No abstract

As in Lake Victoria and Lake Kyoga, most of the native fish species of Lake Nabugabo have either been depleted or have disappeared since Lates niloticus, Nile perch, and Oreochromis niloticus, a tilapiine, were introduced. L. niloticus, O. niloticus, Schilbe intermedius, Brycinus jacksonii, and Rastrineobola argentea are the only species still abundant and widespread in the lake. Of the five haplochromine species endemic to Lake Nabugabo, at least one has vanished and the remaining ones are very rare. The prey of the Nile perch in Lake Nabugabo, with the exception of the prawn Caridina nilotica, which is absent from the lake, are similar to those the predator switched to in Lakes Victoria and Kyoga after haplochromines had been depleted. These consist of ephemeropterans—especially Povilla, the anisopteran nymphs R. argentea, tilapiines, and juvenile Nile perch. Three small lakes adjacent to Lake Nabugabo—Lake Kayanja, Lake Kayugi, and Lake Manywa—contain large numbers of Oreochromis esculentus and two haplochromine species previously known only in Lake Nabugabo. These three lakes should be designated conservation areas for the cichlids and other native species that were previously abundant in Lakes Victoria, Kyoga, and Nabugabo. The swamps around these lakes should be protected so that the Nile perch does not spread into them. Further research should be carried out to ensure the preservation of the endemic species.

Wetlands may protect fishes from introduced predatory fishes by providing both structural and low‐oxygen refugia for prey species tolerant of the conditions that prevail in these habitats. We examined the potential of wetlands as refugia for fishes in Lake Nabugabo, Uganda, where increased numbers of an introduced predator, the Nile perch (Lates niloticus), coincided with the decline or disappearance of many indigenous species in the main lake. In 1993 and 1994 we sampled fishes in three wetland habitats: the marginal wetland ecotones of the lake, wetland lagoons separated from the lake by densely vegetated marsh habitats, and a stream separated from the lake by dense papyrus swamp. Fish distributions in these wetlands were then compared to results from two earlier fish surveys: the 1962 Cambridge expedition to Lake Nabugabo, which was conducted prior to the increase in the Nile perch population; and a 1991–1992 survey of the open lake, which reported the disappearance of 16 indigenous species. In our 1993–1994 surveys 9 of the 16 species not recovered in the 1991–1992 open‐lake survey were found in the wetland ecotones or beyond the margins of the lake in wetland lagoons and tributaries. Three of these species were found only beyond the margins of the lake in the tributaries and lagoons within extensive wetlands. Three endemic haplochromine cichlids were abundant offshore in 1962. Of these species one has disappeared and the others are now largely confined to inshore areas. Other species that were abundant in the open waters of the lake in 1962 (e.g., the lung fish Protopterus aethiopicus) are now found primarily in wetlands. Two species, the characid Brycinus jacksonii and the cyprinid Rastrineobola argentea, are still abundant in the open waters. This study highlights the need for quantitative survey work to identify wetland refugia in the Lake Victoria Basin and suggests that some species thought to have disappeared in the mass extinction of fishes in Lakes Victoria and Kyoga may still survive in refugia. Some fish populations could recover under effective ecosystem management.

No abstract

The fish stocks of Lakes Kyoga and Victoria have changed since Nile perch, Lutes niloticus (L.), was introduced, and this is reflected in the prey ingested by the predator. Initially, haplochromine cichlids constituted the main prey of most sizes of Nile perch. As the stocks of these have declined, Caridina nilotica (Roux) and Anisopteran nymphs have become the dominant food of the juveniles, while Rastrineobola argentea (Pellegrin), juvenile Nile perch and Oreochromis niloticus (L.) have become the main food of larger Nile perch. Apart from R . argentea, most of the native fish species of these lakes have disappeared. The stocks of Nile perch in Lake Kyoga, to which it was introduced earlier than to Lake Victoria, have declined after dominating the fishery since 1965. and have been superseded by 0. niloricus. an introduced herbivore. Similar changes are now occurring in Lake Victoria. The Nile perch might not maintain the high yield realized in the two lakes when haplochromines were abundant. It is therefore necessary to exercise caution with high and long-term investments aimed specifically at developing the Nile perch fishery.

