J. Kolding

Reconsidering the Consequences of Selective Fisheries

Balanced fishing across a range of species, stocks, and sizes could mitigate adverse effects and address food security better than increased selectivity. Concern about the impact of fishing on ecosystems and fisheries production is increasing (1, 2). Strategies to reduce these impacts while addressing the growing need for food security (3) include increasing selectivity (1, 2): capturing species, sexes, and sizes in proportions that differ from their occurrence in the ecosystem. Increasing evidence suggests that more selective fishing neither maximizes production nor minimizes impacts (4–7). Balanced harvesting would more effectively mitigate adverse ecological effects of fishing while supporting sustainable fisheries. This strategy, which challenges present management paradigms, distributes a moderate mortality from fishing across the widest possible range of species, stocks, and sizes in an ecosystem, in proportion to their natural productivity (8), so that the relative size and species composition is maintained.

Are the Lake Victoria fisheries threatened by exploitation or eutrophication? Towards an ecosystem-based approach to management.

Lake Victoria’s ecosystem has shown fundamental changes over its past recorded history in terms of nutrient loadings, productivity, faunal composition and fisheries. As yet, however, no attempt has been made to link the driving processes of eutrophication and fisheries to understand the feedback observed in fish stocks, food webs, exploitation patterns and trade. Single- and multi-species stock assessments, based on steady-state models with effort (and/or predation) as the only driver – still used in the region to advise on management – uniformly indicate overfished stocks of Nile perch that are in danger of collapse. These current views of overfishing are not validated by empirical observations. This chapter presents a holistic integrated ecosystem approach which combines a phenomenological analysis of key processes with a comprehensive set of simple indicators, covering physical, biological and human development, where directionality in time is made explicit to understand ongoing processes in the changing ecosystem. This new approach results in: (i) no signs of overfishing in any of the verifiable indicators; and (ii) biological production increasing over time together with effort and yield as a function of increased eutrophication. The results indicate that continued eutrophication presents a much graver risk to the resource base and thus livelihoods of Lake Victoria’s coastal populations than fishing pressure. Lake Victoria can serve as an interesting case study for the inherent risk of using traditional fish stock assessment in changing ecosystems, and for the development of holistic monitoring systems for ecosystem-based management.

Population dynamics and life-history styles of Nile tilapia,Oreochromis niloticus, in Ferguson's Gulf, Lake Turkana, Kenya

SynopsisThree observed dynamic aspects of the Nile tilapia population around Fergusons Gulf at Lake Turkana, Kenya are evaluated and discussed: the seasonality in catch rates, the enormous inter-annual abundance variations, and the large changes in median size at first maturity. A clear understanding of the regulating mechanisms behind these features has never been achieved, although seasonal changes in the hydrology of shallow sheltered refuges seems to play an important role. This paper suggests a further holistic approach taking the impacts and interrelationships of both the primary productivity and the various predators into account. A synthesizing ecological hypothesis is elaborated, which concludes that most observations on the tilapia dynamics can be explained from changes in the oxygen concentrations and size-specific mortality pressures. Variations in these two proximate factors can ultimately be explained by the floodplain-type fluctuations in the Fergusons Gulf environment.

The Tragedy of Our Legacy: How do Global Management Discourses Affect Small Scale Fisheries in the South?

Modern fisheries management discourse is supported by two fundamental narratives that have global impacts. One is the fear of open access regimes, and the other is the condemnation of catching under-sized and immature fish. These narratives have existed for more than half a century and originate from the common property theory and the maximum yield per recruit theory. Our aim is to critically discuss and evaluate these narratives which have been developed within the context of scientific management of single-species industrial fisheries. We will show that the underlying assumptions can be seriously wrong and particularly absurd in fluctuating multi-species, multi-gear artisanal fisheries. Fishing effort in small scale fisheries is often largely regulated by natural production, like other top predators, and many targeted fish stocks and fish communities display a high degree of resilience. Furthermore, in spite of common belief, small scale unregulated, non-selective, adaptive fishing patterns could be healthier and far more ecosystem conserving than the current imposed single species management strategies. Many of these fisheries are serving as a ‘social security system’ – a common good and thereby function as a ‘last resort’ for economic mishap. Limiting open access will undermine the role of small scale fisheries to provide insurance, particularly for the poorest and least advantaged. The immense pressure to adapt to modern fisheries management thinking and economic theory is based on flawed assumptions and will not only have negative social effects, but also negative biological effects.

On the relationship between hydrology and fisheries in man-made Lake Kariba, central Africa

This paper attempts to show that the hydrological regime of Lake Kariba, especially lake level fluctuations, and the resultant nutrient inputs are important in promoting fish production and hence the exploitation of the fish resources. Experimental, inshore artisanal and offshore pelagic catch per unit effort time series fluctuate synchronously and show a remarkably high correlation with different time-lagged indices of water level changes, particularly water level rises. In contrast, there was little or no correspondence with absolute water levels even during periods of drought. This finding seems to contradict earlier beliefs that fluctuating lake levels have an adverse effect on the fisheries and that the waters should be kept as stable as possible.

