A century of drastic change: Human-induced changes of Lake Victoria fisheries and ecology

摘要

Lake Victoria biophysical and geochemical status has changed dramatically within an unprecedentedly short time scale driven by human actions. These actions can be broadly classified as escalated fisheries exploitation, biomanipulation (characterized by species introduction) and catchment processes. A chronology of these activities since 1901 to present times is described and, where possible, related to changes in the ecology. The impacts of fishing are investigated using catch and effort data that have been consistently recorded for the past two decades. It is evident that water transparency in the offshore decreased from close to 8 m in late 1920s to less than 2 m in the 1990s before improving to about 4 m in the 2010s, a reflection of change in ecosystem functioning. The native tilapias that constituted the commercial fishery initially have since been replaced by introduced species and dagaa (Rastrineobola argentea) following biomanipulation events. Analysis of unified fishing effort index (E) reveals that in the last two decades fishing capacity has increased close to 8-fold in Kenya and Uganda and about three times in Tanzania. Illegal/prohibited gear have increased by 4.7 +/- 1.9 folds whereas legal gear increased by 2.6 +/- 0.8 times in the whole lake during since the year 2000. The catch per unit effort (CPUE) for Nile perch, tilapia and "others" declines with increasing effort while that for dagaa exhibits no clear trend. Haplorochromines catches and catch rates are increasing probably due to predatory release by the Nile perch. Tilapia and other fish catches and catch rates have declined with increasing harvesting capacity clearly indicating that they are over-fished. Catch and effort dynamics indicate that dagaa is still abundant in Lake Victoria. Overall, catches in Lake Victoria increase with increasing fishing effort but catch composition has shifted to one dominated by small pelagic species. It has been clearly shown that events leading to biomanipulation and processes in the catchment can fundamentally change a fishery in a different way as compared to fishing and hence the need for Ecosystem Approach to Fisheries Management (EAFM) in Lake Victoria.