the fish population
of Lake Naivasha
Lakes of the Rift Valley IV, 24/07/02 to 07/08/02
Dr. Robert Britton, UK.
In addition, work was completed on the crayfish population of the lake. These data were collected to supplement the long-term crayfish study in Lake Naivasha. This investigates crayfish density in the littoral vegetation and their mean carapace size in both the littoral vegetation and open water habitats.
Secchi disk readings were also taken at all netting sites.
The principal fish species sampled at all sites was Tilapia zillii (Gervais). The distribution of the other major fish species in the lake, Oreochromis leucosticus (Trewewas), Micropterus salmoides Lacépède and Barbus amphigramma Blgr. was more patchy, with their distribution reliant on the habitat being sampled. O. leucosticus was found mainly in open water sites, M. salmoides in the rocky margins and B. amphigramma in the North Swamp.
There were no samples of C. carpio caught in the Earthwatch gill nets, but samples were collected from the commercial fish-landing beach. It appears from these catches that their population remains low in the lake with fish present between 250 and 500 mm, sizes that may preclude them from the Earthwatch gill nets. These data indicate that C. carpio has either yet to reproduce in Naivasha or has done so with only limited success and, therefore, has yet to establish.
Dietary analysis of M. salmoides revealed the species to be a generalised macro-predator within the lake. Diet composition was similar to previous years. At sizes up to 200 mm, invertebrates - principally Micronecta spp. - were the main food item. As body and gape size increased, there was a dietary shift towards fish and adult crayfish. At sizes of over 300 mm, adult crayfish comprised 89% of all ingested food items.
Stomach samples were taken from T. zillii, O. leucosticus
and C. carpio in order to ascertain if there was any overlap in
their diet at present. This may indicate any potential for competition
between the species should C. carpio become established in Lake
Naivasha in future. These samples will be analysed in the UK.
It is hoped that the fish population data can be used for the following
- a paper on The effect of the commercial fishery closure on the fish population of Lake Naivasha, Kenya, and the implications for management.' This would require analysis of catch statistics from the period before and after closure and Earthwatch gill netting data from before, during and after closure.
Both papers would require collaboration between workers in the UK and Kenya.
The density of crayfish in the littoral vegetation revealed their population was returning to levels last seen in the late 1990s, the period before the boom (~600 m2) and bust (~10 m2) population cycle of 2000 and 2001. The average density for all sites was 45 m2. However, their distribution was patchy in the lake. This was irrespective of actual water hyacinth coverage in the sampled areas. In areas around Joan Root's and Hippo Point, crayfish density in the littoral vegetation was typically between 30 and 150 m2, with an average of 49 and 63 m2 respectively. At the North Swamp, in the margins around the Loldia Hotel, crayfish density was comparatively very low, with an average density of only 18 m2.
These data were in contrast to the crayfish numbers captured by the traps. Numbers of trapped adults at both Joan Root's and Hippo Point were very low, average <1, maximum 4. This contrasts to an average of 19 per trap at Loldia Hotel, with a maximum of 34. At Loldia Hotel, the sex ratio was 27% female and 73% male. This may indicate a breeding area for the crayfish in the North Swamp with subsequent dispersion of juveniles to more favourable nursery areas.
In terms of mean carapace size, the average in the littoral vegetation (quadrant) was 17 mm and in the open water (traps) 45 mm. This reveals the juvenile crayfish preferred the refuge of the littoral vegetation with a gradual dissociation from these vegetated areas with increases in carapace size.
Overall, I believe the work on the fish and crayfish provided a good range of activities for the volunteers to participate in, with both projects complementing each other due to the prevalence of crayfish in aspects of the diet of M. salmoides. I believe both work aspects achieved their intended aims and provided interesting and thought provoking discussions among the volunteers.
Configuration and contours of Lake Naivasha
Hickley, P., Bailey, R.G., Harper, D.M., Kundu, R., Muchiri, S.M., North, R. and Taylor, A. (2003) The status and future of the Lake Naivasha fishery, Kenya. Hydrobiologia, volume 488.
Copyright 2002 - Dr. Robert Britton and Earthwatch Project
Lakes of the Rift Valley under Dr. David Harper
Photos by Naomi Sherer