Denis Tweddle

Lake Malawi: fishes, fisheries, biodiversity, health and habitat

Lake Malawis fishes are a source of food for millions and provide a livelihood for thousands by encouraging tourism, fascinating the scientific fraternity, enchanting aquarists around the world and maintaining ecosystem processes in the lake. From a fisheries and resource assessment perspective, the region is data-poor, but there is sufficient peer-reviewed and grey literature on the limnology, fisheries and ichthyofauna of the lake to provide a good overview of the state of the fishery. There are signs of over exploitation and an increasing fishing effort has resulted in decreased catch rates, depletion of larger, more valuable species in the fishery and species changes. The fishery is harvesting stocks that were formerly thought to be under exploited. Previous attempts to manage the fishery have been ineffective and long term strategies addressing overfishing will need to transform the fishery from an open-access to a limited access system. As important as direct intervention in the management of the fisheries, will be the management of catchment processes. Increased nutrient inputs; changes to the phytoplankton composition; sediment loading; nearshore water quality impacts and changing water levels threaten the ecosystem. Introduction of alien invasive organisms is an ever present threat to the ecosystem as well, due to continued development of small scale aquaculture in the region. The overriding causative factor for all these effects is the poverty of the lakeshore communities which do not have the economic privilege of being able to adapt their utilisation patterns.

An assessment of a proposal to eradicate non-native fish from priority rivers in the Cape Floristic Region, South Africa

Non-native fish are considered the most important threat to the survival of the indigenous freshwater fishes in the Cape Floristic Region (CFR). A pilot project to evaluate the use of the piscicide rotenone to eradicate non-native fish from selected reaches in four rivers has been proposed by CapeNature, the conservation authority of the Western Cape province, South Africa. Each river has unique characteristics and challenges to achieving the eradication of non-native fish and the restoration of its indigenous fish assemblage. The proposal is described and the management methods available for reducing the impact of non-native fish on indigenous species are discussed. An environmental impact assessment (EIA) concluded that the project was justified and necessary, the choice of rivers sound, and supported the use of piscicides. The need for the project, the selection of sites and the findings of the EIA are discussed. It is expected that the project will be successful while having minimal impact on other aquatic fauna. The successful completion of the pilot project could help establish methods to eradicate non-native fish from critical biodiversity areas in South Africa. Such projects must, however, be part of a comprehensive conservation management plan to be implemented by conservation agencies in the CFR.

Overview of the Zambezi River System: Its history, fish fauna, fisheries, and conservation

The Zambezi River has a catchment area of 1.32 million km2, including parts of eight countries. Three divisions of the river are recognised: the Upper Zambezi separated from the Middle Zambezi by Victoria Falls, and the Lower Zambezi below Cahora Bassa gorge. The Okavango River is also linked to the Upper Zambezi system in wet years in an area of complex geomorphological history. Habitats include forested headwater streams, extensive floodplains, deep gorges, two large man-made lakes and an extensive delta. On floodplains, subsistence fisheries exploit the natural seasonal cycles, while the man-made lakes have commercial-scale fisheries for introduced kapenta, Limnothrissa miodon. Aquaculture is on a small scale, though with larger commercial cage culture enterprises on Lake Kariba. This paper summarises current knowledge on the fish faunas and their origins, the status of the different fisheries and their management, and the conservation status of the rivers resources.

Rain and Copper: The Evolution of a Fish Marketing Channel in a Rapidly Changing Region of Southern Africa

This case study of a fish market near the border of Namibia and Zambia examines how economic and environmental changes have affected the marketing channels associated with fishery in southern Africa. We monitored the volume and origin of fish entering the market in Katima Mulilo, Namibia, between 2007 and 2012, and conducted semi-structured interviews with fish committee members and market vendors. Prior to 2010, the market for fish had a simple commodity chain, involving local producers and consumers. Since then, a nearby lake has refilled, providing a new source of fish. The market has expanded and diversified, with large volumes of fish now being sent to consumers in urban centres in Zambia and even to the Democratic Republic of Congo. We consider the sustainability of the fishery in relation to expansion and re-alignment of the marketing channel and the degree to which intra-regional trade in fish promotes or hampers food security and sustainable use of the resource.

Ecology, fish and fishery of Lake Liambezi, a recently refilled floodplain lake in the Zambezi Region, Namibia

Lake Liambezi (300 km2) refilled in 2009 after a prolonged 22-year dry period. Its aquatic macrophyte populations, fish fauna and fishery shortly after refilling are described. The emergent aquatic macrophyte Phragmites australis formed dense stands covering large parts of the lake, while extensive beds of submerged Lagarosiphon ilicifolius and Najas horrida occurred in shallower areas. Twenty-nine fish species were recorded in experimental gillnets. Catches were dominated by Brycinus lateralis and Schilbe intermedius, contributing 39.5% and 38.5% by weight, respectively. Large-meshed (3–4.5 inch) monofilament and multifilament gillnets emulating those used in the commercial fishery caught mostly Oreochromis andersonii (42.7% by weight) and Serranochromis macrocephalus (28.3% by weight), respectively. Commercial gillnet catch per unit effort (CPUE) averaged 7.0 kg 100-m net−1 night−1, and the annual yield for 2011/2012 was estimated at 2 700 tonnes. Aquatic macrophytes and fishes quickly recolonised the lake and, just two years after refilling, the lake supported a thriving fishery that yielded 4.5 times more than the 600 tonnes recorded in 1974.

The composition and health of fishes in residual dry season habitats in southern Africa (Strauch et al. 2015)

A recent paper on the composition and health of fish in refugia habitats in seasonal tributaries of the Zambezi River in southern Africa contains several errors. These include the misidentification of species and a misunderstanding of the zoogeography of the Zambezi River. There were also several weaknesses in the data analysis and some conclusions were based on misinterpretations of their own data and the literature. The authors should have considered regional literature and worked with southern African ichthyologists to prevent these errors.

Could the elongate yellow-orange nostrils of Anguilla bicolor McClelland, 1844 function as fishing lures?

The shortfin eel Anguilla bicolor has elongate, yellow nostrils tipped with orange that protrude forward above the mouth. They are a striking, highly visible feature and it is hypothesised that they function as lures to attract prey, analogous to the illicium and esca of anglerfishes and frogfishes and the lingual appendage of the alligator snapping turtle. Another possible function is as an intraspecific signalling device. The first hypothesis is favoured here.

What factors drive fishery yields in the Lower Shire River, Malawi?

The Shire River drains from Lake Malawi to the Lower Zambezi River. Annual flow is dependent mainly on lake level, partially controlled by the operation of a barrage at Liwonde to regulate flows for hydroelectricity generation in the escarpment reaches of the river. Downstream of the escarpment, the floodplains of the Lower Shire River support important subsistence fisheries that yielded on average 5 100 t y−1 between 1977 and 1993. Catches from the major fishery area, the Elephant Marsh, were positively correlated with mean annual flow for the same year. Since the target fish species (mainly clariid catfishes) were mostly caught in their second and third year, improved recruitment through higher breeding success at higher flow levels is less likely to explain interannual variation in catches than is greater upstream migration of fish from the Lower Zambezi River in high-flow years. In 1978 a sudden closure of Liwonde Barrage resulted in rapid drainage of stagnant water from the Elephant Marsh floodplain vegetation into the river, causing a mass fish mortality and thus a marked decline in catches from 1979, followed by gradual recovery by 1981. The delay in recovery contrasts with the annual direct catch/flow relationship, and raises questions about the most important influences on Lower Shire catch rates.