Inshore fish population and species changes in Lake Kariba, Zimbabwe

Lake Kariba (Fig. 12.1) is artificial and probably one of the most studied freshwater impoundments in Africa, as judged from the bibliographical listings (Marshall, 1979; Sanyanga, 1988; Hutton, 1991). However, relatively few analyses or reviews have been published on the fish population changes (Jackson, 1961; Donnelly, 1970, 1971; Balon, 1974b; Kenmuir, 1984; Marshall, 1984; Moyo, 1986), and these describe mainly the early transformations. The last detailed analysis ends in 1975 (Kenmuir, 1984) and since 1982 no data have been published (Marshall, 1984). Moreover, as with all artificial dams and reservoirs, changes in the fish fauna are to be expected during the initial phases (Petr, 1967; Lelek, 1973; Lowe-McConnell, 1973; Balon, 1974b; Lewis, 1974) and so the critical question is when stabilization will occur and the rate of biological changes will decrease in Lake Kariba.

Changes in species composition and abundance of fish populations in Lake Turkana, Kenya

Lake Turkana, a large holomictic endorheic Rift Valley lake between 2°27′ N and 4°40′N in the arid northern part of Kenya (Fig. 16.1), is relatively little studied and understood. Besides a 1930–31 Cambridge University Expedition (Worthington and Ricardo, 1936), which worked for only a few months on the lake in a localized area, only one major investigation into the biology has been made. This was the Lake Turkana Fisheries Research Project sponsored by the Overseas Development Administration of the UK Government from 1972 to 1975, which submitted an exhaustive report on the limnology, biology and productivity of the lake (Hopson, 1982a). Probably due to its equatorial location, Lake Turkana has commonly been associated with environmental and biological stability (e.g. Beadle, 1981; Merona et al., 1988). It appears, however, under closer scrutiny to be more in a state of perpetual change strongly governed by the hydrological regime (Kolding, 1992).

Fisheries and Water Level Fluctuations in the World's Largest Desert Lake

Hydrological regimes are significant drivers of fisheries production in many African Lakes due to their influence on fish habitat and food availability, breeding success, and catchability. Lake Turkana, Kenya will undergo substantial changes in hydrology due to water regulation and extraction along the Omo River in neighboring Ethiopia, which provides over 90% of its water. The objective of this study was to predict how the lakes fisheries, which provide an important livelihood and protein source in the region, will respond to hydrological change. While variations in fishing effort are poor predictors of fisheries catch in the lake, water levels and their fluctuations strongly influence fisheries production. Seasonal oscillations play a particularly important role and with complete loss of these oscillations the lakes predicted fisheries yield will decrease by over two thirds. The fishery is predicted to collapse at a lake level decline of 25 m, regardless of seasonal amplitude magnitude. The lakes total littoral habitat, where fisheries are currently concentrated, will increase in surface area with lake level declines of <25 m. However, the extent of productive, dynamic littoral habitat will decrease with dampening of the lakes seasonal oscillations. The most severe habitat loss will occur in the lakes Turkwel Sector, which hosts the regions highest human population densities, and North Sector, where inter-tribal conflict over resources is common and likely to be exacerbated by lake level decline. The continued ecological functioning of Lake Turkana necessitates immediate efforts to develop and apply a water resource management plan rooted in science.

Dynamics of the feeding ecology of selected fish species from the Okavango River delta, Botswana

Background. This study contributes to the understanding of the potential impact of changes in flooding patterns and (potential) fish production as a consequence of upstream developments in floodplain systems. Therefore, stomachs of eight fish species from the Okavango Delta were analysed to evaluate the feeding ecology of floodplain fish (and the effect of seasonal flooding), using the delta as a case study. Materials and methods. In total, 2101 fish stomachs of eight species, representing six families, were collected (in all seasons) from the delta using experimental fishing nets, from 2004 to 2009. Frequency of occurrence, Levin’s diet breadth index, Pianka’s overlap index, trophic levels, and Bray–Curtis similarities were used to evaluate feeding preferences. Detrended correspondence analysis was used to study temporal variations in diet. Multiple linear regressions were used to determine the influence of flooding on diet. ANOVA and MANOVA were used to determine the level of significance among variables, while LSD post hoc analysis revealed the source of significance. Results. Cluster analysis and Pianka’s index highlighted interand intra-specific competition for food among different species and age classes, ANOVA highlighted dynamic changes in interand intra-specific trophic level partitioning, while detrended analysis showed that the terrestrial environment is subsidizing the aquatic environment. Regression analysis showed that Schilbe intermedius diet was driven by discharge (F = 7.03; P = 0.045; R2 = 0.58) while that of Marcusenius macrolepidotus was driven by water depth (F = 25.88; P = 0.04; R2 = 0.93). Conclusion. The terrestrial–aquatic ecotone is important in fish growth of seasonal floodplains. Energy uptake is optimised through cannibalism to ensure species survival. Furthermore, species inter-relations are dynamic due to variations in food availability driven by seasonal flooding, which shortens and lengthens the food chain periodically. This dynamic relation is pronounced at low floods when predation and competition increases within the fish community. The evidence from this study has shown that predation, cannibalism, interand intra-specific competition are regulating factors in floodplain fish communities, driven by seasonal flooding.

Effects of ambient oxygen and size-selective mortality on maturation and growth in guppies

Abstract Human activities, e.g. nutrient enrichment (resulting in low oxygen) and selective fishing (i.e. higher mortality of large fish), affect growth and maturation (key characteristics determining fish life history). We investigated combined effects of oxygen and size-dependent mortality on growth and maturation to provide informed management decisions for these events.