths between 5 and 10 m during the cool dry season and post-turnover period, which agrees with recent observations that the thermocline is migrating upwards. The Annual Birgean Heat Budget was 18 869 cal. cm-2 year-1, an increase of 4 800 cal. cm-2 year-1 from the data for 1967-68. It is clear the heat content of the lake has increased by about 25% in the last 45 years consistent with recent results on the possible impact of global climate change on lake temperatures and heat content. The zooplankton composition and densities in the Sanyati Basin were assessed to validate whether zooplankton densities had declined to the extent of limiting food availability of Limnothrissa miodon. The composition and densities were compared with historical records. There were indications in this study that zooplankton densities have declined considerably in Lake Kariba over the past 25 years. Bosmina longirostris, which recorded peak densities of 16 ind./L and 11 ind./L during the turnover period in 1986 and 1987 respectively, only recorded a density of almost 0.5 ind./L in July 2011. In 2011 the Sanyati Basin (Basin 5) was the most productive basin in Lake Kariba with over 60% of the total annual catch recorded on the Zimbabwean side. However the catch per unit effort (CPUE) slumped from 0.6 in 1997 to 0.13 in 2009. The extent of low catches was exemplified by the total mean monthly catch recorded for Basin 4 of 47.0 tonnes, which was lower than the mean catch recorded for a single rig in 2000 (50.2 tonnes). A comparison of MSY estimates with the trend in annual catches indicated that the MSY was surpassed in the mid 1980s. The argument about the infinite resilience of L. miodon given the current fishing pressures needs to be revisited. The trend in catch per unit effort suggests that the lake is currently experiencing a high fishing pressure and there is need for measures to control this. An assessment of the profitability of the industry showed that the industry was viable despite challenges of declining fish stocks. However, it was predicted that if this trend in declining catches continues small companies will shut down as it will not be economically viable. Consequently, the structure of the industry will revert to being inequitable as was the case pre-1990 and the whole diversification of the sector would have failed. From this study it is recommended that the National Parks and Wildlife Authority addresses the problem of illegal fishing and thefts. There is also need to reduce and rationalise fishing effort in Lake Kariba. Further research is recommended to fully understand the food web dynamics in lake given the limnological changes that have taken place.