In Lake Nabugabo, Uganda, a small satellite of the equatorial Lake Victoria, approximately 50% of the indigenous fish species disappeared from the open waters subsequent to establishment of the introduced predatory Nile perch (   Lates niloticus ). However, several of these species persisted in wetland refugia. Over the past decade, Nile perch in Lake Nabugabo have been intensively fished. Herein we report a resurgence of some indigenous species in open waters. In a multiyear study, we used annual transects in inshore and offshore waters of exposed ( no wetland ) and wetland habitats to document the pattern of resurgence. In 1995, haplochromine cichlids were largely confined to inshore areas, particularly wetland ecotones, and were rare in Nile perch stomachs, as were most other indigenous species. By 2000 haplochromine cichlids were abundant in inshore and offshore areas of both exposed and wetland transects. Several indigenous noncichlids also reappeared in the main lake, including three of the four original mormyrid species. Between 1995 and 1999, there was a dramatic increase in the proportion of haplochromines in the diet of Nile perch. When haplochromines were rare ( 1995 ), Nile perch switched from an invertebrate‐dominated diet to piscivory at a large size ( 30 cm total length ). In 2000, however, Nile perch were strongly piscivorous by 5–10 cm total length. The pattern of faunal loss and recovery in Lake Nabugabo demonstrates the importance of refugia in providing the seeds of resurgence and provides a model with which to understand some changes in Lake Victoria.

Fish introductions in Africa have been made at various spatial scales from small fish ponds to the largest lakes, primarily to sustain or increase production, though some were to develop sport fisheries and to control unwanted organisms. Some introductions have fulfilled their objective in the short term, but several "successful" introductions have created uncertainties about their long-term sustainability. Lates niloticus, Oreochromis niloticus, O. leucostictus, Tilapia melanopleura and T. zilli were introduced into Lakes Victoria and Kyoga in 1950's and early 1960's; by the 1980's L. niloticus and O. niloticus dominated the fisheries, having virtually eliminated a number of endemic species. In Lake Victoria, the loss of genetic diversity has been accompanied by a loss of trophic diversity; the transformation of the fish community coincided with profound eutrophication (algal blooms, fish kills, hypolimnetic anoxia) which might be related to alterations of the lake's food-web structure. By contrast, the introduction of a planktivore, Limnothrissa miodon into Lake Kivu and the Kariba reservoir has established highly successful fisheries with little effect on the pre-existing fish community or trophic ecology. The endemic species-rich African Great Lakes may be particularly sensitive to introductions. Species extinctions, introgressive hybridization and ecosystem alterations may occur following introductions.

Fisheries resources support livelihoods of fishing communities but are threatened by over-exploitation, habitat degradation, pollution, invasive species and climate change. Unlike the other threats, climate change has received limited consideration and reducing its risks requires appropriate adaptation strategies. This study used quantitative and qualitative methods to generate knowledge on fishers' perceptions of climate change, changes in climate variables and their impacts on livelihoods, adaptation strategies, constraints to adaptation and required interventions to promote adaptation strategies that would enable fishers to build resilience to sustain their livelihoods. We found that fishers were aware of changes in climate conditions manifested by unpredictable seasons, floods and droughts. Fishing remained the main livelihood activity. However, the dominance of fishes had changed from Nile tilapia (Oreochromis niloticus L.) to the African catfish (Clarias gariepinus Burchell). Floods and droughts were associated with damage to gears, boats, landing sites and changes in fish catches and sizes, income from fishing and fish consumption. The fishers adapted by increasing time on fishing grounds and changing target species and fishing gear among other things. Some innovative fishers diversified to high-value crops and livestock. This increased their income beyond what was solely earned from fishing which provided an incentive for some of them to quit fishing. Livelihood diversification was enhanced by use of communications technology, membership of social groups, increasing fishing days and fishing experience. Adaptation was, however, constrained by limited credit, awareness and access to land, which require interventions such as improving access to credit, irrigation facilities, appropriate planting materials and awareness raising. We identified adaptation strategies, which if promoted and their constraints addressed, could increase resilience of fishers to the influence of climate change and sustain their livelihoods.